Louis Pasteur

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Louis Pasteur

(born December 27, 1822, Dole, France—died September 28, 1895, Saint-Cloud), French chemist and microbiologist who was one of the most important founders of medical microbiology. Pasteur’s contributions to science, technology, and medicine are nearly without precedent. He pioneered the study of molecular asymmetry; discovered that microorganisms cause fermentation and disease; originated the process of pasteurization; saved the beer, wine, and silk industries in France; and developed vaccines against anthrax and rabies.

Louis Pasteur in his laboratory, painting by Albert Edelfelt, 1885.

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Louis Pasteur.

Archives Photographiques, Paris

Pasteur’s academic positions were numerous, and his scientific accomplishments earned him France’s highest decoration, the Legion of Honour, as well as election to the Académie des Sciences and many other distinctions. Today there are some 30 institutes and an impressive number of hospitals, schools, buildings, and streets that bear his name—a set of honours bestowed on few scientists. Early education

Pasteur’s father, Jean-Joseph Pasteur, was a tanner and a sergeant major decorated with the Legion of Honour during the Napoleonic Wars. This fact probably instilled in the younger Pasteur the strong patriotism that later was a defining element of his character. Louis Pasteur was an average student in his early years, but he was gifted in drawing and painting. His pastels and portraits of his parents and friends, made when he was 15, were later kept in the museum of the Pasteur Institute in Paris. After attending primary school in Arbois, where his family had moved, and secondary school in nearby Besançon, he earned his bachelor of arts degree (1840) and bachelor of science degree (1842) at the Royal College of Besançon.

Research career

French chemist and microbiologist Louis Pasteur made many important contributions to science,…

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In 1843 Pasteur was admitted to the École Normale Supérieure (a teachers’ college in Paris), where he attended lectures by French chemist Jean-Baptiste-André Dumas and became Dumas’s teaching assistant. Pasteur obtained his master of science degree in 1845 and then acquired an advanced degree in physical sciences. He later earned his doctorate in sciences in 1847. Pasteur was appointed professor of physics at the Dijon Lycée (secondary school) in 1848 but shortly thereafter accepted a position as professor of chemistry at the University of Strasbourg. On May 29, 1849, he married Marie Laurent, the daughter of the rector of the university. The couple had five children; however, only two survived childhood.

Molecular asymmetry

Soon after graduating from the École Normale Supérieure, Pasteur became puzzled by the discovery of the German chemist Eilhardt Mitscherlich, who had shown that tartrates and paratartrates behaved differently toward polarized light: tartrates rotated the plane of polarized light, whereas paratartrates did not. This was unusual because the compounds displayed identical chemical properties. Pasteur noted that the tartrate crystals exhibited asymmetric forms that corresponded to their optical asymmetry. He made the surprising observation that crystalline paratartrate consisted of a mixture of crystals in a right-handed configuration. However, when these crystals were separated manually, he found that they exhibited right and left asymmetry. In other words, a balanced mixture of both right and left crystals was optically inactive. Thus, Pasteur discovered the existence of molecular asymmetry, the foundation of stereochemistry, as it was revealed by optical activity. Over the course of the next 10 years, Pasteur further investigated the ability of organic substances to rotate the plane of polarized light. He also studied the relationship that existed between crystal structure and molecular configuration. His studies convinced him that asymmetry was one of the fundamental characteristics of living matter.

Germ theory of fermentation

In 1854 Pasteur was appointed professor of chemistry and dean of the science faculty at the University of Lille. While working at Lille, he was asked to help solve problems related to alcohol production at a local distillery, and thus he began a series of studies on alcoholic fermentation. His work on these problems led to his involvement in tackling a variety of other practical and economic problems involving fermentation. His efforts proved successful in unraveling most of these problems, and new theoretical implications emerged from his work. Pasteur investigated a broad range of aspects of fermentation, including the production of compounds such as lactic acid that are responsible for the souring of milk. He also studied butyric acid fermentation.

In 1857 Pasteur left Lille and returned to Paris, having been appointed manager and director of scientific studies at the École Normale Supérieure. That same year he presented experimental evidence for the participation of living organisms in all fermentative processes and showed that a specific organism was associated with each particular fermentation. This evidence gave rise to the germ theory of fermentation.

Pasteur effect

The realization that specific organisms were involved in fermentation was further supported by Pasteur’s studies of butyric acid fermentation. These studies led Pasteur to the unexpected discovery that the fermentation process could be arrested by passing air (that is, oxygen) through the fermenting fluid, a process known today as the Pasteur effect. He concluded that this was due to the presence of a life-form that could function only in the absence of oxygen. This led to his introduction of the terms aerobic and anaerobic to designate organisms that live in the presence or absence of oxygen, respectively. He further proposed that the phenomena occurring during putrefaction were due to specific germs that function under anaerobic conditions.

Pasteurization

Pasteur readily applied his knowledge of microbes and fermentation to the wine and beer industries in France, effectively saving the industries from collapse due to problems associated with production and with contamination that occurred during export. In 1863, at the request of the emperor of France, Napoleon III, Pasteur studied wine contamination and showed it to be caused by microbes. To prevent contamination, Pasteur used a simple procedure: he heated the wine to 50–60 °C (120–140 °F), a process now known universally as pasteurization. Today pasteurization is seldom used for wines that benefit from aging, since it kills the organisms that contribute to the aging process, but it is applied to many foods and beverages, particularly milk.

Learn about the chemistry of beer and the process of brewing from a brewmaster of the Samuel Adams…

Following Pasteur’s success with wine, he focused his studies on beer. By developing practical techniques for the control of beer fermentation, he was able to provide a rational methodology for the brewing industry. He also devised a method for the manufacturing of beer that prevented deterioration of the product during long periods of transport on ships.

Spontaneous generation

Fermentation and putrefaction were often perceived as being spontaneous phenomena, a perception stemming from the ancient belief that life could generate spontaneously. During the 18th century the debate was pursued by the English naturalist and Roman Catholic divine John Turberville Needham and the French naturalist Georges- Louis Leclerc, count de Buffon. While both supported the idea of spontaneous generation, Italian abbot and physiologist Lazzaro Spallanzani maintained that life could never spontaneously generate from dead matter. In 1859, the year English naturalist Charles Darwin published his On the Origin of Species, Pasteur decided to settle this dispute. He was convinced that his germ theory could not be firmly substantiated as long as belief in spontaneous generation persisted. Pasteur attacked the problem by using a simple experimental procedure. He showed that beef broth could be sterilized by boiling it in a “swan-neck” flask, which has a long bending neck that traps dust particles and other contaminants before they reach the body of the flask. However, if the broth was boiled and the neck of the flask was broken off following boiling, the broth, being reexposed to air, eventually became cloudy, indicating microbial contamination. These experiments proved that there was no spontaneous generation, since the boiled broth, if never reexposed to air, remained sterile. This not only settled the philosophical problem of the origin of life at the time but also placed on solid ground the new science of bacteriology, which relied on proven techniques of sterilization and aseptic manipulation.

Work with silkworms

In 1862 Pasteur was elected to the Académie des Sciences, and the following year he was appointed professor of geology, physics, and chemistry at the École des Beaux-Arts (School of Fine Arts). Shortly after this, Pasteur turned his attention to France’s silkworm crisis. In the middle of the 19th century, a mysterious disease had attacked French silkworm nurseries. Silkworm eggs could no longer be produced in France, and they could not be imported from other countries, since the disease had spread all over Europe and had invaded the Caucasus region of Eurasia, as well as China and Japan. By 1865 the silkworm industry was almost completely ruined in France and, to a lesser extent, in the rest of western Europe. Pasteur knew virtually nothing about silkworms, but, upon the request of his former mentor Dumas, Pasteur took charge of the problem, accepting the challenge and seizing the opportunity to learn more about infectious diseases. He soon became an expert silkworm breeder and identified the organisms that caused the silkworm disease. After five years of research, he succeeded in saving the silk industry through a method that enabled the preservation of healthy silkworm eggs and prevented their contamination by the disease-causing organisms. Within a couple of years, this method was recognized throughout Europe; it is still used today in silk-producing countries.

In 1867 Pasteur resigned from his administrative duties at the École Normale Supérieure and was appointed professor of chemistry at the Sorbonne, a university in Paris. Although he was partially paralyzed (left hemiplegia) in 1868, he continued his research. For Pasteur, the study of silkworms constituted an initiation into the problem of infectious diseases, and it was then that he first became aware of the complexities of infectious processes. Accustomed as he was to the constancy and accuracy of laboratory procedures, he was puzzled by the variability of animal life, which he had come to recognize through his observation that individual silkworms differed in their response to disease depending on physiological and environmental factors. By investigating these problems, Pasteur developed certain practices of epidemiology that served him well a few years later when he dealt with animal and human diseases.

Vaccine development

French chemist and microbiologist Louis Pasteur experimenting on a chloroformed rabbit, coloured…

National Library of Medicine, Bethesda, Maryland

In the early 1870s Pasteur had already acquired considerable renown and respect in France, and in 1873 he was elected as an associate member of the Académie de Médecine. Nonetheless, the medical establishment was reluctant to accept his germ theory of disease, primarily because it originated from a chemist. However, during the next decade, Pasteur developed the overall principle of vaccination and contributed to the foundation of immunology.

Pasteur’s first important discovery in the study of vaccination came in 1879 and concerned a disease called chicken cholera. (Today the bacteria that cause the disease are classified in the genus Pasteurella.) Pasteur said, “Chance only favours the prepared mind,” and it was chance observation through which he discovered that cultures of chicken cholera lost their pathogenicity and retained “attenuated” pathogenic characteristics over the course of many generations. He inoculated chickens with the attenuated form and demonstrated that the chickens were resistant to the fully virulent strain. From then on, Pasteur directed all his experimental work toward the problem of immunization and applied this principle to many other diseases.

Pasteur began investigating anthrax in 1879. At that time an anthrax epidemic in France and in some other parts of Europe had killed a large number of sheep, and the disease was attacking humans as well. German physician Robert Koch announced the isolation of the anthrax bacillus, which Pasteur confirmed. Koch and Pasteur independently provided definitive experimental evidence that the anthrax bacillus was indeed responsible for the infection. This firmly established the germ theory of disease, which then emerged as the fundamental concept underlying medical microbiology.

Pasteur wanted to apply the principle of vaccination to anthrax. He prepared attenuated cultures of the bacillus after determining the conditions that led to the organism’s loss of virulence. In the spring of 1881 he obtained financial support, mostly from farmers, to conduct a large-scale public experiment of anthrax immunization. The experiment took place in Pouilly-le-Fort, located on the southern outskirts of Paris. Pasteur immunized 70 farm animals, and the experiment was a complete success. The vaccination procedure involved two inoculations at intervals of 12 days with vaccines of different potencies. One vaccine, from a low-virulence culture, was given to half the sheep and was followed by a second vaccine from a more virulent culture than the first. Two weeks after these initial inoculations, both the vaccinated and control sheep were inoculated with a virulent strain of anthrax. Within a few days all the control sheep died, whereas all the vaccinated animals survived. This convinced many people that Pasteur’s work was indeed valid.

Following the success of the anthrax vaccination experiment, Pasteur focused on the microbial origins of disease. His investigations of animals infected by pathogenic microbes and his studies of the microbial mechanisms that cause harmful physiological effects in animals made him a pioneer in the field of infectious pathology. It is often said that English surgeon Edward Jenner discovered vaccination and that Pasteur invented vaccines. Indeed, almost 90 years after Jenner initiated immunization against smallpox, Pasteur developed another vaccine—the first vaccine against rabies. He had decided to attack the problem of rabies in 1882, the year of his acceptance into the Académie Française. Rabies was a dreaded and horrible disease that had fascinated popular imagination for centuries because of its mysterious origin and the fear it generated. Conquering it would be Pasteur’s final endeavour.

Louis Pasteur, coloured lithograph from Vanity Fair (1887).

National Library of Medicine, Bethesda, Maryland

Pasteur suspected that the agent that caused rabies was a microbe (the agent was later discovered to be a virus, a nonliving entity). It was too small to be seen under Pasteur’s microscope, and so experimentation with the disease demanded the development of entirely new methodologies. Pasteur chose to conduct his experiments using rabbits and transmitted the infectious agent from animal to animal by intracerebral inoculations until he obtained a stable preparation. In order to attenuate the invisible agent, he desiccated the spinal cords of infected animals until the preparation became almost nonvirulent. He realized later that, instead of creating an attenuated form of the agent, his treatment had actually neutralized it. (Pasteur perceived the neutralizing effect as a killing effect on the agent, since he suspected that the agent was a living organism.) Thus, rather unknowingly, he had produced, instead of attenuated live microorganisms, a neutralized agent and opened the way for the development of a second class of vaccines, known as inactivated vaccines.

On July 6, 1885, Pasteur vaccinated Joseph Meister, a nine-year-old boy who had been bitten by a rabid dog. The vaccine was so successful that it brought immediate glory and fame to Pasteur. Hundreds of other bite victims throughout the world were subsequently saved by Pasteur’s vaccine, and the era of preventive medicine had begun. An international fund-raising campaign was launched to build the Pasteur Institute in Paris, the inauguration of which took place on November 14, 1888. Implications of Pasteur’s work

French chemist and microbiologist Louis Pasteur (1852).

The theoretical implications and practical importance of Pasteur’s work were immense. Pasteur once said, “There are no such things as pure and applied science; there are only science and the application of science.” Thus, once he established the theoretical basis of a given process, he investigated ways to further develop industrial applications. (As a result, he deposited a number of patents.)

However, Pasteur did not have enough time to explore all the practical aspects of his numerous theories. One of the most important theoretical implications of his later research, which emerged from his attenuation procedure for vaccines, is the concept that virulence is not a constant attribute but a variable property—a property that can be lost and later recovered. Virulence could be decreased, but Pasteur suspected that it could be increased as well. He believed that increased virulence was what gave rise to epidemics. In Louis Pasteur, Free Lance of Science (1950), American microbiologist René Dubos quoted Pasteur:

Thus, virulence appears in a new light which may be disturbing for the future of humanity unless nature, in its long evolution, has already had the occasions to produce all possible contagious diseases—a very unlikely assumption.What is a microorganism that is innocuous to man or to a given animal species? It is a living being which does not possess the capacity to multiply in our body or in the body of the animal. But nothing proves that if the same microorganism should chance to come into contact with some other of the thousands of animal

species in the Creation, it might invade it and render it sick. Its virulence might increase by repeated passages through that species, and might eventually affect man or domesticated animals. Thus might be brought about a new virulence and new contagions. I am much inclined to believe that such mechanisms would explain how smallpox, syphilis, plague, yellow fever, etc. have come about in the course of time, and how certain great epidemics appear once in a while.

Pasteur was the first to recognize variability in virulence. Today this concept remains relevant to the study of infectious disease, especially with regard to understanding the emergence of diseases such as bovine spongiform encephalopathy (BSE), severe acute respiratory syndrome (SARS), and acquired immunodeficiency syndrome (AIDS).

A tribute from Le Petit Journal, Paris, at the time of Louis Pasteur's death (1895).

After Pasteur’s 70th birthday, which was acknowledged by a large but solemn celebration at the Sorbonne that was attended by several prominent scientists, including British surgeon Joseph Lister, Pasteur’s health continued to deteriorate. His paralysis worsened, and he died on September 28, 1895. He was buried in the cathedral of Notre-Dame de Paris, but his remains were transferred to a Neo-Byzantine crypt at the Pasteur Institute in 1896.

Le Bel Institute, University of Strasbourg, France. From 1971 to 2009, this portion of the school…

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During Pasteur’s career, he touched on many problems, but a simple description of his achievements does not do justice to the intensity and fullness of his life. He never accepted defeat, and he always tried to convince skeptics, though his impatience and intolerance were notorious when he believed that truth was on his side. Throughout his life he was an immensely effective observer and readily integrated relevant observations into conceptual schemes.

Agnes Ullmann Additional Reading

Studies of Pasteur’s life and work include ÉMILE DUCLAUX, Pasteur: The History of a Mind (1920, reissued 1973; originally published in French, 1896), a scientific and philosophical work written by a collaborator of Pasteur;

FRANÇOIS DAGOGNET, Méthodes et doctrine dans l’œuvre de Pasteur (1967), primarily a detailed work of

methodology; RENÉ J. DUBOS, Louis Pasteur, Free Lance of Science (1950, reprinted 1993), more philosophical

than scientific; ELIE METCHNIKOFF (I.I. MECHNIKOV), The Founders of Modern Medicine: Pasteur, Koch, Lister (1939, reprinted special edition, 2006; originally published in French, 1933), written by an important scholar who

worked with Pasteur; JACQUES NICOLLE, Louis Pasteur: A Master of Scientific Enquiry (1961; originally published in French, 1953), and Louis Pasteur: The Story of His Major Discoveries (1961), both works giving a complete authoritative review of Pasteur’s discoveries, and Pasteur: sa vie, sa méthode, ses découvertes (1969), an

account of Pasteur’s life and work; and RENÉ VALLERY-RADOT, The Life of Pasteur, 2 vol. (1902, reissued 1960; originally published in French, 1900), written by Pasteur’s son-in-law, who was also his secretary, a fundamental work on the life of Pasteur but weak from the scientific point of view. A modern account of Pasteur’s life and

contributions to science is PATRICE DEBRÉ, Louis Pasteur (1998; originally published in French, 1994).

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©bacteriology 2020 Encyclopædia Britannica, Inc. 11 of 43 Britannica LaunchPacks | Louis Pasteur

bacteriology

branch of microbiology dealing with the study of bacteria.

Technician examining bacteria culture in a laboratory.

The beginnings of bacteriology paralleled the development of the microscope. The first person to see microorganisms was probably the Dutch naturalist Antonie van Leeuwenhoek, who in 1683 described some animalcules, as they were then called, in water, saliva, and other substances. These had been seen with a simple lens magnifying about 100–150 diameters. The organisms seem to correspond with some of the very large forms of bacteria as now recognized.

As late as the mid-19th century, bacteria were known only to a few experts and in a few forms as curiosities of the microscope, chiefly interesting for their minuteness and motility. Modern understanding of the forms of bacteria dates from Ferdinand Cohn’s brilliant classifications, the chief results of which were published at various periods between 1853 and 1872. While Cohn and others advanced knowledge of the morphology of bacteria, other researchers, such as Louis Pasteur and Robert Koch, established the connections between bacteria and the processes of fermentation and disease, in the process discarding the theory of spontaneous generation and improving antisepsis in medical treatment.

The modern methods of bacteriological technique had their beginnings in 1870–85 with the introduction of the use of stains and by the discovery of the method of separating mixtures of organisms on plates of nutrient media solidified with gelatin or agar. Important discoveries came in 1880 and 1881, when Pasteur succeeded in immunizing animals against two diseases caused by bacteria. His research led to a study of disease prevention and the treatment of disease by vaccines and immune serums (a branch of medicine now called immunology). Other scientists recognized the importance of bacteria in agriculture and the dairy industry.

Bacteriological study subsequently developed a number of specializations, among which are agricultural, or soil, bacteriology; clinical diagnostic bacteriology; industrial bacteriology; marine bacteriology; public-health bacteriology; sanitary, or hygienic, bacteriology; and systematic bacteriology, which deals with taxonomy.

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fermentation

chemical process by which molecules such as glucose are broken down anaerobically. More broadly, fermentation is the foaming that occurs during the manufacture of wine and beer, a process at least 10,000 years old. The frothing results from the evolution of carbon dioxide gas, though this was not recognized until the 17th century. French chemist and microbiologist Louis Pasteur in the 19th century used the term fermentation in a narrow sense to describe the changes brought about by yeasts and other microorganisms growing in the absence of air (anaerobically); he also recognized that ethyl alcohol and carbon dioxide are not the only products of fermentation.

At this plant in South Dakota, starch from corn is processed via fermentation for the production of…

In the 1920s it was discovered that, in the absence of air, extracts of muscle catalyze the formation of lactate from glucose and that the same intermediate compounds formed in the fermentation of grain are produced by muscle. An important generalization thus emerged: that fermentation reactions are not peculiar to the action of yeast but also occur in many other instances of glucose utilization.

The generation of pyruvate through the process of glycolysis is the first step in fermentation.

Encyclopædia Britannica, Inc.

Glycolysis, the breakdown of sugar, was originally defined about 1930 as the metabolism of sugar into lactate. It can be further defined as that form of fermentation, characteristic of cells in general, in which the six-carbon sugar glucose is broken down into two molecules of the three-carbon organic acid, pyruvic acid (the nonionized form of pyruvate), coupled with the transfer of chemical energy to the synthesis of adenosine triphosphate (ATP). The pyruvate may then be oxidized, in the presence of oxygen, through the tricarboxylic acid cycle, or in the absence of oxygen, be reduced to lactic acid, alcohol, or other products. The sequence from glucose to pyruvate is often called the Embden–Meyerhof pathway, named after two German biochemists who in the late 1920s and ’30s postulated and analyzed experimentally the critical steps in that series of reactions.

The term fermentation now denotes the enzyme-catalyzed, energy-yielding pathway in cells involving the anaerobic breakdown of molecules such as glucose. In most cells the enzymes occur in the soluble portion of the cytoplasm. The reactions leading to the formation of ATP and pyruvate thus are common to sugar transformation in muscle, yeasts, some bacteria, and plants. Industrial fermentation

The role that controlled spoilage has played in the development of cuisine.

Industrial fermentation processes begin with suitable microorganisms and specified conditions, such as careful adjustment of nutrient concentration. The products are of many types: alcohol, glycerol, and carbon dioxide from yeast fermentation of various sugars; butyl alcohol, acetone, lactic acid, monosodium glutamate, and acetic acid from various bacteria; and citric acid, gluconic acid, and small amounts of antibiotics, vitamin B , and riboflavin 12 (vitamin B ) from mold fermentation. Ethyl alcohol produced via the fermentation of starch or sugar is an 2 important source of liquid biofuel.

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germ theory

in medicine, the theory that certain diseases are caused by the invasion of the body by microorganisms, organisms too small to be seen except through a microscope. The French chemist and microbiologist Louis Pasteur, the English surgeon Joseph Lister, and the German physician Robert Koch are given much of the credit for development and acceptance of the theory. In the mid-19th century Pasteur showed that fermentation and putrefaction are caused by organisms in the air; in the 1860s Lister revolutionized surgical practice by utilizing carbolic acid (phenol) to exclude atmospheric germs and thus prevent putrefaction in compound fractures of bones; and in the 1880s Koch identified the organisms that cause tuberculosis and cholera.

Louis Pasteur.

Archives Photographiques, Paris

Joseph Lister, 1857.

Courtesy of the Wellcome Trustees, London

Although the germ theory has long been considered proved, its full implications for medical practice were not immediately apparent; bloodstained frock coats were considered suitable operating-room attire even in the late 1870s, and surgeons operated without masks or head coverings as late as the 1890s.

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microbiology

study of microorganisms, or microbes, a diverse group of generally minute simple life-forms that include bacteria, archaea, algae, fungi, protozoa, and viruses. The field is concerned with the structure, function, and classification of such organisms and with ways of both exploiting and controlling their activities.

The 17th-century discovery of living forms existing invisible to the naked eye was a significant milestone in the history of science, for from the 13th century onward it had been postulated that “invisible” entities were responsible for decay and disease. The word microbe was coined in the last quarter of the 19th century to describe these organisms, all of which were thought to be related. As microbiology eventually developed into a specialized science, it was found that microbes are a very large group of extremely diverse organisms.

Photomicrograph of Streptococcus pyogenes, a bacterium that can cause scarlet fever. (Magnified…

Centers for Disease Control and Prevention (CDC) (Image Number: 2110)

Daily life is interwoven inextricably with microorganisms. In addition to populating both the inner and outer surfaces of the human body, microbes abound in the soil, in the seas, and in the air. Abundant, although usually

unnoticed, microorganisms provide ample evidence of their presence—sometimes unfavourably, as when they cause decay of materials or spread diseases, and sometimes favourably, as when they ferment sugar to wine and beer, cause bread to rise, flavour cheeses, and produce valued products such as antibiotics and insulin. Microorganisms are of incalculable value to Earth’s ecology, disintegrating animal and plant remains and converting them to simpler substances that can be recycled in other organisms. Historical background

Microbiology essentially began with the development of the microscope. Although others may have seen microbes before him, it was Antonie van Leeuwenhoek, a Dutch draper whose hobby was lens grinding and making microscopes, who was the first to provide proper documentation of his observations. His descriptions and drawings included protozoans from the guts of animals and bacteria from teeth scrapings. His records were excellent because he produced magnifying lenses of exceptional quality. Leeuwenhoek conveyed his findings in a series of letters to the British Royal Society during the mid-1670s. Although his observations stimulated much interest, no one made a serious attempt either to repeat or to extend them. Leeuwenhoek’s “animalcules,” as he called them, thus remained mere oddities of nature to the scientists of his day, and enthusiasm for the study of microbes grew slowly. It was only later, during the 18th-century revival of a long-standing controversy about whether life could develop out of nonliving material, that the significance of microorganisms in the scheme of nature and in the health and welfare of humans became evident.

Spontaneous generation versus biotic generation of life

The early Greeks believed that living things could originate from nonliving matter (abiogenesis) and that the goddess Gea could create life from stones. Aristotle discarded this notion, but he still held that animals could arise spontaneously from dissimilar organisms or from soil. His influence regarding this concept of spontaneous generation was still felt as late as the 17th century, but toward the end of that century a chain of observations, experiments, and arguments began that eventually refuted the idea. This advance in understanding was hard fought, involving series of events, with forces of personality and individual will often obscuring the facts.

Although Francesco Redi, an Italian physician, disproved in 1668 that higher forms of life could originate spontaneously, proponents of the concept claimed that microbes were different and did indeed arise in this way. Such illustrious names as John Needham and Lazzaro Spallanzani were adversaries in this debate during the mid- 1700s. In the early half of the 1800s, Franz Schulze and Theodor Schwann were major figures in the attempt to disprove theories of abiogenesis until Louis Pasteur finally announced the results of his conclusive experiments in 1864. In a series of masterful experiments, Pasteur proved that only preexisting microbes could give rise to other microbes (biogenesis). Modern and accurate knowledge of the forms of bacteria can be attributed to German botanist Ferdinand Cohn, whose chief results were published between 1853 and 1892. Cohn’s classification of bacteria, published in 1872 and extended in 1875, dominated the study of these organisms thereafter.

Microbes and disease

Girolamo Fracastoro, an Italian scholar, advanced the notion as early as the mid-1500s that contagion is an infection that passes from one thing to another. A description of precisely what is passed along eluded discovery until the late 1800s, when the work of many scientists, Pasteur foremost among them, determined the role of bacteria in fermentation and disease. Robert Koch, a German physician, defined the procedure (Koch’s postulates) for proving that a specific organism causes a specific disease.

The foundation of microbiology was securely laid during the period from about 1880 to 1900. Students of Pasteur, Koch, and others discovered in rapid succession a host of bacteria capable of causing specific diseases (pathogens). They also elaborated an extensive arsenal of techniques and laboratory procedures for revealing the ubiquity, diversity, and abilities of microbes.

Progress in the 20th century

Hear a scientist discuss the potential for interdisciplinary research in microbiology, biochemistry, …

University College Cork, Ireland

All of these developments occurred in Europe. Not until the early 1900s did microbiology become established in America. Many microbiologists who worked in America at this time had studied either under Koch or at the Pasteur Institute in Paris. Once established in America, microbiology flourished, especially with regard to such related disciplines as biochemistry and genetics. In 1923 American bacteriologist David Bergey established that science’s primary reference, updated editions of which continue to be used today.

Since the 1940s microbiology has experienced an extremely productive period during which many disease- causing microbes have been identified and methods to control them developed. Microorganisms have also been effectively utilized in industry; their activities have been channeled to the extent that valuable products are now both vital and commonplace.

The study of microorganisms has also advanced the knowledge of all living things. Microbes are easy to work with and thus provide a simple vehicle for studying the complex processes of life; as such they have become a powerful tool for studies in genetics and metabolism at the molecular level. This intensive probing into the functions of microbes has resulted in numerous and often unexpected dividends. Knowledge of the basic metabolism and nutritional requirements of a pathogen, for example, often leads to a means of controlling disease or infection. Types of microorganisms

The major groups of microorganisms—namely bacteria, archaea, fungi (yeasts and molds), algae, protozoa, and viruses—are summarized below. Links to the more detailed articles on each of the major groups are provided.

Bacteria (eubacteria and archaea)

Microbiology came into being largely through studies of bacteria. The experiments of Louis Pasteur in France, Robert Koch in Germany, and others in the late 1800s established the importance of microbes to humans. As stated in the Historical background section, the research of these scientists provided proof for the germ theory of disease and the germ theory of fermentation. It was in their laboratories that techniques were devised for the microscopic examination of specimens, culturing (growing) microbes in the laboratory, isolating pure cultures

from mixed-culture populations, and many other laboratory manipulations. These techniques, originally used for studying bacteria, have been modified for the study of all microorganisms—hence the transition from bacteriology to microbiology.

The organisms that constitute the microbial world are characterized as either prokaryotes or eukaryotes; all bacteria are prokaryotic—that is, single-celled organisms without a membrane-bound nucleus. Their DNA (the genetic material of the cell), instead of being contained in the nucleus, exists as a long, folded thread with no specific location within the cell.

Until the late 1970s it was generally accepted that all bacteria are closely related in evolutionary development. This concept was challenged in 1977 by Carl R. Woese and coinvestigators at the University of Illinois, whose research on ribosomal RNA from a broad spectrum of living organisms established that two groups of bacteria evolved by separate pathways from a common and ancient ancestral form. This discovery resulted in the establishment of a new terminology to identify the major distinct groups of microbes—namely, the eubacteria (the traditional or “true” bacteria), the archaea (bacteria that diverged from other bacteria at an early stage of evolution and are distinct from the eubacteria), and the eukarya (the eukaryotes). Today the eubacteria are known simply as the true bacteria (or the bacteria) and form the domain Bacteria. The evolutionary relationships between various members of these three groups, however, have become uncertain, as comparisons between the DNA sequences of various microbes have revealed many puzzling similarities. As a result, the precise ancestry of today’s microbes is very difficult to resolve. Even traits thought to be characteristic of distinct taxonomic groups have unexpectedly been observed in other microbes. For example, an anaerobic ammonia-oxidizer—the “missing link” in the global nitrogen cycle—was isolated for the first time in 1999. This bacterium (an aberrant member of the order Planctomycetales) was found to have internal structures similar to eukaryotes, a cell wall with archaean traits, and a form of reproduction (budding) similar to that of yeast cells.

Schematic drawing of the structure of a generalized bacterium.

Encyclopædia Britannica, Inc.

Bacteria have a variety of shapes, including spheres, rods, and spirals. Individual cells generally range in width from 0.5 to 5 micrometres (μm; millionths of a metre). Although unicellular, bacteria often appear in pairs, chains, tetrads (groups of four), or clusters. Some have flagella, external whiplike structures that propel the organism through liquid media; some have capsule, an external coating of the cell; some produce spores— reproductive bodies that function much as seeds do among plants. One of the major characteristics of bacteria is their reaction to the Gram stain. Depending upon the chemical and structural composition of the cell wall, some bacteria are gram-positive, taking on the stain’s purple colour, whereas others are gram-negative.

Through a microscope the archaea look much like bacteria, but there are important differences in their chemical composition, biochemical activities, and environments. The cell walls of all true bacteria contain the chemical

substance peptidoglycan, whereas the cell walls of archaeans lack this substance. Many archaeans are noted for their ability to survive unusually harsh surroundings, such as high levels of salt or acid or high temperatures. These microbes, called extremophiles, live in such places as salt flats, thermal pools, and deep-sea vents. Some are capable of a unique chemical activity—the production of methane gas from carbon dioxide and hydrogen. Methane-producing archaea live only in environments with no oxygen, such as swamp mud or the intestines of ruminants such as cattle and sheep. Collectively, this group of microorganisms exhibits tremendous diversity in the chemical changes that it brings to its environments.

Algae

The cells of eukaryotic microbes are similar to plant and animal cells in that their DNA is enclosed within a nuclear membrane, forming the nucleus. Eukaryotic microorganisms include algae, protozoa, and fungi. Collectively algae, protozoa, and some lower fungi are frequently referred to as protists (kingdom Protista, also called Protoctista); some are unicellular and others are multicellular.

Representative algae.

Encyclopædia Britannica, Inc.

Unlike bacteria, algae are eukaryotes and, like plants, contain the green pigment chlorophyll, carry out photosynthesis, and have rigid cell walls. They normally occur in moist soil and aquatic environments. These eukaryotes may be unicellular and microscopic in size or multicellular and up to 120 metres (nearly 400 feet) in length. Algae as a group also exhibit a variety of shapes. Single-celled species may be spherical, rod-shaped, club-shaped, or spindle-shaped. Some are motile. Algae that are multicellular appear in a variety of forms and degrees of complexity. Some are organized as filaments of cells attached end to end; in some species these filaments intertwine into macroscopic, plantlike bodies. Algae also occur in colonies, some of which are simple aggregations of single cells, while others contain different cell types with special functions.

Fungi

Fungi are eukaryotic organisms that, like algae, have rigid cell walls and may be either unicellular or multicellular. Some may be microscopic in size, while others form much larger structures, such as mushrooms and bracket fungi that grow in soil or on damp logs. Unlike algae, fungi do not contain chlorophyll and thus cannot carry out photosynthesis. Fungi do not ingest food but must absorb dissolved nutrients from the environment. Of the fungi classified as microorganisms, those that are multicellular and produce filamentous, microscopic structures are frequently called molds, whereas yeasts are unicellular fungi.

In molds cells are cylindrical in shape and are attached end to end to form threadlike filaments (hyphae) that may bear spores. Individually, hyphae are microscopic in size. However, when large numbers of hyphae accumulate—for example, on a slice of bread or fruit jelly—they form a fuzzy mass called a mycelium that is visible to the naked eye.

The unicellular yeasts have many forms, from spherical to egg-shaped to filamentous. Yeasts are noted for their ability to ferment carbohydrates, producing alcohol and carbon dioxide in products such as wine and bread.

Protozoa

Representative protozoans. The phytoflagellate Gonyaulax is one of the dinoflagellates responsible…

Protozoa, or protozoans, are single-celled, eukaryotic microorganisms. Some protozoa are oval or spherical, others elongated. Still others have different shapes at different stages of the life cycle. Cells can be as small as 1 μm in diameter and as large as 2,000 μm, or 2 mm (visible without magnification). Like animal cells, protozoa lack cell walls, are able to move at some stage of their life cycle, and ingest particles of food; however, some phytoflagellate protozoa are plantlike, obtaining their energy via photosynthesis. Protozoan cells contain the typical internal structures of an animal cell. Some can swim through water by the beating action of short, hairlike appendages (cilia) or flagella. Their rapid, darting movement in a drop of pond water is evident when viewed through a microscope.

Amoeba (magnified).

Russ Kinne/Photo Researchers

The amoebas (also amoebae) do not swim, but they can creep along surfaces by extending a portion of themselves as a pseudopod and then allowing the rest of the cell to flow into this extension. This form of locomotion is called amoeboid movement. The sporozoans (phylum Apicomplexa) are so named because they form dormant bodies called spores during one phase of their life cycle. Protozoa occur widely in nature, particularly in aquatic environments.

Viruses

Schematic structure of the tobacco mosaic virus. The cutaway section shows the helical ribonucleic…

Encyclopædia Britannica, Inc.

Viruses, agents considered on the borderline of living organisms, are also included in the science of microbiology, come in several shapes, and are widely distributed in nature, infecting animal cells, plant cells, and microorganisms. The field of study in which they are investigated is called virology. All viruses are obligate parasites; that is, they lack metabolic machinery of their own to generate energy or to synthesize proteins, so they depend on host cells to carry out these vital functions. Once inside a cell, viruses have genes for usurping the cell’s energy-generating and protein-synthesizing systems. In addition to their intracellular form, viruses have an extracellular form that carries the viral nucleic acid from one host cell to another. In this infectious form, viruses are simply a central core of nucleic acid surrounded by a protein coat called a capsid. The capsid protects the genes outside the host cell; it also serves as a vehicle for entry into another host cell because it binds to receptors on cell surfaces. The structurally mature, infectious viral particle is called a virion.

With the electron microscope it is possible to determine the morphological characteristics of viruses. Virions generally range in size from 20 to 300 nanometres (nm; billionths of a metre). Since most viruses measure less than 150 nm, they are beyond the limit of resolution of the light microscope and are visible only by electron microscopy. By using materials of known size for comparison, microscopists can determine the size and structure of individual virions.

Prions

Even smaller than viruses, prions (pronounced “pree-ons”) are the simplest infectious agents. Like viruses they are obligate parasites, but they possess no genetic material. Although prions are merely self-perpetuating proteins, they have been implicated as the cause of various diseases, including bovine spongiform encephalopathy (mad cow disease), and are suspected of playing a role in a number of other disorders.

Lichens

Elegant sunburst lichen (Xanthoria elegans) and green lichen (Risocarpen geographica).

Lichens represent a form of symbiosis, namely, an association of two different organisms wherein each benefits. A lichen consists of a photosynthetic microbe (an alga or a cyanobacterium) growing in an intimate association with a fungus. A simple lichen is made up of a top layer consisting of a tightly woven fungal mycelium, a middle layer where the photosynthetic microbe lives, and a bottom layer of mycelium. In this mutualistic association, the photosynthetic microbes synthesize nutrients for the fungus, and in return the fungus provides protective cover for the algae or cyanobacteria. Lichens play an important role ecologically; among other activities they are capable of transforming rock to soil.

Slime molds

The slime molds are a biological and taxonomic enigma because they are neither typical fungi nor typical protozoa. During one of their growth stages, they are protozoa-like because they lack cell walls, have amoeboid movement, and ingest particulate nutrients. During their propagative stage they form fruiting bodies and sporangia, which bear walled spores like typical fungi. Traditionally, the slime molds have been classified with the fungi. There are two groups of slime molds: the cellular slime molds and the acellular slime molds. The study of microorganisms

As is the case in many sciences, the study of microorganisms can be divided into two generalized and sometimes overlapping categories. Whereas basic microbiology addresses questions regarding the biology of microorganisms, applied microbiology refers to the use of microorganisms to accomplish specific objectives.

Basic microbiology

The study of the biology of microorganisms requires the use of many different procedures as well as special equipment. The biological characteristics of microorganisms can be summarized under the following categories: morphology, nutrition, physiology, reproduction and growth, metabolism, pathogenesis, antigenicity, and genetic properties.

Morphology

Morphology refers to the size, shape, and arrangement of cells. The observation of microbial cells requires not only the use of microscopes but also the preparation of the cells in a manner appropriate for the particular kind of microscopy. During the first decades of the 20th century, the compound light microscope was the instrument commonly used in microbiology. Light microscopes have a usual magnification factor of 1000 × and a maximum useful magnification of approximately 2000 ×. Specimens can be observed either after they have been stained by one of several techniques to highlight some morphological characteristics or in living, unstained preparations as a “wet mount.”

Light microscopy

Several modifications of light microscopy are available, such as:

bright field

The specimen is usually stained and observed while illuminated; useful for observation of the gross morphological features of bacteria, fungi, algae, and protozoa.

dark field

The specimen is suspended in a liquid on a special slide and can be observed in a living condition; useful for determining motility of microorganisms or some special morphological characteristic such as spiral or coiled shapes.

fluorescence

The specimen is stained with a fluorescent dye and then illuminated; objects that take up the fluorescent dye will “glow.”

phase contrast

Special condenser lenses allow observation of living cells and differentiation of cellular structures of varying density.

Electron microscopy

Scanning electron micrograph of the spirochete Treponema pallidum attached to testicular cell…

ASM/Science Source/Photo Researchers

The development of the electron microscope and complimentary techniques vastly increased the resolving power beyond that attainable with light microscopy. This increase is possible because the wavelengths of the electron beams are so much shorter than the wavelengths of light. Objects as small as 0.02 nm are resolvable by electron microscopy, compared with 0.25 μm—allowing, for instance, the observation of virions and viral structures. Specimens are observed by either transmission electron microscopy or scanning electron microscopy. In TEM the electron beam passes through the specimen and registers on a screen forming the image; in SEM the electron beam moves back and forth over the surface of microorganisms coated with a thin film of metal and registers a three-dimensional picture on the screen.

Advances in microscopes and microscopic techniques continue to be introduced to study cells, molecules, and even atoms. Among these are confocal microscopy, the atomic force microscope, the scanning tunneling microscope, and immunoelectron microscopy. These are particularly significant for studies of microorganisms at the molecular level.

Nutritional and physiological characteristics

Microorganisms as a group exhibit great diversity in their nutritional requirements and in the environmental conditions that will support their growth. No other group of living organisms comes close to matching the versatility and diversity of microbes in this respect. Some species will grow in a solution composed only of inorganic salts (one of the salts must be a compound of nitrogen) and a source of carbon dioxide (CO ); these 2 are called autotrophs. Many, but not all, of these microbes are autotrophic via photosynthesis. Organisms requiring any other carbon source are called heterotrophs. These microbes commonly make use of carbohydrates, lipids, and proteins, although many microbes can metabolize other organic compounds such as hydrocarbons. Others, particularly the fungi, are decomposers. Many species of bacteria also require specific additional nutrients such as minerals, amino acids, and vitamins. Various protozoans, fungi, and bacteria are parasites, either exclusively (obligate parasites) or with the ability to live independently (facultative parasites).

If the nutritional requirements of a microorganism are known, a chemically defined medium containing only those chemicals can be prepared. More complex media are also routinely used; these generally consist of peptone (a partially digested protein), meat extract, and sometimes yeast extract. When a solid medium is desired, agar is added to the above ingredients. Agar is a complex polysaccharide extracted from marine algae. It has several properties that make it an ideal solidifying substance for microbiological media, particularly its resistance to microbial degradation.

Microorganisms vary widely in terms of the physical conditions required for growth. For example, some are aerobes (require oxygen), some are anaerobes (grow only in the absence of oxygen), and some are facultative (they grow in either condition). Eukaryotic microbes are generally aerobic. Microorganisms that grow at temperatures below 20 °C (68 °F) are called psychrophiles; those that grow best at 20–40 °C (68–104 °F) are called mesophiles; a third group, the thermophiles, require temperatures above 40 °C. Those organisms which grow under optimally under one or more physical or chemical extremes, such as temperature, pressure, pH, or salinity, are referred to as extremophiles. Bacteria exhibit the widest range of temperature requirements. Whereas bacterial (and fungal) growth is commonly observed in food that has been refrigerated for a long period, some isolated archaea (e.g., Pyrodictium occultum and Pyrococcus woesei) grow at temperatures above 100 °C (212 °F).

Other physical conditions that affect the growth of microorganisms are acidity or basicity (pH), osmotic pressure, and hydrostatic pressure. The optimal pH for most bacteria associated with the human environment is in the

neutral range near pH 7, though other species grow under extremely basic or acidic conditions. Most fungi are favoured by a slightly lower pH (5–6); protozoa require a range of pH 6.7–7.7; algae are similar to bacteria in their requirements except for the fact that they are photosynthetic.

Reproduction and growth

Bacteria reproduce primarily by binary fission, an asexual process whereby a single cell divides into two. Under ideal conditions some bacterial species may divide every 10–15 minutes—a doubling of the population at these time intervals. Eukaryotic microorganisms reproduce by a variety of processes, both asexual and sexual. Some require multiple hosts or carriers (vectors) to complete their life cycles. Viruses, on the other hand, are produced by the host cell that they infect but are not capable of self-reproduction.

The study of the growth and reproduction of microorganisms requires techniques for cultivating them in pure culture in the laboratory. Data collected on the microbial population over a period of time, under controlled laboratory conditions, allow a characteristic growth curve to be constructed for a species.

Metabolism

Collectively, microorganisms show remarkable diversity in their ability to produce complex substances from simple chemicals and to decompose complex materials to simple chemicals. An example of their synthetic ability is nitrogen fixation—the production of amino acids, proteins, and other organic nitrogen compounds from atmospheric nitrogen (N ). Certain bacteria and blue-green algae (cyanobacteria) are the only organisms 2 capable of this ecologically vital process. An example of microbes’ ability to decompose complex materials is shown by the white and brown rot fungi that decompose wood to simple compounds, including CO . 2

Laboratory procedures are available that make it possible to determine the biochemical capability of a species qualitatively and quantitatively. Routine techniques can identify which compounds or substances are degraded by a specific microbe and which products are synthesized. Through more elaborate experimentation it is possible to determine step-by-step how the microbe performs these biochemical changes. Studies can be performed in a number of ways using growing cultures, “resting cells” (suspensions of cells), cell-free extracts, or enzyme preparations from cells.

Certain biochemical tests are routinely used to identify microbes—though more in the case of bacteria than algae, fungi, or protozoa. The adoption of routine sets of laboratory tests has allowed automated instrumentation to perform the tests. For instance, technicians often simply inoculate individual units of a “chamber” that is preloaded with a specific chemical substance (the substrate) and then place the chamber into an apparatus that serves as an incubator and analyzer. The apparatus automatically records the results and is frequently capable of calculating the degree of accuracy of the identification.

Pathogenesis

Some microorganisms cause diseases of humans, other animals, and plants. Such microbes are called pathogens. Pathogens are identified by the hosts they infect and the symptoms they cause; it is also important to identify the specific properties of the pathogen that contribute to its infectious capacity—a characteristic known as virulence. The more virulent a pathogen, the fewer the number needed to establish an infection.

Antigenic characteristics

An antigen is a substance that, when introduced into an animal body, stimulates the production of specific substances (antibodies) that react or unite with the antigen. Microbial cells and viruses contain a variety of antigenic substances. A significant feature of antigen-antibody reactions is specificity; the antibodies formed as

a result of inoculating an animal with one microbe will not react with the antibodies formed by inoculation with a different microbe. Antibodies appear in the blood serum of animals, and laboratory tests of antigen-antibody reactions are performed by using sera—hence the term serological reactions. Thus, it is possible to characterize a microorganism by its antigenic makeup as well as to identify microorganisms by using one of many different serological tests. Antigens and antibodies are important aspects of immunity, and immunology is included in the science of microbiology.

Genetic characterization

Since the last quarter of the 20th century, researchers have accumulated a vast amount of information elucidating in precise detail the chemical composition, synthesis, and replication of the genetic material of cells. Much of this research has been done by using microorganisms, and techniques have been developed that permit experimentation at the molecular level. For instance, experiments determining the degree of similarity between different organisms’ DNA and RNA have provided new insights for the classification of microorganisms. Test kits are available for the identification of microorganisms, particularly bacteria, by DNA probes.

Since the invention of recombinant DNA technology in 1973, techniques have been developed whereby genes from one cell can be transferred to an entirely different cell, as when a gene is transferred from an animal cell to a bacterium or from a bacterium to a plant cell. Recombinant DNA technology opened the door to many new medical and industrial applications of microbiology, and it plays a central role in genetic engineering.

Applied microbiology

Genetic engineering is an example of how the fields of basic and applied microbiology can overlap. Genetic engineering is primarily considered a field of applied microbiology (that is, the exploitation of microorganisms for a specific product or use). The methods used in genetic engineering were developed in basic research of microbial genetics. Conversely, methods used and perfected for applied microbiology can become tools for basic microbiology. Applied microbiology can, however, be divided under the following headings.

Soil microbiology

However “dead” soil may appear, it is in fact teeming with millions or billions of microbial cells per gram, depending upon soil fertility and the environment. Dead vegetation, human and animal wastes, and dead animals are deposited in or on soil. In time they all decompose into substances that contribute to soil, and microbes are largely responsible for these transformations.

Two great pioneer soil microbiologists were Martinus W. Beijerinck (1851–1931), a Dutchman, and Sergey N. Winogradsky (1856–1953), a Russian. These researchers isolated and identified new types of bacteria from soil, particularly autotrophic bacteria, that use inorganic chemicals as nutrients and as a source of energy. The relationship between legumes and bacteria in the nodules of legume roots was discovered by other scientists in 1888. The nodules contain large numbers of bacteria (Rhizobium) that are capable of fixing atmospheric nitrogen into compounds that can be used by plants.

The ecology of fertile soil consists of plant roots, animals such as rodents, insects, and worms, and a menagerie of microorganisms—viruses, bacteria, algae, fungi, and protozoa. The role of this microbial flora can be conveniently expressed in Earth’s natural cycles. In the nitrogen cycle, for example, microorganisms capture nitrogen gas from the atmosphere and convert it into a combined form of nitrogen that plants can use as a nutrient; the plant synthesizes organic nitrogen compounds that are consumed by humans and animals; the consumed nitrogen compounds eventually reach the soil; microorganisms complete the cycle by decomposing these compounds back to atmospheric nitrogen and simple inorganic molecules that can be used by plants. In

similar cycles for other elements such as carbon, sulfur, and phosphorus, microbes play a role; this makes them essential to maintaining life on Earth.

Microbiology of water supplies, wastewater, and other aquatic environments

Long before the establishment of microbiology as a science, water was suspected of being a carrier of disease- producing organisms. But it was not until 1854, when an epidemic of cholera was proved to have had its origin in polluted water, that contaminated water was considered more seriously as a source of disease. Since that time there has been continuous research on the microbiology of public water supplies, including the development of laboratory procedures to determine whether the water is potable, or safe for human consumption. At the same time, purification procedures for these supplies have emerged.

A highly standardized and routine laboratory procedure to determine the potability of water is based upon detecting the presence or absence of the bacterium Escherichia coli. E. coli is a normal inhabitant of the intestinal tract of humans; its presence in water indicates that the water is polluted with intestinal wastes and may contain disease-producing organisms.

The principal operations employed in a municipal water-purification plant are sedimentation, filtration, and chlorination. Each of these operations removes or kills microorganisms, and the microbiological quality of the treated water is monitored at frequent intervals.

The used water supply of a community, commonly referred to as sewage, is microbiologically significant in two ways. First, sewage is a potential carrier of pathogenic microorganisms, so measures such as chlorination must be implemented to prevent these microbes from contaminating drinking-water supplies. Second, sewage- treatment plants purify water by exploiting the biochemical abilities of microbes to metabolize contaminants. Raw sewage is processed through large tanks, first for anaerobic degradation of complex substrates and later for aerobic oxidation of soluble products. This “activated sludge” treatment is dependent upon incubation conditions that favour the growth and metabolic activity of appropriate microorganisms.

Another aspect of the microbiology of water pertains to natural bodies of water such as ponds, lakes, rivers, and oceans. Aquatic microbes perform a host of biochemical transformations and are an essential component of the food chain in these environments. For example, the microbial flora of the sea comprises bacteria, algae, fungi, and protozoa. The microorganisms inhabiting aquatic environments are collectively referred to as plankton; phytoplankton refers to the photosynthetic microbes (primarily algae), whereas protozoa, and other small animals, are zooplankton. Phytoplankton is responsible for converting solar energy into chemical energy—the components of plankton cells that serve as food for higher aquatic life. The magnitude of this process can be appreciated by calculations indicating that it takes 1,000 tons of phytoplankton to support the growth of one ton of fish.

Large populations of archaea live in volcanic ridges 2,600 metres (8,500 feet) below the ocean surface in areas immediately surrounding hydrothermal vents (deep-sea hot springs). The vents spew superheated water (350 °C [662 °F]) that contains hydrogen sulfide (H S); the water surrounding the vents has a temperature range of 10– 2 20 °C (50–68 °F). Many bacteria concentrate in this region because of the availability of H S, which they can use 2 for energy. The abundance of animal life that also inhabits this region is completely dependent on the microbes for food.

There is a growing interest in other ecological aspects of aquatic microbiology, such as the role of microbes in global warming and oxygen production. Experimental approaches are being developed to study the complex biology and ecology of biofilms and microbial mats. These assemblages of microbes and their products, while

potentially useful in several ways, are complex. In many instances the microbial flora involved must sometimes be studied in its natural environment because the environment cannot be reproduced in the laboratory.

Food microbiology

Microorganisms are of great significance to foods for the following reasons: (1) microorganisms can cause spoilage of foods, (2) microorganisms are used to manufacture a wide variety of food products, and (3) microbial diseases can be transmitted by foods.

Food spoilage

Learn about scientific research into smart tags, small gel-like tags on food packaging that change…

Foods can be considered as a medium for microbial growth. Considering the vast array of sources, substances, and methods with which food is produced, practically every kind of microbe is a potential contaminant. Given a chance to grow, microbes will produce changes in appearance, flavour, odour, and other qualities of the food. The changes vary according to the type of food degraded but can be summarized by examining the fates of the major nutrients found in food: proteins, carbohydrates, and fats.

Protein-containing foods, particularly meats, are putrefied by organisms (e.g., Proteus, Pseudomonas, and Clostridium bacteria) that break down the long peptide chains of proteins into amino acids and foul-smelling compounds such as amines, ammonia, and hydrogen sulfide (H S). 2

Carbohydrates (sugars and starches) are fermented into acids (e.g., the acetic acid in vinegar), alcohols, and gases, especially carbon dioxide. This process is responsible for the bursting of spoiled chocolate cream candies by yeasts.

Fat-containing foods such as dairy products are spoiled by microbes that break down lipids into fatty acids and glycerol. Rancid milk, which can be caused by bacteria, yeast, or mold, is an example of this process.

Improperly canned foods are also subject to spoilage by bacteria, yeasts, and molds. Bacteria such as Bacillus and Clostridium are of particular significance in the canning industry because of the high level of resistance that their spores possess. One example of microbial spoilage of canned foods is “sulfide spoilage” caused by C. nigrificans, in which contents are blackened and have the odour of rotten eggs. Another example is called “flat sour,” in which the spoiled product has an abnormal odour, a cloudy appearance, and a sour taste owing to its lowered pH. Putrefaction caused by C. sporogenes may cause a can to swell and burst, releasing its partially digested contents and a putrid odour.

Food preservation

All methods of food preservation are based upon one or more of the following principles: (1) prevention of contamination and removal of microorganisms, (2) inhibition of microbial growth and metabolism, and (3) killing of microorganisms. Prevention—or, more accurately, minimization—of contamination is achieved by the sanitary handling of raw food products, inhibition of growth by low temperatures (refrigeration or freezing), dehydration by evaporation or by high concentrations of salt or sugar, and killing of microbes by the application of high temperatures and, in some instances, radiation.

Food products from microorganisms

Important food items produced in whole or in part by the biochemical activities of microorganisms include pickles, sauerkraut, olives, soy sauce, certain types of sausage, all unprocessed cheeses except cream cheese, and many fermented milk products such as yogurt and acidophilus milk. In each instance a raw food item, such as cucumbers in the case of pickles or milk protein in the case of cheeses, is inoculated with microorganisms known to produce the changes required for a desirable product. The initial food item thus serves as a substrate that is acted upon by microorganisms during the period of incubation. Frequently the manufacturer uses a “starter culture”—a commercial population of microorganisms already known to produce a good product.

Industrial microbiology and genetic engineering

Many substances of considerable economic value are products of microbial metabolism. From an industrial viewpoint the substrate may be regarded as a raw material and the microorganism as the “chemical factory” for converting the raw material into new products. If an organism can be shown to convert inexpensive raw material into a useful product, it may be feasible to perform this reaction on a large industrial scale if the following conditions can be met.

The organism.

The organism to be employed (a virus, bacterium, yeast, or mold) must have the capacity to produce appreciable amounts of the product. It should have relatively stable characteristics and the ability to grow rapidly and vigorously, and it should be nonpathogenic.

The medium.

The medium, including the substrate from which the organism produces the new product, must be cheap and readily available in large quantities.

The product.

A feasible method of recovering and purifying the desired end product must be developed. Industrial fermentations are performed in large tanks, some with capacities of 190,000 litres (50,000 gallons) or more. The product formed by the metabolism of the microorganism must be removed from a heterogeneous mixture that also includes a tremendous crop of microbial cells and unused constituents of the medium, as well as products of metabolism other than those being sought. Traditional products of industrial microbiology are antibiotics, alcoholic beverages, vaccines, vinegar, and miscellaneous chemicals such as acetone and butyl alcohol.

The development of recombinant DNA technology, however, has made it possible to conceive of virtually unlimited new products made by genetically engineered microorganisms. One example of what can be achieved

via recombinant DNA technology is the production of human insulin by a genetically altered strain of E. coli. By inserting the human gene coding for insulin into the E. coli cell, biotechnologists give this bacterium the ability to synthesize the hormone on an industrial scale.

The scientific advances that have made genetic engineering a reality have broad implications for the future. By introducing foreign genes into microorganisms, it may be possible to develop strains of microbes that offer new solutions to such diverse problems as pollution, food and energy shortages, and the treatment and control of disease.

Medical and public health microbiology

Following the establishment of the germ theory of disease in the mid-1880s and the development of laboratory techniques for the isolation of microorganisms (particularly bacteria), the causative agents of many common diseases were discovered in rapid succession. Some common diseases and the date of discovery of their causative agent illustrate this point: anthrax (1876), gonorrhea (1879), typhoid fever (1880), malaria (1880), tuberculosis (1882), diphtheria (1883), cholera (1884), and tetanus (1884). Some of the most notable successes of medical microbiology include the development of vaccines beginning in the 1790s, antibiotics during the mid- 20th century, and the global eradication of smallpox by 1977.

Despite such great advances in identifying and controlling agents of disease and in devising methods for their control, the world still faces the threat of diseases such as AIDS and hantavirus pulmonary syndrome (HPS), the reemergence of old scourges such as tuberculosis, cholera, and diphtheria, and the increasing resistance of microbes to antibiotics. (See alsopublic health; human disease; antibiotic resistance.)

Plant pathology

Plants are subject to infection by thousands of species of very diverse organisms, most of which are microbes. These disease-producing plant pathogens cause significant agricultural losses and include viruses, bacteria, and mycoplasma-like organisms and fungi. The study of plant diseases is called plant pathology.

Michael J. PelczarRita M. Pelczar Additional Reading

General works

DAVID B. DUSENBERY, Life at Small Scale: The Behavior of Microbes (1996), illustrates descriptions of the environments and survival strategies of microscopic organisms.

BERNARD DIXON, Power Unseen: How Microbes Rule the World (1994, reissued 1996), is a collection of 75 vignettes, each describing one of the myriad roles played by microorganisms throughout human and geologic history.

JOHN POSTGATE, Microbes and Man, 4th ed. (2000), surveys microorganisms and their role in our environment.

Intimate Strangers: Unseen Life on Earth (1999), produced by JULIO EMILIO MELINE, is a series of four video

documentaries aired in the United States by the Public Broadcasting System; a companion text by CYNTHIA

NEEDHAM et al. with the same title was published (2000) by the American Society for Microbiology.

WOLFGANG K. JOKLIK et al. (ed.), Microbiology: A Centenary Perspective (1999), is a collection of the 20th century’s most significant research papers with explanations of the significance and context of each paper.

DORION SAGAN and LYNN MARGULIS, Garden of Microbial Delights: A Practical Guide to the Subvisible World (1988, reissued 1993), examines how microbes affect ecology, industry, medicine, agriculture, and biotechnology.

PAUL DE KRUIF, Microbe Hunters (1926, reissued 1996), offers an account of early microbiology and the individuals who shaped it. This text can be supplemented by another classic volume on the subject,

WILLIAM BULLOCH, The History of Bacteriology (1938, reprinted 1979).

HUBERT A. LECHEVALIER and MORRIS SOLOTOROVSKY, Three Centuries of Microbiology (1965, reprinted with corrections, 1974), illuminates a definitive history covering the important discoveries beginning with the invention of the microscope.

Textbooks and advanced works

MICHAEL J. PELCZAR, JR., E.C.S. CHAN, and NOEL R. KRIEG, Microbiology: Concepts and Applications (1993), is an introduction to the science of microbiology.

RALPH MITCHELL (ed.), Environmental Microbiology (1992), describes the role of microorganisms in water, soil, and the atmosphere and also discusses methods for controlling pollution.

RONALD M. ATLAS and RICHARD BARTHA, Microbial Ecology: Fundamentals and Applications, 4th ed. (1998), addresses both the roles and applications of microorganisms in various environments.

BIBEK RAY, Fundamental Food Microbiology (1996), covers the biological and technological aspects of miroorganisms involved in food production and spoilage.

RICHARD H. BALTZ, GEORGE D. HEGEMAN, and PAUL L. SKATRUD (eds.), Industrial Microorganisms: Basic and Applied Molecular Genetics (1993), gives an account of the uses of microorganisms in industry.

ARNOLD L. DEMAIN, JULIAN E. DAVIES , and RONALD M. ATLAS (eds.), Manual of Industrial Microbiology and Biotechnology, 2nd ed., (1999), provides information on specialized aspects of the use of microorganisms for industrial production.

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pasteurization

heat-treatment process that destroys pathogenic microorganisms in certain foods and beverages. It is named for the French scientist Louis Pasteur, who in the 1860s demonstrated that abnormal fermentation of wine and beer could be prevented by heating the beverages to about 57 °C (135 °F) for a few minutes. Pasteurization of milk, widely practiced in several countries, notably the United States, requires temperatures of about 63 °C (145 °F) maintained for 30 minutes or, alternatively, heating to a higher temperature, 72 °C (162 °F), and holding for 15 seconds (and yet higher temperatures for shorter periods of time). The times and temperatures are those determined to be necessary to destroy Mycobacterium tuberculosis and other, more heat-resistant, non-spore- forming, disease-causing microorganisms found in milk. The treatment also destroys most of the microorganisms that cause spoilage and so prolongs the storage time of food.

Equipment for the high-temperature short-time (HTST) pasteurization of milk.

Ultra-high-temperature (UHT) pasteurization involves heating milk or cream to 138–150 °C (280–302 °F) for one or two seconds. Packaged in sterile, hermetically sealed containers, UHT milk may be stored without refrigeration for months. Ultrapasteurized milk and cream are heated to at least 138 °C for at least two seconds, but, because of less stringent packaging, they must be refrigerated. Shelf life is extended to 60–90 days. After opening, spoilage times for both UHT and ultrapasteurized products are similar to those of conventionally pasteurized products.

Pasteurization of some solid foods involves a mild heat treatment, the exact definition of which depends on the food. Radiation pasteurization refers to the application of small amounts of beta or gamma rays to foods to increase their storage time.

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experiments disproving spontaneous generation

The hypothesis of spontaneous generation posited that living organisms develop from nonliving matter. This idea was disproved following experiments conducted in 1668 by Italian physician Francesco Redi and in 1859 by French chemist and microbiologist Louis Pasteur.

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Louis Pasteur

French chemist and microbiologist Louis Pasteur experimenting on a chloroformed rabbit, coloured wood engraving, 1885.

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Louis Pasteur

French chemist and microbiologist Louis Pasteur (1852).

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Louis Pasteur

Louis Pasteur, coloured lithograph from Vanity Fair (1887).

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Louis Pasteur

A tribute from Le Petit Journal, Paris, at the time of Louis Pasteur's death (1895).

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Examine how bacteria can be beneficial ecological agents of decomposition but harmful spoilers of food and agents of disease

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Louis Pasteur originated pasteurization, a heat-treatment process that destroys harmful bacteria and other microbes in certain foods and beverages.

Video Transcript

Bacteria live in almost every habitat on Earth. They owe part of this success to their tremendous reproductive output. Through the process of binary fission, a bacterium duplicates its genetic material and divides into two new bodies. A single colonizing cell can soon give rise to thousands of descendants. Just how fast do colonies of bacteria grow? Some have doubling times of just 10 minutes, which means that every 10 minutes the number of cells in the culture doubles. Rapid reproduction makes bacteria well suited for their main ecological role, as decomposers, but it also makes them a danger to unprotected food supplies. Preventing food from spoiling means preventing bacteria from growing. The most fail-safe method is to destroy all the bacteria in and on the food through heat pasteurization and canning. Cans prevent new bacteria from reaching the food from the outside, while the high temperatures of pasteurization kill any bacteria already trapped inside. If heat pasteurization is not complete, dormant spores of microbes, such as Clostridium botulinum, can begin to grow. Clostridium botulinum—the agent of botulism—produces a potent toxin that can cause fatal paralysis. The illnesses associated with bacteria do not stop with food spoilage. Disease-causing bacteria, or pathogens, are responsible for a long list of ailments, including typhoid, cholera, and pneumonia. As bacteria cells enter the human body, they are attacked by immune-system cells called phagocytes. When a phagocyte overtakes the bacteria, it engulfs and dissolves the microbe. The phagocytes in our immune system tend to be very busy, because bacteria are persistent and widespread in our world—part of every meal, every breath, and every touch.

Learn about bacteria as agents of decomposition, food spoilage, and disease (pathogens).

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Know about ingredients and fermentation in the process of brewing beer

Video Transcript

GRANT WOOD: Hi, my name is Grant Wood. I'm one of to brewers with Sam Adams. Welcome to our brewery. I have a degree in food science and technology from Texas A&M University. And I've been a brewmaster with Sam Adams for 15 years. Can't wait to show you around. We'll show you some ingredients, show you how we make the beer. You guys like, we'll do a little tasting as well. Come on into the brewery. [MUSIC PLAYING] What I've got here are the basic ingredients in our beer. The first one, at Sam Adams we use two-row malting barley. This is our pale malt. All of our beers have this malt in it in some fashion-- somewhere, anywhere between from 50% to 100%. The other major ingredient that we have right here is our hops. The hop is a bine, not a vine. It likes to wind around wires. It's got kind of a gold dust in the interior. Those are the lupulin glands. That's where all the flavor compounds-- the humulones, the humulenes, cohumulone, adhumulone, all the things that add flavor and bitterness to beer are packed into those little resin beads. Next one obviously is water. We want to make sure that the water that we have is clean and free of all flavors-- organics that might have come out of ground water. The other thing that is important about water is the amount of minerals that are in there. You can really affect the flavor and mouth feel of the beer with the amount of minerals. The most important ingredient in the beer is the yeast. The yeast decide whether the beer is going to be an ale or whether it's going to be a lager. Those are the basic four ingredients. Now that we know a little bit about those four ingredients, we can head over to the brewhouse, and we can talk about how we put those into the process. We think of it as the four-vessel brewing process. We do what's called a decoction mashing. Mash is the process of breaking down complex carbohydrates, the starches, into simple sugars. So what we do. We've measured out our amount of malt. We

ground it in a mill to crush it, but not turn it into a flour. And then we take a portion of that malt and add that with water into the first vessel, which we call the mash kettle. That we start at 122 degrees, turn the steam on, and bring that to a boil. Meanwhile, in the next vessel-- which is called the mash tun-- we take the rest of the malt; again, some more water at 122 degrees; and we place that into the mash tun and allow that to rest. It's called the protein rest. When the match kettle reaches boil, we transfer that over into the mash tun in a process called decoction. You're pushing this over, raising the temperature very rapidly inside the mash tun. This does a couple of things. One is it holds the enzymes back. And so by limiting and killing the enzymes and slowing them down a bit by this rapid temperature rise, we hold onto some of the bigger chunks of carbohydrate that are called dextrans, which give the beer body and a very mild sweetness. So now, this mash looks like oatmeal, basically. There's a lot of husk material in there. There's some grain material. We have to take it over to the strainer, which is this vessel right here called the lauter tun. The mash goes over into that, and we allow the wort at this point to trickle down through this bed of spent grain. And we collect that solution of simple sugars. So we come back over to this vessel, we turn the steam on, bring the wort to a boil. And at that point, the third ingredient goes in, which is the hops. As you subject hops to the heat, those lupulin glands that I talked about, that I showed you-- that gold dust that's in there-- those melt. It extracts these compounds out that are called alpha acids that give beer its characteristic bitterness. And the longer you boil them, the more of those are converted and the more bitter your beer will become. And then at the end, we're adding more hops in that don't get as long a boil, and that part of the hop contributes the volatile oils that have the flavor compounds of hops in them. So, boiling for an hour and a half, send it over to the whirlpool where we allow the hot material to settle out of the beer. And then decant this clear wort that has, again, the solution of simple sugars. There's some proteins in there. And the hop compounds that have also come out. And it's transferred into one of these stainless steel vessels over here. Yeast is added, and fermentation begins. So what we've done in the brewhouse is we've created this yeast food, this solution of simple sugars. Yeasts, living, breathing microorganism that eats the sugars-- the simple sugars like glucose, maltose, and maltotriose that we made in the brewhouse-- and converts that into alcohol, carbon dioxide, and about 600 other flavor compounds that the yeast produce from their metabolism. And so this process of fermentation takes four, five days until it's complete. All that's left is the unfermentable sugar, the dextrans, that give beer body. There's a lot of sulphur. There's some aldehydes. It's just rough and really, not really ready for consumption. So, what we do is we actually run it through a second fermentation process. Krausening is like sending in a cleanup crew. It helps smooth off some of the rough edges, get rid of some of the harsh characteristics, and add some smoothness and sweetness to the beer. And then the beer is sent over into aging, where we actually add more hops in a cold process to the aging process. And it adds these very light and delicate hop aromas that we might have lost in the brewhouse back into the beer in the aging process. And now we're ready to filter it, and put it in a keg or put it in a bottle, and send it out to you guys to taste. Modern microbiology was led by Louis Pasteur. To Pasteur worked with brewers primarily. Yeasts, you could see under the microscope. You couldn't really see bacteria, but you could see the yeasts. And so with the advent of that knowledge, you could actually select what yeasts you wanted and produce what kind of beer and be very specific about the flavors that you wanted and also about the consistency. Because you could reproduce that same yeast over and over again and not have contaminants come in. Modern brewing technology, if we think of beer as it is today, that was really also the advent of modern science. The selection of yeasts happens in the late 1800s with the creation of the lager beer. Ales had been made since the dawn of man. Lager is a creation of science and the selection of yeast specifically good for brewing in cooler temperatures and producing clear and crisp beers. Ah. It's a good day's work. [MUSIC PLAYING]

Learn about the chemistry of beer and the process of brewing from a brewmaster of the Samuel Adams Brewery, Boston, Massachusetts.

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Know about ingredients and fermentation in the process of brewing beer. Video. Britannica LaunchPacks: Louis Pasteur, Encyclopædia Britannica, 19 Feb. 2021. packs-preview.eb.com. Accessed 10 Aug. 2021.

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