University of Georgia Cooperative Extension Bulletin 817 1 What are Bacteria, Yeasts & Molds? Table of Contents
Looking Beyond What Your Eyes Can See...... 3
The Universal Cell...... 3
Bacteria...... 4 Bacterial Eating Habits...... 5 Growth and Importance to Food Industry...... 5
Sugar-Loving Yeasts...... 6 Growth and Importance to Food Industry...... 6
Moisture and Air-Loving Molds...... 7 Growth and Importance to Food Industry...... 7
Additional Information...... 8
This publication is part of the UGA EFS “Starting a New Food Business” packet. For more informa- tion, please contact the UGA Food Science specialists.
Extension Food Science Outreach Program 240 Food Science Building The University of Georgia Athens, GA 30602-7610
(706) 542-2574 fax (706) 542-9066 Website: http://www.EFSonline.uga.edu
Grateful appreciation is expressed to the individuals and organizations who contributed to this publication.
Prepared by William C. Hurst, Extension Food Scientist, James A. Christian, Former Extension Food Scientist, and George A. Schuler, Former Extension Food Scientist
What are Bacteria, Yeasts & Molds? 2 University of Georgia Cooperative Extension Bulletin 817 Looking Beyond What Your Eyes Can See
Just what are microorganisms? They are small living forms of life which we cannot see with the naked eye. Bacteria, yeasts and molds are three types of microorganisms. Some people often confuse, and almost always misunderstand, their functions, but they are just as real and alive as you are. They eat and grow. They reproduce and die.
Have you ever wondered just how small microorganisms really are? Molds can be seen with only slight magnification -- use of an ordinary magnifying glass. Yeasts must be viewed through a mi- croscope that magnifies several hundred times. Bacteria can best be seen when studied with a more powerful scope that enlarges 1,000 times.
Bacteria, yeasts and molds can be found everywhere. Scientists have gathered them from clouds above mountain tops and in the deepest parts of the ocean. They are present on animals, people and even in the air we breathe.
Microorganisms have a direct impact on our daily lives. Some are helpful. They aid our bodily pro- cesses by helping break down complex foods into simpler substances. Some, called germs, are harm- ful to us by the role they play in causing diseases.
The Universal Cell All living organisms, large and small, have one thing in common – the cell. This is a tiny living fac- tory capable of converting simple food substances into energy and new cell material and of reproduc- ing itself.
Larger organisms, including people, are composed of billions of cells with many different roles. They make up your body’s parts from your brain to your big toe.
Microorganisms, on the other hand, are made up of a very few or even a single cell capable of carrying on all of life’s processes.
So a basic understanding of cell structure and func- tion is essential to understanding the actions of bacte- ria, yeasts and molds.
Since the cell is the basic unit of all living things, you might think it is a simple structure. Nothing could be farther from the truth. The cell is complex in its make- up and its function. Many scientists have spent their lives studying it.
The main parts of the cell are the nucleus, cytoplasm and cell wall. The nucleus is the control center. It directs cell division or the formation of new cells. They cytoplasm contains the parts which convert food material into energy and new cell material. The cell wall or membrane holds everything together and controls the passage of material into and out of the cell.
University of Georgia Cooperative Extension Bulletin 817 3 What are Bacteria, Yeasts & Molds? Bacteria Bacteria make up the largest group of microorganisms. People often think of them only as germs and the harm they do. Actually, only a small number of bacteria types are pathogenic (disease causing). Most are harmless and many are helpful.
There are thousands of different kinds of bacteria. Some differ only slightly and it takes a highly trained person to identify them. There are also groups which differ greatly in growth habits and appearance and are quite easily identified. But regardless of minor differences, most bacteria can be classified according to the five basic cell shapes illustrated.
In addition to their different shapes, their cell arrangement varies. For example, some cocci are always grouped in pairs (diplococci). Others are arranged in chains (streptococci). Still others are bunched (staphylococci). Diplococci are the kind which cause pneumonia. Streptococci are often associated with “strep throat”. Staphylococci are familiar to many because of their role in “staph infections” and some types of food poisoning.
Bacteria also vary somewhat in size, but average about 1/25,000 inch. In other words, 25,000 bacteria laid side by side would occupy only one inch of space. One cubic inch is big enough to hold nine tril- lion average size bacteria -- about 3,000 bacteria for every person on earth.
Microorganisms, including bacteria, can also be grouped according to their requirement for oxygen. Some grow only in the presence of oxygen (aerobes). Others grow only in the absence of oxygen (an- aerobes). Some are able to grow with or without oxygen (facultative anaerobes).
Under natural conditions, anaerobes grow only in places protected from the air, such as deep in the soil or under water. They can also grow under man-made anaerobic conditions, such as in canned or vacuum packed foods which have not been processed or handled properly.
What are Bacteria, Yeasts & Molds? 4 University of Georgia Cooperative Extension Bulletin 817 Bacterial Eating Habits Bacteria and other microorganisms need food in order to grow and multiply. They vary in their food needs, but nearly everything we consider as food can also be used as food by some type of bacteria.
To be used by bacteria, a food substance must pass into the cell where it can be processed into energy and new cell material. Because most foods are too complex to move into a bacterial cell, they must be broken down into simpler substances. Enzymes do this by increasing the rate of biochemical reac- tions.
Produced within the bacterial cell, enzymes move through the cell wall to break down the food on the outside into a form bacteria can use.
Growth and Importance to Food Industry Bacteria reproduce by a process called binary fission -- one cell divides and becomes two. Some can reproduce at a very rapid rate under proper conditions. If food and moisture are adequate and the temperature is right, certain bacteria can reproduce in as little as 20 minutes. ithinW 20 minutes, one cell becomes two; in 40 minutes, there will be four, and so on. In only eight hours, the original cell will have multiplied to nearly 17 million new bacteria. Of course, conditions don’t remain favorable for such a rate of reproduction for long. If they did, we could be buried in bacterial cells.
We must use a microscope capable of magnifying 500 times in order to see a single bacterial cell. However, if that cell is al- lowed to grow on suitable food or solid media, it will reproduce rapidly into a colony consisting of millions of cells. The colony is visible to the naked eye. The photo shows hundreds of bacte- rial colonies which grew and reproduced within 24 hours after a dirty apron was touched to a place of nutrient agar.
Plate counting, a technique by which colonies of bacteria are grown from single cells, makes it possible to determine the num- ber of bacteria in a food sample without the aid of a microscope.
An important thing to consider in the growth of bacteria is the Bacterial colonies from a dirty apron after ability of certain types to produce spores. A spore is a dormant 24 hours. or resting state of a bacterial cell.
There are certain basic differences between spores andactive or vegetative bacterial cells. Spores are not easily killed. In fact, conditions which will quickly kill active bacteria have little or no effect on spores. A temperature of 180oF will kill bacterial cells within minutes, but bacterial spores can resist this temperature indefinitely. This is important to us in that all processing times for canned foods are calculated by using both the time and temperature required to kill bacterial spores.
Another important method of classifying bacteria is by their ability to grow and reproduce at various temperatures. Some are cold loving and grow well under refrigeration. Called psychrophilic bacteria, this group grows at temperatures less that 50oF. They are responsible for many types of spoilage in refrigerated foods, such as slime formation on meats and ropiness in milk.
The second group – mesophilic bacteria – includes those which cause disease and food poisoning. They grow only at moderate temperatures of 50oF to 110oF. You can see, therefore, why it’s so impor- tant to store unprocessed foods under refrigeration. University of Georgia Cooperative Extension Bulletin 817 5 What are Bacteria, Yeasts & Molds? The third group – called thermophiles – grows well at temperatures between 110oF to 140oF. These bacteria can cause spoilage in under-processed canned foods.
As noted earlier, microorganisms are essential in the production of certain foods. Bacteria are respon- sible in whole or in part for the following: Buttermilk Cheeses Cocoa Coffee Pickles Sauerkraut Vanilla Vinegar Yogurt
Sugar-Loving Yeasts Yeasts are small, single-celled plants. They are members of the family fungi (singular, fungus), which also includes mushrooms. Fungi differ from other plants in that they have no chlorophyl.
Bacteria thrive on many different types of food. But most yeasts can live only on sugars and starches. From these, they produce carbon dioxide gas and alcohol. Thus, they have been useful to man for centuries in the production of certain foods and beverages. They are responsible for the rising of bread dough and the fermentation of wines, whiskey, brandy and beer. They also play the initial role in the production of vinegar.
Growth and Importance to Food Industry Some yeasts are psychrophilic and so they can grow at relatively low temperatures. In fact, the fer- mentation of wines and beer is often carried out at temperatures near 40oF. Because some kinds are psychrophiles, they can create a spoilage problem in meat coolers and other refrigerated storage areas.
Unlike bacteria, which multiply by binary fission, yeasts reproduce by a method called budding. A small knob or bud forms on the parent cell, grows and finally separates to become a new yeast dell. Although this is the most com- mon method of reproduction, yeasts also multiply by the formation of spores.
Because they can grow under conditions of high salt or sugar content, they can cause the spoilage of certain foods in which bacteria would not grow. Examples are honey, jellies, maple syrup and sweetened condensed milk. Foods produced by the bacterial fermentation process, such as pickles and sauerkraut, can also be spoiled by yeasts which interfere with the normal fermentative process.
Certain yeasts are pathogenic. However, yeast infections are much less common than are bacterial infections.
What are Bacteria, Yeasts & Molds? 6 University of Georgia Cooperative Extension Bulletin 817 Moisture and Air-Loving Molds Probably the best known microorganisms, molds are widely distributed in nature and grow under a variety of conditions in which air and moisture are present. They are also plants and a part of the fungi family.
Nearly everyone has seen mold growth on damp clothing and old shoes. So, many may find it hard to believe that mold is a microorganism. However, the mold we see with the naked eye is actually a colony of millions of mold cells growing together.
Molds vary in appearance. Some are fluffy and filament-like; others are moist and glossy; still others are slimy.
Growth and Importance to Food Industry Unlike bacteria, molds are made up of more than one cell. Vegetative cells sustain the organism by taking in food substances for energy and the production of new cell material. Reproductive cells pro- duce small “seed” cells called spores. Unlike bacterial spores, mold spores are the source of new mold organisms. Bacterial spores generally form only when environmental conditions are unfavorable.
Molds produce a stem consisting of several cells. Together, these cells form a “fruiting body”. The fruiting body produces the spores, which detach and are carried by air currents and deposited to start new mold colonies whenever conditions are favorable.
Mold spores are quite abundant in the air. So, any food allowed to stand in the open soon becomes contaminated with mold if adequate moisture is present. Some types of molds are also psychrophiles and can cause spoilage of refrigerated foods.
Molds are important to the food industry. Among their many contributions are the flavor and color they add to cheeses and the making of soy sauce. They also play a role in the making of such chemi- cals as citric and lactic acid and many enzymes.
Molds can also cause problems in foods. Certain kinds can produce poisons called mycotoxins. Mycotoxins have only recently been discovered and little is known about what causes molds to produce them.
Probably the best known use of molds is in the drug industry, where they help produce such anti- biotics as penicillin.
University of Georgia Cooperative Extension Bulletin 817 7 What are Bacteria, Yeasts & Molds? Additional Information This publication is a brief introduction to the field of microbiology -- the study of all invisible forms of life. If you would like to know more about this field, we recommend one or more of the following books. Two of them deal directly with the role microbiology plays in the food we eat.
Food Microbiology. Frazier, W. C. McGraw-Hill Publishing Company, 1967.
Microbiology (Second Edition). Pelczar, Michael J., Jr. and Roger D. Reid. McGraw-Hill, 1965.
Practical Food Microbiology and Technology. Weiser, Harry H. AVI Publishing Company, Inc., 1962.
Bulletin 817 / Reviewed March 2010
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What are Bacteria, Yeasts & Molds? 8 University of Georgia Cooperative Extension Bulletin 817