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MICROBIAL DIVERSITY 4 PART 1 | Acellular and Procaryotic Microbes

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MICROBIAL DIVERSITY 4 PART 1 | Acellular and Procaryotic Microbes 18283_CH04.qxd 8/23/09 3:33 AM Page 40 MICROBIAL DIVERSITY 4 PART 1 | Acellular and Procaryotic Microbes CHAPTER OUTLINE Mimivirus Pathogenicity Plant Viruses Genetic Composition INTRODUCTION Viroids and Prions Unique Bacteria ACELLULAR MICROBES THE DOMAIN BACTERIA Rickettsias, Chlamydias, and Closely Viruses Characteristics Related Bacteria Origin of Viruses Cell Morphology Mycoplasmas Bacteriophages Staining Procedures Especially Large and Especially Animal Viruses Motility Small Bacteria Latent Virus Infections Colony Morphology Photosynthetic Bacteria Antiviral Agents Atmospheric Requirements THE DOMAIN ARCHAEA Oncogenic Viruses Nutritional Requirements Human Immunodeficiency Virus Biochemical and Metabolic Activities LEARNING OBJECTIVES INTRODUCTION AFTER STUDYING THIS CHAPTER, YOU SHOULD BE ABLE TO: Imagine the excitement that Anton van Leeuwenhoek experienced as he gazed through his tiny glass lenses • Describe the characteristics used to classify viruses (e.g., and became the first person to see live microbes. In the DNA vs. RNA) years that have followed his eloquently written late • List five specific properties of viruses that distinguish 17th to early 18th century accounts of the bacteria and them from bacteria protozoa that he observed, tens of thousands of mi- • List at least three important viral diseases of humans crobes have been discovered, described, and classified. • Discuss differences between viroids and virions, and the In this chapter and the next, you will be introduced to diseases they cause the diversity of form and function that exists in the • List various ways in which bacteria can be classified microbial world. • State the three purposes of fixation As you will recall, microbiology is the study of • Define the terms diplococci, streptococci, staphylococci, microbes, which are too small to be seen by the naked tetrad, octad, coccobacilli, diplobacilli, streptobacilli, eye. Microbes can be divided into those that are truly and pleomorphism cellular (bacteria, archaea, algae, protozoa, and fungi) • Define the terms obligate aerobe, microaerophile, facul- and those that are acellular (viruses, viroids, and prions). tative anaerobe, aerotolerant anaerobe, obligate anaer- The cellular microorganisms can be subdivided into obe, and capnophile those that are procaryotic (bacteria and archaea) and • State key differences among rickettsias, chlamydias, and those that are eucaryotic (algae, protozoa, and fungi). mycoplasmas For a variety of reasons, acellular microorganisms are • Identify several important bacterial diseases of humans not considered by most scientists to be living organisms. • State several ways in which archaea differ from bacteria Thus, rather than using the term microorganisms to 40 18283_CH04.qxd 8/23/09 3:33 AM Page 41 CHAPTER 4 ■ Microbial Diversity 41 describe them, viruses, viroids, and prions are more oncoviruses—cause specific types of cancer, including correctly referred to as acellular microbes or infectious human cancers such as lymphomas, carcinomas, and particles. some types of leukemia. Viruses are said to have five specific properties that distinguish them from living cells: ACELLULAR MICROBES • The vast majority of viruses possess either DNA or Viruses RNA, unlike living cells, which possess both. Complete virus particles, called Viruses are extremely • They are unable to replicate (multiply) on their own; virions, are very small and sim- small. They are their replication is directed by the viral nucleic acid ple in structure. Most viruses observed using once it has been introduced into a host cell. range in size from 10 to 300 nm electron microscopes. • Unlike cells, they do not divide by binary fission, mito- in diameter, although some— sis, or meiosis. like Ebola virus—can be up to 1 ␮m in length. The small- • They lack the genes and enzymes necessary for energy est virus is about the size of the large hemoglobin mole- production. cule of a red blood cell. Scientists were unable to see • They depend on the ribo- Except in very rare viruses until electron microscopes were invented in the somes, enzymes, and meta- cases, a particular virus 1930s. The first photographs of viruses were obtained in bolites (“building blocks”) of contains either DNA or 1940. A negative staining procedure, developed in 1959, the host cell for protein and RNA—not both. revolutionized the study of viruses, making it possible to nucleic acid production. observe unstained viruses against an electron-dense, dark A typical virion consists of The simplest of human background. a genome of either DNA or viruses consists of No type of organism is safe Viruses are not alive. RNA, surrounded by a capsid nothing more than from viral infections; viruses To replicate, viruses (protein coat), which is com- nucleic acid surrounded infect humans, animals, plants, must invade live host posed of many small protein by a protein coat (the fungi, protozoa, algae, and bac- cells. units called capsomeres. capsid). The capsid plus terial cells (Table 4-1). Many Together, the nucleic acid and the enclosed nucleic human diseases are caused by viruses (refer back to Table the capsid are referred to as acid are referred to as 1-1). Many of the viruses that infect humans are shown in the nucleocapsid (Fig. 4-2). the nucleocapsid. Figure 4-1. Some viruses—called oncogenic viruses or Some viruses (called en- veloped viruses) have an outer envelope composed of Relative Sizes and Shapes lipids and polysaccharides (Fig. 4-3). Bacterial viruses TABLE 4-1 of Some Viruses may also have a tail, sheath, and tail fibers. There are no ribosomes for protein synthesis or sites of energy pro- NUCLEIC SIZE RANGE duction; hence, the virus must invade and take over a VIRUSES ACID TYPE SHAPE (nm) functioning cell to produce new virions. Viruses are classified by the following characteris- Animal Viruses Vaccinia DNA Complex 200 ϫ 300 tics: (a) type of genetic material (either DNA or RNA), Mumps RNA Helical 150–250 (b) shape of the capsid, (c) number of capsomeres, Herpes simplex DNA Polyhedral 100–150 (d) size of the capsid, (e) presence or absence of an en- Influenza RNA Helical 80–120 velope, (f) type of host that it infects, (g) type of disease Retroviruses RNA Helical 100–120 it produces, (h) target cell, and (i) immunologic or anti- Adenoviruses DNA Polyhedral 60–90 genic properties. Retroviruses RNA Polyhedral 60–80 There are four categories of viruses, based on the Papovaviruses DNA Polyhedral 40–60 type of genome they possess. The genome of most Polioviruses RNA Polyhedral 28 viruses is either double-stranded DNA or single- Plant Viruses stranded RNA, but a few viruses possess single-stranded Turnip yellow RNA Polyhedral 28 DNA or double-stranded RNA. Viral genomes are usu- mosaic ally circular molecules, but some are linear (having two Wound tumor RNA Polyhedral 55–60 ends). Capsids of viruses have various shapes and sym- Alfalfa mosaic RNA Polyhedral 18 ϫ 36–40 metry. They may be polyhedral (many sided), helical ϫ Tobacco mosaic RNA Helical 18 300 (coiled tubes), bullet shaped, spherical, or a complex Bacteriophages combination of these shapes. Polyhedral capsids have T2 DNA Complex 65 ϫ 210 20 sides or facets; geometrically, they are referred to as L DNA Complex 54 ϫ 194 icosahedrons. Each facet consists of several capsomeres; Fx-174 DNA Complex 25 thus, the size of the virus is determined by the size of each facet and the number of capsomeres in each. 18283_CH04.qxd 8/23/09 3:33 AM Page 42 42 SECTION II ■ Introduction to Microbes and Cellular Biology RNA VIRUSES Nonenveloped Enveloped Single stranded Single stranded Single stranded Double stranded Retrovirus Positive sense Positive sense Negative sense Astroviruses Reoviruses Togaviruses Linear Lentiviruses Caliciviruses Rotaviruses Flaviviruses Rhabdoviruses Oncoviruses Picornaviruses Coronaviruses Paramyxoviruses Segmented Arenaviruses Bunyaviruses Orthomyxoviruses DNA VIRUSES Nonenveloped Enveloped Single stranded Double stranded Double stranded Double stranded Double stranded Linear Linear Circular Linear Circular Parvoviruses Adenoviruses Papillomaviruses Herpesviruses Hepadnaviruses Polymaviruses Poxviruses FIGURE 4-1. Some of the viruses that infect humans. Note that some viruses contain RNA, whereas others contain DNA, and that the nucleic acid that they possess may either be single or double stranded. Within the host cell, single-stranded positive sense RNA functions as messenger RNA (mRNA), whereas single-stranded negative sense RNA serves as a template for the production of mRNA. Some of the viruses possess an envelope, whereas others do not. (From Engleberg NC et al. Schaechter’s Mechanisms of Microbial Diseases, 4th ed. Philadelphia: Lippincott Williams & Wilkins, 2007.) Frequently, the envelope around the capsid makes the vaded. A list of some viruses, their characteristics, and virus appear spherical or irregular in shape in electron diseases they cause is presented in Table 4-2. Sizes of micrographs. The envelope is acquired by certain ani- some viruses are depicted in Figure 4-6. mal viruses as they escape from the nucleus or cyto- plasm of the host cell by budding (Figs. 4-4 and 4-5). In Origin of Viruses other words, the envelope is derived from either the Where did viruses come from? Two main theories have host cell’s nuclear membrane or cell membrane. been proposed to explain the origin of viruses. One theory Apparently, viruses are then able to alter these mem- states that viruses existed before cells, but this seems un- branes by adding protein fibers,
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