Chapter 1 Infection

The major classes of microorganisms in terms of

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The major classes of microorganisms in terms of ascending size and complexity are viruses, bacteria, fungi, and parasites. Parasites exist as single or multicellular structures with the same eukaryotic cell plan of our own cells. Fungi are also eukaryotic, but have a rigid external wall that makes them seem more like plants than animals. Bacteria also have a cell wall, but their cell plan is prokaryotic and lacks the organelles of eukaryotic cells. Viruses have a genome and some structural elements but must take over the machinery of another living cell (eukaryotic or prokaryotic) to replicate. The four classes of infectious agents are summarized in Table 1–1, and generic examples of each are shown in Figure 1–3. Viruses Viruses are strict intracellular parasites of other living cells, not only of mammalian and plant cells, but also of simple unicellular organisms, including bacteria (the bacteriophages). Viruses are simple forms of replicating, biologically active particles that carry genetic information in either DNA or RNA molecules, but never in both. Most mature viruses have a protein coat over their nucleic acid and sometimes a lipid surface membrane derived from the cell they infect. Because viruses lack the protein-synthesizing enzymes and structural apparatus necessary for their own replication, they bear essentially no resemblance to a true eukaryotic or prokaryotic cell. Viruses replicate by using their own genes to direct the metabolic activities of the cell they infect to bring
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about the synthesis and reassembly of their component parts. A cell infected with a single viral particle may thus yield many thousands of viral particles, which can be assembled almost simultaneously under the direction of the viral nucleic acid. With many viruses, cell death and infection of other cells by the newly formed viruses results. Sometimes, viral reproduction and cell reproduction proceed simultaneously without cell death, although cell physiology may be affected. The close association of the virus with the cell sometimes results in the integration of viral nucleic acid into the functional nucleic acid of the cell, producing a latent infection that can be transmitted intact to the progeny of the cell. Bacteria Bacteria are the smallest (0.1 to 10 m) living cells. They have a cytoplasmic membrane surrounded by a cell wall; a unique interwoven polymer called peptidoglycan makes the wall rigid. The simple prokaryotic cell plan includes no mitochondria, lysosomes, endoplasmic reticulum, or other organelles (Table 1–2). In fact, most bacteria are about the size of mitochondria. Their cytoplasm contains only ribosomes and a single, double-stranded DNA chromosome. Bacteria have no nucleus, but all the chemical elements of nucleic acid and protein synthesis are present. Although their nutritional requirements vary greatly, most bacteria are free-living if given an appropriate energy source. Tiny
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The major classes of microorganisms in terms of ascending...

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