Porth's Essentials of Pathophysiology, 4e

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Infection and Immunity

U N I T 4

Animal cell

Absorption

Malignancy

Penetration

Nucleus

Cytoplasm

Host cell lysis

Budding release of enveloped viruses

Uncoating

Oncogenesis

Viral maturation

Viral replication

FIGURE 14-2. Schematic representation of the many possible consequences of viral infection of host cells, including cell lysis (poliovirus), continuous release of budding viral particles, or latency (herpesviruses) and oncogenesis (papovaviruses).

Latency

Protein coat synthesis

the Epstein-Barr virus, hepatitis B virus, and human papillomavirus (see Chapter 7). Bacteria Bacteria are autonomously replicating unicellular organisms known as prokaryotes because they lack an organized nucleus. Compared with nucleated eukary- otic cells (see Chapter 1), the bacterial cell is small and structurally primitive (Fig. 14-3). Similar to eukaryotic cells but unlike viruses, bacteria contain both DNA and RNA, although their genome is considerably smaller than eukaryotes and typically is encoded on a single

chromosome. Many bacteria transiently harbor smaller extrachromosomal pieces of circular DNA called plas- mids. Occasionally, plasmids contain genetic informa- tion that increases the virulence or antibiotic resistance of the organism. The prokaryotic cell is organized into an internal compartment called the cytoplasm , which contains the reproductive and metabolic machinery of the cell. The cytoplasm is surrounded by a flexible lipid mem- brane, called the cytoplasmic membrane. This in turn is enclosed within a rigid cell wall. The structure and synthesis of the cell wall determine the microscopic shape of the bacterium (e.g., spherical [cocci], helical [spirilla], or elongate [bacilli], Fig. 14-4). Most bacteria produce a cell wall composed of a distinctive polymer known as peptidoglycan. This polymer is produced only by prokaryotes and is therefore an attractive target for antibacterial therapy. For example, the penicillin antibi- otics target the peptidoglycan cell wall. Several bacteria synthesize an extracellular capsule composed of protein or carbohydrate. The capsule protects the organism from environmental hazards such as the immunologic defenses of the host. Certain bacteria are motile as the result of external whip-like appendages called flagella. The flagella rotate like a propeller, transporting the organism through a liquid environment. Bacteria can also produce hairlike structures projecting from the cell surface called pili or fimbriae , which enable the organism to adhere to sur- faces such as mucous membranes or other bacteria. Reproduction. Most prokaryotes reproduce asexu- ally by simple cellular division. The manner in which an organism divides can influence the microscopic mor- phology. For instance, when the cocci divide in chains, they are called streptococci; in pairs, diplococci ; and in clusters, staphylococci (Fig. 14-5). The growth rate of bacteria varies significantly among different species and depends greatly on physical growth conditions and the availability of nutrients. In the laboratory, a single bac- terium placed in a suitable growth environment, such as an agar plate, reproduces to the extent that it forms a

FIGURE 14-3. Electron micrograph of the rod-shaped, gram- positive bacterium Listeria monocytogenes showing the simple prokaryotic cell structure including the cytoplasm, the cytoplasmic membrane, and the rigid cell wall. (From the Centers for Disease Control and Prevention Public Health Images Library. No. 10828. Courtesy of Balasubr Swaminathan, Peggy Hayes.)