PRINCIPLES OF MODERN MICROBIOLOGY

Mark Wheelis

ANSWERS TO STUDYQUESTIONS

Chapter 19

Microbial Pathogenesis

2. Intracellular pathogens initially gain access to their host cells by being phagocytosed. In some cases they attack phagocytic cells like macrophages, for which phagocytosis is a normal activity. In other cases they attack cells that do not normally phagocytose, and in this case the pathogen induces the host cell to phagocytose it by one of several different means.

Once phagocytosed, intracellular pathogens avoid being killed by their host cell in several different ways. They may destroy the phagosome membrane and grow in the cytoplasm, or they may modify the phagosome membrane such that it does not fuse with lysosomes.

Pathogens that destroy the phagosome membrane often transfer from host cell to host cell by co-opting the cytoskeleton to drive them into the new host cell at the tip of a bundle of microfilaments.

4. Antigenic variation is usually the result of one of two different mechanisms. Some pathogens have multiple genes for surface proteins, and these are switched at a frequency such that every several days, a new variant is expressed. In other cases, particularly in the viruses, high mutation rates generate considerable random variation in surface epitopes. In either case, as the surface epitopes change, the developing immune response to the previous epitopes becomes useless.

6. Exotoxins are proteins secreted from cells. They have a defined mechanism of action (usually as a result of their enzymatic activity), and they generally affect only one specific physiological system (although they may affect that system in many different tissues). Endotoxin is lipopolysaccharide, a structural component of the cell envelope, and it is released from cells in large amounts only when the cell dies. It has a wide variety of effects, caused by the release of cytokines by phagocytes that take it up.

8. Exotoxins include neurotoxins, which affect nerve cells; enterotoxins, which affect cells of the intestinal tract; and cytotoxins affect multiple different types of cells. A subcategory of cytotoxins, the cytolytic toxins, kill cells by disrupting their membranes.