Wrm

Prevent encounters with the phagocyte

• C5a peptidase

• Cytolytic toxins

Avoid recognition and attachment

Survive within the phagocyte

• Escape from the phagosome

• Prevent phagosome-lysosome fusion

• Survive within the phagosome

Figure 19.9 Avoiding Destruction by Phagocytes

Avoid recognition and attachment

Survive within the phagocyte

• Escape from the phagosome

• Prevent phagosome-lysosome fusion

• Survive within the phagosome

Microbe is by destroying the complement component that attracts phagocytes; the other way is to kill phagocytic cells as they arrive. The mechanisms involved include:

■ C5a peptidase. This enzyme degrades the complement component C5a, a chemoattractant that recruits phagocytic cells to an area where complement has been activated. Streptococcus pyogenes, which causes strep throat, is an example of a bacterium that makes C5a peptidase. ■ C5a, p. 383 ■ Streptococcus pyogenes, p. 565

■ Membrane-damaging toxins. These kill phagocytes and other cells, often by forming pores in their membranes. S. pyogenes makes a membrane-damaging toxin called streptolysin O. ■ membrane-damaging toxins, p. 474

Avoiding Recognition and Attachment

Recall that recognition and attachment of phagocytes to foreign material is most efficient if either the complement component C3b or certain classes of antibody molecules have first opsonized the material. Mechanisms that bacteria use to avoid recognition and attachment include:

■ Capsules. Capsules have long been recognized for their ability to prevent phagocytosis. Often, they interfere with the alternative pathway of complement activation.

They do this by binding to host cell complement regulatory proteins that inactivate C3b that has bound to a surface; this mechanism is identical to that described earlier for serum-resistant bacteria (see figure 19.8). Rapid inactivation of C3b also prevents it from being an effective opsonin. Streptococcus pneumoniae is an example of an organism that produces a capsule that prevents phagocytosis (figure 19.10).

■ M protein. This component of the cell wall of Streptococcus pyogenes functions in a manner similar to that of the Streptococcus pneumoniae capsule; it binds to a complement regulatory protein that inactivates C3b, interfering with complement activation and the subsequent formation of more C3b. Rapid inactivation of C3b also prevents it from being an effective opsonin.

■ Fc receptors. These proteins are found on the surface of Staphylococcus aureus (protein A) and Streptococcus pyogenes (protein G). The Fc receptors foil opsonization by antibodies; they outfox the immune system by binding the Fc region of antibody molecules, reversing the intended orientation of the molecule on the cell (figure 19.11). Recall that antibodies have two parts— the Fab region, which binds specifically to antigens, and the Fc region, which functions as a "red flag," directing the fate of the antigen. Bacterial cells that

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