How Bacterial Pathogens Penetrate Host Defenses

Capsules

Some pathogens have capsules that prevent them from being phagocytized.

Components of the Cell Wall

M protein of Streptococcus pyogenes.

Heat- and acid-resistant protein in the cell wall and fimbriae that facilitate adherence to epithelial cells and prevent phagocytosis by leukocytes.

Opa - outer membrane protein Neisseria gonorrhoea uses with fimbriae to attach to host cells and gain entry (live inside epithelial cells and leukocytes).

Waxes in cell wall of Mycobacterium tuberculosis.

Some microbes can reproduce inside phagocytes.

Enzymes

Leukocidins destroy neutrophils and macrophages.

Hemolysins lyse red blood cells.

Local infections can be protected in a fibrin clot caused by the bacterial enzyme coagulase.

Bacteria can spread from a focal infection by means of kinases (which destroy blood clots), hyaluronidase (which destroys a mucopolysaccharide that holds cells together), and collagenase (which hydrolyzes connective tissue collagen).

IgA proteases destroy IgA antibodies.

Antigenic Variation

Some organisms change their antigens, either by mutation or by changing expression of the gene that codes for the antigen, to evade host immune responses.

Penetration into the Host Cell Cytoskeleton

Salmonella bacteria produce invasins, proteins that cause the actin of the host cell’s cytoskeleton to form a "basket" (membrane ruffling) to carry the bacteria into the cell.

Salmonella entering epithelial cells