Staph evades immunity in numerous ways

• it is the number one agent of systemic disease

• has enzymes that prevent the assembly of the C3 convertase complex

• proteolysin

• protein A can bind antibodies in a non-opsonic fashion, making Fc segment unavailable

• clumping factors prevent against phagocytic clearance

• secretes toxins directly lytic to immune cells

S. aureus ECM binding proteins

• bacteria produce surface proteins that can act as adhesins

• they can bind to ECM or to cells directly

Clumping factor (ClfA)

• closely related to coagulase

• binds fibrinogen at multiple sites

• causes S. aureus to clump in serum, which can help S. aureus resist phagocytic clearance

• it is likely a virulence factor in endocarditis and septic arthritis

Protein A: interference with opsonization

• protein A has a very high affinity for immunoglobulins

• physiologically, Fab can recognize an epitope of antigen and Fc can attract leukocytes

• however, S. aureus protein A has an even higher affinity for Fc than Fc receptors on leukocytes. protein A on the surface of bacteria binds Fc and antibodies then cannot mediate humoral immunity

Capsule: interference with phagocytosis

• polysaccharide capsule

• there are 11 serotypes

• types 5 and 8 are 70% of invasive Staph

• impairs complement deposition on surface

α-hemolysin

• this is the most potent S. aureus hemolysin

• heptameric mushroom structure forms pores in cell membranes

Category: Microbiology Notes