Peritoneal Defence Mechanisms and Staphylococcus Aureus in Patients Treated with Continuous Ambulatory Peritoneal Dialysis (CAPD)

Peritonitis in continuous ambulatory peritoneal dialysis (CAPD) patients due to S. aureus is associated with an adverse clinical outcome, suggesting impaired clearance of this organism by the host. The ability of peritoneal macrophages (PMO) derived from CAPD patients to take up S. aureus and mount a respiratory burst was investigated. Whilst significant activity was observed in the absence of opsonin, both parameters of phagocytosis were augmented by addition of 20% pooled human serum (PHS), complement-depleted PHS, and fibronectin. When used as sole opsonin, fibronectin resulted in a dose-related increase in chemiluminescent response by both blood neutrophils and PMO.

The opsonic activity of dialysis effluent, as judged by neutrophil chemiluminescence, correlated with IgG and fibronectin content, but not with complement as assessed by C3 levels. The addition of urokinase to dialysate improved its opsonic properties whilst having no effect on the activity of PHS-20%; this would suggest that the formation of fibrin in dialysate, promoted by S. aureus, interferes with phagocytosis. This and the low IgG, complement and fibronectin levels in dialysate may explain in part the relatively poor clearance of this organism from the peritoneum.

Functional blocking of Staphylococcus aureus adhesins following growth in ex vivo media

Defining the role of Staphylococcus aureus adhesins in disease pathogenesis may depend on the use of bacteria grown in culture media that more closely reflect the human milieu than conventional broth. This study examined the functional effect on S. aureus adhesins following growth in an ex vivo medium containing a complex mixture of human proteins (used peritoneal dialysate) relative to growth in Todd-Hewitt broth. The adherence of S. aureus, cultured in dialysate, to fibronectin and fibrinogen was markedly reduced despite the expresion of full-length ClfA, ClfB, and fibronectin-binding proteins. Growth in dialysate resulted in the acquisition of a surface coat, as visualized by transmission electron microscopy, which was shown to contain fibronectin, fibrinogen, and immunoglobulins. Adherence of S. aureus to fibrinogen following growth in dialysate was significantly reduced by expression of protein A but was restored following growth in immunoglobulin-depleted dialysate. We conclude that bacterial adherence to solid-phase protein is critically dependent on the culture medium, that S. aureus adhesins may become saturated with target protein prior to contact with solid surfaces, and that there is an interaction between fibrinogen-binding proteins and immunoglobulin bound to protein A following contact with host proteins. These findings have important implications for future studies of S. aureus adhesins.