Opsonic activity of the alternative complement pathway in infected human intra-abdominal fluid

The complement-mediated opsonic activity (CMOA) in intra-abdominal exudates collected during laparotomy from peritonitis patients was examined by an in vitro phagocytic bactericidal assay. It was found that infected intra-abdominal exudates barely promoted detectable killing of Escherichia coli 01 by polymorphonuclear leukocytes. Only the minority of bacteria recovered by differential centrifugation from the infected exudates had C3 on their surfaces. Such bacteria were not optimally opsonized in vivo, whereas they became fully coated with C3 during a brief incubation in vitro in normal human serum. The low level of CMOA in the peritoneal fluid paralleled a depletion of complement in the peripheral blood. Thus, in cases complicated by sepsis with gram-negative bacteria, the CMOA in the blood was extremely low. These data suggest that the high susceptibility of the peritoneum to infection by gut flora, despite the normal phagocytic response, may be partly explained by the low local level of functional complement which is inadequate for optimal opsonization of the bacteria.