FURTHER CHARACTERIZATION OF A COMPLEMENT-SENSITIVE MUTANT OF A VIRULENT AVIAN ESCHERICHIA-COLI ISOLATE

An attempt was made to characterize the mechanism of complement resist ance operating in a virulent avian Escherichia coli isolate. Using flo w cytometry to detect antibody to C3, we found that there was signific antly more antibody bound to a complement-sensitive mutant of this wil d type than to the parent organism, suggesting that more C3 subunits w ere bound to the wild type. Neither the wild type nor the mutant degra ded C3. Further, the mutant: was phagocytosed to a significantly great er degree than the wild type by cultured phagocytes in the presence of C5-deficient serum. These data suggest that the wild type is resistan t to complement, at least in part, because of its ability to restrict C3 deposition on its surface. Therefore, the decrease in virulence see n in the mutant may be related to its increased sensitivity to complem ent-mediated bacteriolysis or its enhanced susceptibility to complemen t-opsonized phagocytosis or both.