Antibodies raised against rough mutants of Escherichia coli and Salmonella strains are opsonic only in the presence of complement.

The opsonic capacity of antisera raised in rabbits against rough (R) mutants and smooth (S) parent strains of Escherichia coli and Salmonella typhimurium were studied. All specific antibodies in the antisera belonged to the immunoglobulin G (IgG) class. Radioactively labeled bacteria were preincubated in various dilutions of antisera, in which complement was inactivated. Fresh normal rabbit serum, as a standard complement source, was used in some experiments. After preincubation, washed bacteria were added to normal human neutrophils. Opsonization of R mutants for 5 min in 5% fresh normal rabbit serum resulted in effective phagocytosis; S strains needed at least a 30-min opsonization time or 20 to 50% serum. After incubation for 5 min in diluted, homologous antisera, phagocytosis of S strains was optimal, but preincubation of R mutants in diluted, homologous antisera did not lead to amelioration of phagocytosis compared with that of bacteria preincubated in buffer only. However, when fresh normal serum was added to homologous antisera, uptake of R mutants occurred at a faster rate than that of bacteria opsonized in fresh serum alone. Using six clinical isolates of members of the family Enterobacteriaceae, we found that, with or without complement, antisera raised against E. coli J5 or S. typhimurium Re had, with the exception of one strain, no opsonic activity for these strains. Thus, the protective effect of R antisera in gram-negative bacteremia, as shown by several investigators, is unlikely to be mediated through enhanced opsonization of invading bacteria by IgG antibodies directed against these R mutants.