Enhanced killing of group B streptococci in vitro by penicillin and opsonophagocytosis with intravenous immunoglobulin.

Combined effects of intravenous immunoglobulin (IVIG) and antibiotics in killing bacteria are of interest with broadening clinical use of IVIG. Since the kinetics of killing by these agents differ and each may influence the outcome of the other, it is difficult to evaluate combination effects in vitro. Conditions were developed to measure killing of group B streptococci (GBS), type III strain M732, by an opsonic mixture with IVIG, fresh serum, and human polymorphonuclear leukocytes (PMNL) with or without penicillin. Bacterial killing was observed with the opsonic IVIG mixture, penicillin, and the opsonic IVIG mixture plus penicillin at 1 h. The effect of the combination was greater than the sum of the effects of two separate incubations. The enhanced killing was evident for up to 18 h. By 24 h, the killing by the combination was no greater than that by penicillin alone. A similar pattern of GBS killing was observed with cord blood PMNL, six different GBS type III strains, and pretreatment of GBS with either IVIG or penicillin. These effects suggest that the combination of IVIG and penicillin has potential for use in the treatment of neonatal GBS infections.