Susceptibility of group B streptococcus to beta-lactam antibiotics

To evaluate the relative activity of fourteen beta-lactam agents, including seven new agents, the susceptibility of 77 clinical isolates of group B streptococci were measured using an agar dilution technique. Thienamycin, cefotaxime and azlocillin had better activity than penicillin, with MIC₅₀ of ≤ 0.015 μg/ml. Ampicillin, cefamandole, cefazolin, cefoperazone, mezlocillin and piperacillin had activity similar to penicillin with MIC₅₀ of 0.03-0.06 μg/ml. Oxacillin, ticarcillin, cefoxitin and moxalactam were less active than penicillin with MIC₅₀ of 0.25-2 μg/ml. Susceptibility to five of the more active antibiotics was markedly reduced, however, when the inoculum size was increased from 10⁴ to 10⁸ colony forming units/ml.