Serious infection in an adult due to penicillin-tolerant group B streptococcus

This report describes a patient with group B streptococcal (GBS) bacteremia with pyelonephritis and septic arthritis whose condition failed to improve after two weeks of therapy with penicillin G sodium. The organism was found to be tolerant to penicillin (minimal inhibitory concentration, 0.06 IU/mL; minimal bactericidal concentration [MBC], 10 IU/mL). Antimicrobial synergy with gentamicin sulfate was demonstrated (MBC of penicillin was 0.07 IU/mL in the presence of 2.5 micrograms/mL of gentamicin). Addition of gentamicin to penicillin therapy was associated with clinical improvement. It is suggested that bactericidal rather than inhibitory susceptibility tests be employed as a guide to therapy in serious GBS infections. Where penicillin tolerance is found in association with a poor clinical response to penicillin, addition of an aminoglycoside should be considered. Antimicrobial synergy studies should be performed to demonstrate that a beneficial effect is possible at clinically attainable antibiotic concentrations.