Antimicrobial tolerance in group C and group G streptococci.

The in-vitro activity of various antimicrobial agents against 61 clinical isolates (35 group G streptococci and 26 group C streptococci) was studied. Penicillin, cephalothin and vancomycin were the most active agents with MIC90 values of 0.03, 0.06 and 0.12 mg/l respectively. Tolerance, defined as an MIC to MBC ratio of 32 or greater, was observed in six isolates (four group G--11.4% and two group C-7.7%). Two isolates were tolerant to penicillin alone and one each to cephalothin, vancomycin, latamoxef (moxalactam) and cefotaxime. No cross-tolerance was observed. The addition of gentamicin to the beta-lactam agents and of gentamicin or rifampicin to vancomycin resulted in bactericidal activity. None of the combinations were antagonistic. Our results demonstrate that various bactericidal antimicrobial combinations are available for therapy against tolerant organisms.