In vitro susceptibility of group G streptococci to ten antimicrobial agents with broad gram-positive spectra.

Minimal inhibitory and bactericidal concentrations (MICs, MBCs) of ten antibiotics with broad spectra against grampositive cocci were determined for 25 group G streptococci (GGS). Penicillin G, cefotaxime, and ampicillin were the most active bactericidal agents, with 100% of MBCs less than or equal to 0.6 micrograms/ml. Among the nonpenicillin, noncephalosporin agents, vancomycin had the lowest MBCs, with all strains killed by less than or equal to 2.5 micrograms/ml. Despite good in vitro activity against other streptococci, erythromycin, clindamycin, and chloramphenicol had disappointing bactericidal activity against GGS. Three strains were tolerant to erythromycin, while 15 strains were tolerant to clindamycin. Chloramphenicol was the least active inhibitory agent, with an MIC90 of 13.8 micrograms/ml; this agent was also the least active bactericidal agent against GGS. with no MBCs less than 2.5 micrograms/ml and 9 strains with MBCs greater than or equal to 20 micrograms/ml. There were no GGS strains tolerant to either penicillin G or vancomycin.