In vitro susceptibility of Streptococcus bovis to six antibiotics.

A recent report of two isolates of Streptococcus bovis resistant to killing by low levels of penicillin G prompted this study of the in vitro susceptibility of 100 clinical isolates of S bovis to six antibiotics. Using a microdilution method, we found the minimum bactericidal concentrations for 99% of the isolates to be as follows: penicillin G, 0.5 microgram/ml; methicillin, 16 micrograms/ml; ampicillin, 0.5 microgram/ml; cephalothin, 2 micrograms/ml; clindamycin, 2 micrograms/ml; and vancomycin, 2 micrograms/ml. The high-level resistance of S bovis to penicillin G previously described was not seen. This study suggests that penicillin G can still prove effective in treating most infections caused by S bovis, including infective endocarditis, and that cephalothin and vancomycin are useful alternative agents.