Efficacy of rifamycin SV and vancomycin againstBacteroides fragilis in vitro and in experimentally infected mice

The efficacy of rifamycin SV and vancomycin againstBacteroides fragilis was compared with that of metronidazole in vitro and in vivo. The in vitro comparison was based on the MIC and on short-term growth curves, the in vivo comparison on a mixed infection withB. fragilis andEscherichia coli in a thigh infection model in mice. Rifamycin SV was about 11 times less effective than metronidazole in vivo, according to dose, and about 30 times less effective according to the mean plasma concentration. This lower efficacy could have been anticipated on the basis of the effect on in vitro grwoth curves, but not from the MIC. Vancomycin was about nine times less effective than metronidazole in vivo according to dose and about 18 times less effective according to the mean plasma concentration. The latter difference was less than would have been predicted from the results obtained in vitro, i.e., from both the effect on the growth curves and the MIC.