Comparative efficacy of 10 antimicrobial agents in experimental infections with Bacteroides fragilis

The comparative efficacy of 10 antimicrobial agents against 15 strains of Bacteroides was examined in vivo using an experimental model of subcutaneous abscesses in mice. Results were evaluated by bacterial counts per lesion with the antimicrobial agents administered beginning 1 hr after challenge. Six drugs reduced counts (mean +/- SEM log decrease) significantly compared with values in untreated control animals: metronidazole, 6.7 +/- 0.6; clindamycin, 5.0 +/- 0.6; moxalactam, 3.8 +/- 0.5; cefoxitin, 3.5 +/- 0.5; chloramphenicol, 1.6 +/- 0.5; and carbenicillin, 1.0 +/- 0.3. Antimicrobial agents that had no significant effect compared with values in untreated control animals were cephalothin, cefoperazone, ceforanide, and rosaramicin. Evaluation of several parameters based on in vitro activity and pharmacokinetic properties at the infected site indicated that the time during which the level of antimicrobial agent exceeded the minimal inhibitory concentration correlated best with in vivo antibacterial activity. A delay in the time that treatment was initiated resulted in a marked reduction in in vivo activity.