Antimicrobial effect of beta-lactam antibiotic combinations against Bacteroides fragilis in vitro

Carbenicillin, piperacillin, and mezlocillin, when combined with cefoxitin, exhibited synergism against 20, 13, and 10 isolates of Bacteroides fragilis, respectively, and no antagonism was observed in 19 of 20 strains. These results suggest that if beta-lactamase was induced from B. fragilis by cefoxitin, it was ineffective against the antibiotics tested.