Inactivation of cephalosporins by Bacteroides

We investigated the relationship between beta-lactamases of Bacteroides fragilis organisms and their resistance to cephalosporins. Timed killing curves were used to study the in vitro activity of three cephalosporins, cephalothin, cefazolin, and cefamandole, and a semisynthetic cephamycin, cefoxitin. Measurements of residual antibiotic concentrations in culture supernatants were made, and they were compared with the beta-lactamase activity of the microorganism. A cephalosporin-susceptible strain was rapidly killed by cephalothin, cefazolin, cefamandole, and cefoxitin. Four cephalosporin-resistant strains were not killed by cephalothin, cefazolin, or cefamandole but were killed by cefoxitin. An inoculum effect was noted with cefazolin and not with cefoxitin. The resistant strains of Bacteroides inactivated the three cephalosporins, but there was no inactivation of cefoxitin. A constitutive beta-lactamase was detected in all the isolates of the B. fragilis group that were resistant to the cephalosporins. There was no distinction of the species based on isoelectric focusing of the enzyme. These data suggest that inactivation by beta-lactamase may be the mechanism for resistance of B. fragilis to the cephalosporins and would explain the enhanced in vitro activity of cefoxitin.