Cefoxitin resistance in Bacteroides species: evidence indicating two mechanisms causing decreased susceptibility

Clinical isolates of Bacteroides species resistant to cefoxitin (not due to beta-lactamase) were obtained from two sources and examined for PBP affinity (to cefoxitin) and outer membrane proteins. Mutants of Bacteroides fragilis with decreased susceptibility to cefoxitin were obtained in the laboratory with the mutagen N-methyl-N'-nitro-N-nitrosoguanidine. No spontaneous mutants resistant to cefoxitin could be selected. Changes in the affinity of PBP 1 or PBP 2 were correlated with a decrease in susceptibility to cefoxitin. Two clinical isolates showing decreased affinity of PBPs for cefoxitin also showed outer membrane protein changes with a protein of 49-50,000 daltons apparently absent. Three strains did not have altered PBP affinity or altered outer membrane protein profile, one strain being a laboratory mutant.