The effect of clavulanic acid on the susceptibility of Bacteroides fragilis to three acyl-ureidopenicillins, ampicillin and carbenicillin

The susceptibilities of 50 clinical isolates of the Bacteroides fragilis group to ampicillin, carbenicillin, mezlocillin, azlocillin and piperacillin were similar, modal MICs being 8-32 mg/l. However, in the presence of clavulanic acid, 45 and 43 strains became susceptible (MIC less than or equal to 4 mg/l) to ampicillin and carbenicillin respectively whilst 27, 37 and 35 strains became susceptible (MIC less than or equal to 4 mg/l) to mezlocillin, azlocillin and piperacillin. Clavulanic acid did not influence the susceptibility of six (12%) strains. The isoelectric points of the beta-lactamases from Bact. fragilis and Bact. thetaiotaomicron were similar to those found by others.