Characterization of β-lactamases in clinical isolates of Bacteroides

A total of 78 strains of Bacteroides was freshly isolated from clinical specimens; 60 of the strains were identified as B. fragilis and 16 as B. thetaiotaomicron. Overall, imipenem and the combination of benzylpenicillin and clavulanic acid were found to be considerably more active than cefoxitin or latamoxef (moxalactam) against these strains. Nearly all the strains exhibited weak beta-lactamase activity and 13 were judged to produce elevated levels of enzyme. These 13 strains fell into three groups: four strains that produced enzymes of low specific activity which were very susceptible to beta-lactamase inhibitors; three strains that produced enzymes of intermediate specific activity which hydrolysed cefoxitin and latamoxef at a variable rate; two of these enzymes rapidly inactivated imipenem and were insusceptible to beta-lactamase inhibitors; six strains that produced enzymes of high specific activity which were susceptible to beta-lactamase inhibitors; these strains did not inactivate beta-lactamase-stable beta-lactam antibiotics, but two of the strains nonetheless exhibited reduced susceptibility to latamoxef and imipenem.