CHARACTERIZATION OF BETA-LACTAMASES FROM NON-BACTEROIDES-FRAGILIS GROUP BACTEROIDES SPP BELONGING TO 7 SPECIES AND THEIR ROLE IN BETA-LACTAM RESISTANCE

Twelve β-lactamase-positive non-Bacteroides fragilis group Bacteroides spp. belonging to seven different species were examined by MIC determination and enzyme characterization. MICs of most β-lactams except cefoxitin, cefotetan, imipenem, and meropenem were relatively high or very high. All enzymes hydrolyzed cephaloridine (V(max), 100%; K(m), 12 to 70 μM), cephalothin (V(max), 25 to 826%; K(m), 8 to 143 μM), cefamandole (V(max), 13 to 158%; K(m), 17 to 170 μM), and cefuroxime (hydrolysis rate, 19 to 98%), and 11 of 12 hydrolyzed cefotaxime (V(max), 26 to 145%; K(m), 13 to 127 μM); no hydrolysis of cefoxitin or moxalactam was observed. Penicillins were hydrolyzed at lower rates, with V(max) values ≤20% of that obtained with cephaloridine. Addition of clavulanate, sulbactam, or tazobactam led to a 4- to 2,048-fold lowering of MICs of penicillins as well as cephalosporins. All enzymes were inhibited by clavulanate (50% inhibitory concentration [IC50], 0.01 to 1.8 μM), sulbactam (IC50, 0.02 to 1.9 μM), tazobactam (IC50, 0.001 to 0.9 μM), cefoxitin (IC50, 0.002 to 0.35 μM), and moxalactam (IC50, 0.03 to 6.6 μM). No enzymes were inhibited by 100 μM EDTA or p-chloromercuribenzoic acid; an enzyme of one strain of B. loescheii was inhibited by 100 μM cloxacillin (IC50, 2.35 μM). Ten enzymes had optimal activity at pH 5.0 to 6.0, and two had optimal activity at pH 8.0. Isoelectric focusing revealed pIs between 4.2 and 5.6. These enzymes seem to belong to a previously unclassified group of β-lactamases, related (but not identical) to β-lactamases of the B. fragilis group.