Characterization of a beta-lactamase found in Eikenella corrodens

Eleven strains of Eikenella corrodens with beta-lactamase activity were isolated from a patient with refractory periodontitis who had previously been treated with penicillin antibiotics. These strains were relatively resistant to benzylpenicillin, amoxicillin, and ampicillin (MICs, greater than or equal to 64 micrograms/ml); susceptible to amoxicillin-clavulanate (2:1) (MICs, less than or equal to 4 micrograms/ml); and moderately susceptible to cephalothin and cephaloridine (MICs, 0.12 to 16 micrograms/ml). The addition of 1 microgram of potassium clavulanate, a beta-lactamase inhibitor, per ml resulted in a significant increase in the susceptibilities of these strains to penicillins but not to cephalosporins. Potassium clavulanate had no effect on non-beta-lactamase-producing strains. Enzyme production was constitutive since activity was not increased when cells were cultivated in the presence of benzylpenicillin. Enzyme activity was strongly inhibited by potassium clavulanate, sulbactam, and iodine; weakly inhibited by cloxacillin, imipenem, and moxalactam; but not inhibited by aztreonam, EDTA, or p-chloromercuribenzoate. By gel infiltration, the enzyme had an estimated molecular mass of 29 kDa. Isoelectric focusing of the partially purified enzyme gave a major beta-lactamase band at pH 5.50 and a minor band at pH 5.60. Plasmids were not detected in any of the 11 beta-lactamase-positive strains. This enzyme is considered to belong to class 2a of the Bush classification scheme.