Studies on beta-lactamase in Bacteroides fragilis

Fifteen strains of Bacteroides fragilis, five highly resistant, five moderately resistant and five sensitive to benzylpenicillin and cephaloridine, were tested for beta-lactamase production. In the highly resistant and the moderately resistant groups of strains, a correlation between formation of beta-lactamase and MIC-values was found. Culture under controlled conditions in stirred fermentors gave higher yields of beta-lactamase than static cultures in bottles. The enzyme had its maximum activity at pH 5.0 and was stable between pH 5.5-8.5. Isoelectric focusing in polyacrylamide gels gave an isoelectric point of 4.9 +/- 0.2 for the enzyme. In the presence of the beta-lactamase inhibitors clavulanic acid and CP-45.899 at concentrations of 1 microgram/ml, the susceptibility to cephaloridine increased fourfold or more in the beta-lactamase-producing strains. Crypticity measurements (beta-lactamase activity broken/intact cells) with cephaloridine as substrate could indicate a diffusion barrier in the cell wall.