Factors Contributing to Resistance to Beta-Lactam Antibiotics in Bacteroides fragilis

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Abstract
Fifteen strains of Bacteroides fragilis , five highly resistant, five moderately resistant, and five susceptible to benzylpenicillin and cephaloridine, were tested for β-lactamase production. In the highly resistant and the moderately resistant groups of strains, a correlation between formation of β-lactamase and minimal inhibitory concentrations was demonstrated. In the presence of the β-lactamase inhibitors clavulanic acid and CP-45,899 at concentrations of 1 μg/ml, the susceptibility to cephaloridine increased fourfold or more in the β-lactamase-producing strains. Crypticity measurements (β-lactamase activity broken/intact cells) with cephaloridine as substrate could indicate a diffusion barrier in the cell wall.