Clavulanic acid-potentiated activity of amoxicillin against Bacteroides fragilis.

Seventy-seven clinical isolates of Bacteroides fragilis from animal sources were examined for beta-lactamase production and resistance to amoxicillin. Sixty-five (84%) of the isolates produced detectable amounts of beta-lactamase. Concentrations of amoxicillin greater than 8 micrograms/ml were required to inhibit all but 13 (17%) of the isolates evaluated. Clavulanic acid, a beta-lactamase inhibitor, minimally inhibited growth of B fragilis when used alone. However, a synergistic effect was found when amoxicillin and clavulanic acid were used together against B fragilis. Ninety-nine percent of the isolates were inhibited by concentrations of 1 microgram of amoxicillin/ml in combination with 0.5 microgram of clavulanic acid/ml.