In vitro activity of ceftizoxime against Bacteroides fragilis: comparison with benzylpenicillin, cephalothin, and cefoxitin.

Minimum inhibitory concentrations of ceftizoxime for seven clinical isolates of Bacteroides fragilis were found to be markedly affected by inoculum size; the very low minimum inhibitory concentration values (less than or equal to 0.25 microgram/ml) obtained in tests with most strains were not sustained when the inoculum was raised 100-fold. Benzylpenicillin and cephalothin were also affected by inoculum size, but cefoxitin was not. Antibiotic assays showed that the strains inactivated ceftizoxime and cephalothin completely, benzylpenicillin partially, and cefoxitin not at all. Morphological investigations revealed that populations of B. fragilis responded differently to each of the four beta-lactam agents during the early stages of the encounter between B. fragilis and the antibiotic. Benzylpenicillin and cefoxitin induced spheroplast formation at lower concentrations than did ceftizoxime. Cephalothin evoked a novel morphological response that was not seen with other beta-lactam agents.