Comparative antibacterial activities of 7 alpha-methoxy cephalosporins and 7 beta-methoxyiminoacetamido cephalosporins against Bacteroides fragilis.

The in vitro antibacterial activities of the newly developed 7 alpha-methoxy cephalosporins and 7 beta-methoxyiminoacetamido cephalosporins against 67 clinical isolates of Bacteroides fragilis and their resistance to the hydrolytic action of a beta-lactamase produced by B. fragilis were simultaneously compared. The minimal inhibitory concentrations that inhibited 90% of the 7 alpha-methoxy cephalosporins, cefoxitin, cefmetazole, moxalactam, and cefotetan, against the isolates were 4, 8, 8, and 16 micrograms/ml, respectively, and these antibiotics were entirely resistant to hydrolysis by beta-lactamases (0.10 mumol/h per mg of protein) of the isolates. By contrast, 7 beta-methoxyiminoacetamido cephalosporins represented by cefotaxime, ceftizoxime, and cefmenoxime were not effective, as indicated by the minimal inhibitory concentrations that inhibited 90%, 64, 32, and 128 micrograms/ml, respectively. Their antibacterial activities clearly corresponded to their resistance to the hydrolytic action of the beta-lactamase: namely, the correlation coefficients in regression curves of cefotaxime, ceftizoxime, and cefmenoxime, which were expressed by the antibacterial activity (x axis) and the beta-lactamase activity (y axis) were 0.098, 0.034, and 0.163, respectively.