Susceptibility of anaerobes to cefoxitin sodium and cephalothin

The susceptibility in vitro of 295 strains of anaerobic bacteria to cefoxitin sodium ' and cephalothin was determined by use of an agar-dilution technique. The strains were isolated from clinical specimens during 1975 and 1976. At 32µg/ml or less, cefoxitin inhibited 100% of Bacteroides melaninogenicus, Fusobacterium spp., anaerobic cocci, and Clostridium perfringens; 91 % of the B. fragilis group, 98% of other Bacteroides, 96% of non-spore-forming Gram-positive bacilli, and 64% of other clostridia. Cephalothin showed similar activity against most of the anaerobes. However, only 18% of the B. fragilis group and 89% of other Bacteroides were inhibited by cephalothin at 32 µg/ml or less, whereas 77% of clostridia were inhibited by cephalothin at this concentration. It appears that, despite widespread use of cephalosporins, the incidence of organisms resistant to these antibiotics is not increasing.