Susceptibility of intestinal anaerobes to new beta-lactam antibiotics.

In vitro susceptibility of 473 isolates of anaerobic bacteria derived from patients hospitalized for treatment of abdominal trauma was tested against third-generation cephalosporins--moxalactam, cefotaxime, ceftizoxime, cefmenoxime--and a penicillin--N-formimidoyl thienamycin--by serial agar dilution method in the anaerobic glove box. There were 208 Bacteroides isolates including 80 Bacteroides fragilis and 100 peptococci and peptostreptococci. The minimal inhibitory concentration (MIC) 75% of B. fragilis was less than or equal to 32 micrograms/ml for the cephalosporins tested and less than or equal to 8 micrograms/ml for thienamycin. The MIC of B. fragilis and Bacteroides vulgatus was not much influenced by increased inoculum size. Characteristically, peptostreptococci and peptococci were resistant to all four cephalosporins tested with MIC 75% being 32-64 micrograms/ml. All five antibiotics in this study appear suitable for therapy of intraabdominal suppurative infections, but the poor activity of third-generation cephalosporins on anaerobic cocci can be a cause of concern.