Susceptibility of anaerobic bacteria to beta-lactam antibiotics and beta-lactamase production.

We examined the susceptibility of various anaerobes to four beta-lactamase susceptible (ampicillin, amoxycillin, cyclacillin, and penicillin G) and two beta-lactamase resistant (moxalactam, and N-F-thienamycin) beta-lactam antibiotics and measured beta-lactamase production. Members of the Bacteroides groups were most resistant to the six antibiotics. N-F-thienamycin was the most effective antimicrobial agent against all the test strains, moxalactam the next most effective, and penicillin G the least. Beta-lactamase production was mainly confined to Bacteroides species. Cephalosporinase was the most common beta-lactamase produced; penicillinase was detected less often. About two thirds of the penicillin-resistant isolates produced cephalosporinase. N-F-thienamycin and moxalactam were the most active agents against those anaerobes that were resistant to many beta-lactam antibiotics.