Resistance to beta-lactam antibiotics in anaerobic bacteria.

The known mechanisms of beta-lactam resistance in anaerobic bacteria involve production of beta-lactamases, alteration of penicillin-binding proteins, and blocked penetration of beta-lactam antibiotics through bacterial outer membranes. The most important factor in beta-lactam resistance is production of beta-lactamases, which have been found in various Bacteroides, Fusobacterium, and Clostridium species. beta-Lactam resistance in Bacteroides fragilis is commonly mediated by beta-lactamases that are mainly cephalosporinase in character. B. fragilis strains can also produce penicillinases and enzymes inactivating cefoxitin and imipenem. The non-fragilis species of Bacteroides produce beta-lactamases that are mainly penicillinase in character. Penicillinases are also isolated from Fusobacterium nucleatum. Among the clostridia, Clostridium butyricum, Clostridium clostridioforme, and Clostridium ramosum have been shown to produce penicillinases.