Mechanisms of beta-lactam resistance in anaerobic bacteria.

The known mechanisms of beta-lactam resistance in anaerobic bacteria involve production of beta-lactamases, alteration of penicillin-binding proteins, and blocking of the penetration of beta-lactams through the outer membranes. The most important factor in beta-lactam resistance is the production of beta-lactamase. beta-Lactamases in various Bacteroides, Fusobacterium, and Clostridium species have been described. beta-Lactam resistance in Bacteroides fragilis is most commonly mediated by the production of beta-lactamase, primarily of the cephalosporinase type. Studies have also shown that B. fragilis can produce a penicillinase that inactivates piperacillin and carbenicillin. Enzymes that inactivate cefoxitin and imipenem have also been found in B. fragilis. The nonfragilis Bacteroides species produce beta-lactamases mainly of the penicillinase type. Recently a penicillinase from Fusobacterium nucleatum has been characterized. Among the clostridia, Clostridium butyricum, clostridium clostridiiforme and Clostridium ramosum have been shown to produce penicillinases.