The need for and choice of chemotherapy for anaerobic infections.

The Clostridia remain a frequent cause of anerobic infections and they also remain fully sensitive to benzylpenicillin, which continues to be the antibiotic of choice in their treatment. Penicillin is also the first choice for the treatment of dental and pulmonary infections due to anaerobic bacteria. However, currently the anerobic pathogen most frequently encountered in infections related to the gastro-intestinal and genital tracts is Bacteroides fragilis, an organism which produces beta-lactamase active against both the penicillins and cephalosporins. The antibiotics available for the treatment of B. fragilis infections are the tetracyclines, chloramphenicol, clindamycin, metronidazole and cefoxitin. Since at least 30% of strains of B. fragilis are now resistant to tetracycline and in view of the serious potential toxicity of chloramphenicol, the choice of agent thus lies with the latter three antimicrobial agents. If the in vitro activity of these agents is considered in relation to their pharmacodynamics, it is clear that only clindamycin and metronidazole can be expected to be effective in over 90% of infections with B. fragilis. Studies of the bactericidal activity of these agents against B fragilis have shown a kill of greater than 10(7) in 6 hours in metronidazole, of 10(5) to 10(6) in 30 hours in clindamycin and of 10(2) in 24 hours with cefoxitin.