Carbenicillin for treatment of Bacteroides fragilis infections: why not penicillin G?

Carbenicillin has been advocated for treatment of infections caused by Bacteroides fragilis and other anaerobic bacteria. Wide-scale use of the drug in this setting could result in a substantial increase in carbenicillin-resistant Pseudomonas aeruginosa, an effect that would have serious implications. Thirty-four strains of B. fragilis, one-half from bacteremic infections, were tested in vitro, and penicillin G was found to be twice as active as carbenicillin on an equal weight basis; 94% of the strains were inhibited by 32 microgram of penicillin/ml, a level easily achieved therapeutically. Penicillin killed B. fragilis organisms as rapidly as carbenicillin. In two subjects given equivalent doses (100 mg/kg intravenously) of carbenicillin and aqueous penicillin G, the bactericidal activity of serum against B. fragilis after administration of each drug was the same. Controlled clinical trials of treatment of anaerobic bacterial infections with penicillin G in high dosage, carbenicillin (or closely related ticarcillin), clindamycin, and chloramphenicol should be undertaken. Carbenicillin (and ticarcillin) for the present would seem better reserved for P. aeruginosa infections.