Speed of Bactericidal Action of Penicillin G, Ampicillin, and Carbenicillin on Bacteroides fragilis

In order to determine the therapeutic value of penicillins in the management of anaerobic infections caused by Bacteroides fragilis, the antibacterial activity of penicillin G, ampicillin, and carbenicillin against 10 strains of B. fragilis subsp. fragilis, isolated from clinical material, was studied. This was undertaken by using a triple-layer technique, enzymatic inactivation of the antibiotics, and an automatic colony counter, which allowed a quantitative and a qualitative approach to the problem. Results indicated in vitro bactericidal activity with 40 mug of penicillin G, ampicillin, and carbenicillin per ml against all strains tested. However, carbenicillin had the most rapid activity.