COMPARATIVE BACTERICIDAL ACTIVITIES OF BETA-LACTAM ANTIBIOTICS DETERMINED IN AGAR AND BROTH MEDIA

Abstract

Comparative bactericidal activities were determined utilizing a relatively large number of test strains, in both agar and broth media, with special reference to the time of exposure of the bacteria to certain β-lactam antibiotics. It was apparent that the activities increase with time. The concentrations producing a 99.9% kill with cephalothin for Escherichia coli, Klebsiella sp., and carbenicillin for Pseudomonas aeruginosa were higher in broth than in agar. In contrast, those of benzylpenicillin for α-streptococcus( non-enterococcal) were higher in agar than in broth. If the bactericidal concentrations with 3-hour or 6-hour exposuret o antibiotics were used as the criterion, these concentrations of carbenicillin for P. aeruginosa, and benzylpenicillin for α-streptococcus were, in particular, unusually high compared with the conventionally determined bacteriostatic concentrations( MICs).