Comparative bactericidal activities of beta-lactam antibiotics determined in agar and broth media.

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Abstract
Comparative bactericidal activities were determined utilizing a relatively large number of test strains, in agar and broth media, with special reference to the time of exposure of the bacteria to certain .beta.-lactam antibiotics. 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. Those of benzylpenicillin for .alpha.-streptococcus (non-enterococcal) were higher in agar than in broth. If the bactericidal concentrations with 3- or 6-h exposure to antibiotics were used as the criterion, these concentrations of carbenicillin for P. aeruginosa, and benzylpenicillin for .alpha.-streptococcus were, in particular, unusually high compared with the conventionally determined bacteriostatic concentrations (MIC).