Postantibiotic and bactericidal effect of imipenem against Pseudomonas aeruginosa.

The postantibiotic effect of imipenem on Pseudomonas aeruginosa was studied at different inocula using one ATCC strain and four clinical isolates. The postantibiotic effect was measured using two different methods: viable counts and bioluminescence assay of intracellular bacterial ATP. The postantibiotic effect could be demonstrated with both methods (viable counts 1-2 h, ATP assay 3-5 h) for all strains at an inoculum of 10(6) CFU/ml. When the inoculum was raised to 10(8) CFU/ml, no postantibiotic effect could be observed with either method using routine growth conditions. This disappearance of the postantibiotic effect coincided with a loss of bactericidal effect of imipenem when high inocula were used. Improved oxygenation of the cultures restored the bactericidal and postantibiotic effects of imipenem at high inocula.