Absence of a postantibiotic effect in experimental Pseudomonas endocarditis treated with imipenem, with or without gentamicin.

We found an in vitro postantibiotic effect (PAE) of 3-4 h for imipenem and of approximately 5 h for imipenem plus gentamicin against Pseudomonas aeruginosa. We therefore evaluated these antibiotics in a rat model of pseudomonas endocarditis. A rapid bactericidal effect was initially observed in vegetations from rats treated with imipenem alone or in combination with gentamicin. Bacterial counts rose rapidly, however, as soon as levels of imipenem in vegetations fell below the minimal inhibitory concentration (i.e., no PAE was demonstrated). Levels of gentamicin in vegetations were similar to those that had enhanced the bactericidal effect of imipenem and had resulted in a 5-h PAE in vitro. The presence in vitro of a PAE for imipenem, with or without gentamicin, does not necessarily predict its presence in vivo in pseudomonas endocarditis in the rat.