ISOLATION AND IN-VITRO AND IN-VIVO CHARACTERIZATION OF A MUTANT OF PSEUDOMONAS-AERUGINOSA PAO1 THAT EXHIBITED A REDUCED POSTANTIBIOTIC EFFECT IN RESPONSE TO IMIPENEM

The postantibiotic effect (PAE) is the persistent inhibition of bacter ial growth after a brief exposure to an antibiotic. Most beta-lactams do not induce a PAE for Gram-negative bacteria, but PAEs have been rep orted for carbapenems and penems. This study investigated the effect o f sequential doses of imipenem on the PAE for Pseudomonas aeruginosa a nd Escherichia coil cultures in a chemostat. The PAE for the bacterial population did not change even after six successive exposures to imip enem. Nevertheless, screening of colonies isolated after repeated drug exposure identified a single P. aeruginosa mutant whose imipenem PAE was shortened, although the MIC was unchanged. The PAEs for the parent and mutant were studied in vitro in batch culture by monitoring: (i) viable counts; (ii) electrical impedance of the culture medium; (iii) incorporation of radiolabelled N-acetyl-D-glucosamine and (iv) cell vo lume changes. PAEs for the parent and mutant were found to be signific antly different by all in-vitro methods used. Moreover, the median cel l volume in antibiotic-exposed cultures remained much smaller and less heterogeneous than in the control cultures, even though both cultures were growing at the same rate. The mutant was found to have a reduced expression of a 52 kDa outer membrane protein. These observations sug gest that factors in addition to suppression of bacterial growth shoul d be considered when studying the PAE. The PAEs of imipenem for the pa rent and mutant were studied in a thigh infection model in leucopenic mice. Similar PAEs were observed in vivo for both parent and mutant in one experiment and no PAEs for either organism were found in a second experiment. This study showed that although the PAE is a stable in-vi tro phenomenon, the lack of correlation between the in-vitro and in-vi vo results warrants caution in attributing clinical significance to th e PAE of imipenem.