Isolation and in-vitro and in-vivo characterisation of a mutant of Pseudomonas aeruginosa PAO1 that exhibited a reduced postantibiotic effect in response to imipenem

Abstract

The postantibiotic effect (PAE) is the persistent inhibition of bacterial growth after a brief exposure to an antibiotic. Most /J-lactams do not induce a PAE for Gram-negative bacteria, but PAEs have been reported for carbapenems and penems. This study investigated the effect of sequential doses of imipenem on the PAE for Pseudomonas aeruginosa and Escherichia coli cultures in a chemostat. The PAE for the bacterial population did not change even after six successive exposures to imipenem. 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 volume changes. PAEs for the parent and mutant were found to be significantly different by all in-vitro methods used. Moreover, the median cell 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 suggest that factors in addition to suppression of bacterial growth should be considered when studying the PAE. The PAEs of imipenem for the parent 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-vitro phenomenon, the lack of correlation between the in-vitro and in-vivo results warrants caution in attributing clinical significance to the PAE of imipenem.

Pharmacokinetics of imipenem-cilastatin in critically ill patients undergoing continuous venovenous hemofiltration

“…For more resistant organisms there is an additional concentration dependence for the observed effect (15). Although a postantibiotic effect has been reported for imipenem in vitro, the clinical significance of this effect has not been evaluated (21). Therefore, it is desirable to maintain the concentration of the antibiotic in plasma over the MIC throughout the whole dosing interval.…”

Population pharmacokinetics of imipenem in burn patients

“…However, a postantibiotic effect has been described for imipenem against Pseudomonas aeruginosa [5]. The variability of this effect against bacteria of the same species [6] and the lack of correlation between the in vitro and in vivo results observed in several studies [7,8] raise questions in attributing clinical significance of this postantibiotic effect.…”

The issue of the true postantibiotic effect

“…A further fundamental criticism of the currently used simple definition of the PAE stems from the fact that it assumes that the cell population is homogeneous and that recovery of each individual cell follows an identical time-course. However, it has been shown that the cell size distribution of a bacterial population was significantly reduced, even though viable counts of the culture were increasing at the normal rate (Majcherczyk et al 1994). Such physiologically altered populations are likely to differ in their susceptibility to phagocytosis, and in their ability to produce toxins and other pathogenic properties.…”