Postantibiotic effect of meropenem on members of the family Enterobacteriaceae determined by five methods

Abstract

The postantibiotic effect (PAE) of meropenem was determined for 11 strains, both clinical isolates and reference strains of members of the family Enterobacteriaceae. The study compares PAE results obtained by five methods used to monitor bacterial regrowth, including viable counting, alone and in combination with impedance; bioluminescence, alone and in combination with impedance; and a morphological technique. After exposure of the test organisms to meropenem (0.1 x to 100 x MIC) for 2 h, concentration-dependent differences in counts by bioluminescence and viable counts were observed, the latter always being lower. The differences varied with the test organism. For example, after exposure ofProvidentia stuartii NCTC 10318 to 0.1 x MIC, the counts were 5.5 x 105 and 2.0 x l05 whereas after exposure of Citrobacter freundii MR76 to 0.1x MIC of meropenem the counts were 2.3 x 106 and 6.8 x 103 by bioluminescence and viable counting, respectively. The discrepancies were probably due to the relative inability of the viable counting procedure to detect fragile aberrant morphologies and resulted in differences in the calculated PAE values. With methods which do not detect fragile morphologies, the PAE may be underestimated. A general trend was observed for the order of magnitude of the PAEs by the following methods (in order of decreasing magnitude of PAE): (i) morphological technique, (ii) bioluminescence technique alone, (iii) bioluminescence in combination with impedance, (iv) viable counting in combination with impedance, and (v) viable counting alone. It is our opinion that of the methods examined in this study, bioluminescence in combination with impedance best reflects the true values for PAEs, and these results were examined more closely.Postantibiotic effect (PAE), the delayed regrowth of bacteria after antibiotic exposure, is now a well-established phenomenon that was first described almost 50 years ago (1). The effect appears to be a feature of virtually all antimicrobial agents and has been observed for a wide variety of bacteria and yeasts. It is, however, the specific antimicrobial agent-organism combination which determines the presence and duration of the PAE. Investigators using both old and new ,-lactams have consistently observed a PAE with gram-positive cocci (2). In contrast, after exposure of gram-negative bacilli to ,-lactams, PAEs, if any, are of short duration or have a negative value. PAE values obtained are, however, very dependent on the methods used to quantify the effect (8, 12). Bearing in mind that gram-negative bacilli generally form fragile spheroplasts when exposed to meropenem, the suitability of performing viable counts on these cell forms has been questioned, and it was found that an underestimation of the number of cells by viable counting led to an underestimation of the PAE (9,13,15). This study extends these observations of some members of the family Enterobacteriaceae by using an impedance technique (3) in conjunction with a bioluminescence assay of bacterial ATP (6) and compa...

Postantibiotic effect of meropenem in combination with gentamicin or sparfloxacin on Gram-positive and Gram-negative organisms

“…time of exposure and antimicrobial concentrations, may heavily influence this value. Moreover, different methodologies recently employed in assessing the PAE have produced some conflicting results [8–10]. It has been shown that the PAEs of carbapenems are highly method dependent [8] with a consistent underestimation when the viable count method is adopted instead of others such as bioluminescence or morphologic techniques.…”

“…Moreover, different methodologies recently employed in assessing the PAE have produced some conflicting results [8–10]. It has been shown that the PAEs of carbapenems are highly method dependent [8] with a consistent underestimation when the viable count method is adopted instead of others such as bioluminescence or morphologic techniques. Recent reports have shown decreased susceptibility to carbapenems of clinical isolates of P. aeruginosa [11,12]: this resistance is conferred by the altered permeability of the outer membrane, due to the loss of D2 porin, and by the expression of chromosomal β‐lactamase.…”

Pharmacodynamic Effects of Nitroimidazoles Alone and in Combination with Clarithromycin on Helicobacter pylori

“…By relating the ERT for the drug-exposed culture to the ERT for the control culture we have developed the CERT parameter, which is obtained independently of methods used to quantitate bacteria, antibiotic elimination, and growth rate (15). PAEs obtained by different methods sometimes give conflicting results (11,14,21). However, it has been shown that the use of ERT and CERT can circumvent these methodological problems (14,15).…”

The post-antibiotic effects of rokitamycin (a 16-membered ring macrolide) on susceptible and erythromycin-resistant strains of Streptococcus pyogenes

“…It has also been used and validated for quantifying the PAEs of antibiotics [4,[18][19][20][21]. This technique does not need viability counts and is not affected by changes in growth conditions from liquid to solid media, or the clumping typical of S. pyogenes, whose culture under normal conditions is a mixture of single and interlaced bacterial chains of different lengths (from 3 or 4 to 30 or 40 cells and sometimes more) [22,23].…”

“…This period can also be a measure of the delayed regrowth of bacteria after a short period of antibiotic exposure [4].…”