POSTANTIBIOTIC EFFECT OF MEROPENEM ON MEMBERS OF THE FAMILY ENTEROBACTERIACEAE DETERMINED BY 5 METHODS

The postantibiotic effect (PAE) of meropenem was determined for 11 str ains, both clinical isolates and reference strains of members of the f amily Enterobacteriaceae. The study compares PAE results obtained by f ive methods used to monitor bacterial regrowth, including viable count ing, alone and in combination with impedance; bioluminescence, alone a nd 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 di fferences varied with the test organism. For example, after exposure o f Providentia stuartii NCTC 10318 to 0.1 x MIC, the counts were 5.5 x 10(5) and 2.0 x 10(5) whereas after exposure of Citrobacter freundii M R76 to 0.1 x MIC of meropenem the counts were 2.3 x 10(6) and 6.8 x 10 (3) by bioluminescence and viable counting, respectively. The discrepa ncies were probably due to the relative inability of the viable counti ng procedure to detect fragile aberrant morphologies and resulted in d ifferences in the calculated PAE values. With methods which do riot de tect fragile morphologies, the PAE may be underestimated. A general tr end was observed for the order of magnitude of the PAEs by the followi ng methods (in order of decreasing magnitude of PAE): (i) morphologica l technique, (ii) bioluminescence technique alone, (iii) bioluminescen ce in combination,vith 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 resul ts were examined more closely.