The postantibiotic effect (PAE) of the carbapenem antibiotic meropenem was determined for the reference strains of Escherichia coli NCTC 4174 and E. coli NCTC 12210. Regrowth of bacteria after antibiotic exposure was determined by viable counting and bioluminescence alone and in combination with an impedance technique and a morphological technique was also employed. Different methods of calculating the PAE were also used. After exposure of E. coli to 0.1-100 x MIC of meropenem for 2 h, concentration dependent differences in counts by bioluminescence, and viable counting were observed, the latter always being lower. The unexposed control of E. coli NCTC 4174 yielded counts of 1.1 x 10(6) +/- 1.1 x 10(5) and 1.3 x 10(6) +/- 4.7 x 10(5) by viable counting and bioluminescence respectively and E. coli NCTC 12210 gave counts of 4.2 x 10(6) +/- 1.8 x 10(6) and 1.1 x 10(7) +/- 4.3 x 10(6) by the same methods. After exposure to 100 x MIC of meropenem, NCTC 4174 yielded counts of 1.28 x 10(3) +/- 5.35 x 10(2) and 2.59 x 10(5) +/- 8.61 x 10(4) and NCTC 12210 gave counts of 5.22 x 10(3) +/- 9.74 x 10(2) and 5.21 x 10(6) +/- 1.45 x 10(6) by viable counting and bioluminescence, respectively. The discrepancies were due to the inability of the viable counting procedure to detect spheroplasts. Falsely low post exposure counts led to falsely low determinations of PAE by viable counting alone and in combination with the impedance technique.(ABSTRACT TRUNCATED AT 250 WORDS)