The in vitro postantibiotic effects (PAE) of aztreonam, ceftazidime, cefuroxime, imipenem, and piperacillin on Escherichia coli ATCC 25922 were studied by a bioluminescence assay of bacterial ATP. In parallel with the PAE investigation, viability and morphology studies were performed. The strain was exposed for 2 h to different concentrations of beta-lactam antibiotics. The antibiotic activity was eliminated by 10(-4) dilutions, and regrowth of bacteria was monitored hourly by the bioluminescence assay of bacterial ATP. The length of PAE was dose dependent for ceftazidime (0.5 to 2.6 h), cefuroxime (0.4 to 2.6 h), and imipenem (0.3 to 4.5 h). The long PAE for these antibiotics at higher concentrations was associated with a potent initial killing and the presence of spheroplasts. Aztreonam and piperacillin produced a short, non-dose-dependent PAE (0.4 to 0.95 h). Short PAEs (below 1 h) were seen concomitantly with production of filaments, except in the case of imipenem, which only produced spheroplasts. The bioluminescence method was not jeopardized by filament formation, in contrast to the viable count assay which is normally used for PAE investigations. This makes it possible to study PAE for beta-lactam antibiotics on gram-negative bacteria with bioluminescence.