Post-antibiotic effect of beta-lactam antibiotics on gram-negative bacteria in relation to morphology, initial killing and MIC

The in vitro post-antibiotic effect (PAE) of cefepime, cefotaxime, ceftazidime and imipenem on reference strains ofEscherichia coli, Enterobacter cloacae, Klebsiella pneumoniae, Pseudomonas aeruginosa andSerratia marcescens were evaluated by bioluminescence assay of bacterial ATP. In parallel with the PAE determination, initial killing and morphology studies were performed. Imipenem produced>1 h PAE on all strains tested, cefepime and cefotaxime on four strains and ceftazidime only on one of the strains tested. The length of the PAE on different strains did not correlate in the same way to MIC. Imipenem induced>1 h PAE at 1/4-2 MIC while the cephalosporins caused>1 h PAE at 4−256 × MIC. A PAE exceeding 1.2 h was seen concomitantly with spheroplasts but there was not necessarily strong (≥99 %) initial killing at the same time. The PAE duration at≥99 % initial killing varied between 2.0 h and 5.0 h. When the cephalosporins produced<1 h PAEs, this was seen concomitantly with production of filaments and weak initial killing. The bioluminescence method was not jeopardized by filament formation and no negative PAE was found in contrast to the viable count method. The study showed that neither a certain multiple of MIC, the presence of spheroplasts nor strong initial killing can predict the length of PAE for β-lactam antibiotics on gram-negative bacteria.