Comparison of methodologies used in assessing the postantibiotic effect

• Published in: Journal of antimicrobial chemotherapy 1994-08, Vol.34 (2), p.223-230
• Main Authors: MacKenzie, F. M., Gould, I. M., Chapman, D. G., Jason, D.
• Format: Article
• Language: English
• Subjects: Anti-Bacterial Agents - pharmacology; Antibacterial agents; Antibiotics. Antiinfectious agents. Antiparasitic agents; Bacteriological Techniques; Biological and medical sciences; Colony Count, Microbial; Electric Conductivity; Escherichia coli; Escherichia coli - drug effects; Escherichia coli - growth & development; Escherichia coli - ultrastructure; Luminescent Measurements; Medical sciences; Meropenem; Pharmacology. Drug treatments; Spheroplasts - drug effects; Thienamycins - pharmacology
• ISSN: 0305-7453; 1460-2091
• DOI: 10.1093/jac/34.2.223

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×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 1.1×106 ± 1.1×105 and 1.3×106 ± 4.7×105 by viable counting and bioluminescence respectively and E. coli NCTC 12210 gave counts of4.2×106 ± 1.8×106 and 1.1×107 ± 4.3×106 by the same methods. After exposure to 100×MIC of meropenem, NCTC 4174 yielded counts of 1 28×103 ± 5.35×102 and 2.59×105 ± 8.61×104 and NCTC 12210 gave counts of 5.22×103 ± 9.74×102 and 5.21× 106 ± 1.45×106 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. For example, after exposing E. coli NCTC 4174 to 100×MIC of meropenem the PAE values were 0.2 ± 0.3 h by viable counting, 0.4 ± 0 2 h by viable counting/impedance, 2.6 ± 0.3 h by bioluminescence, 2.3 ± 0.4 h by bioluminescence/impedance and 4.0 ± 0.6 h by the morphological technique. The PAEs were less than 0.5 h for both organisms by viable counting alone and in combination with impedance, and values varied little with the antibiotic concentration. The PAE values obtained from morphology varied from 0 h to 4.6 h and depended to some extent, on the antibiotic concentration. Bioluminescence alone and in combination with impedance yielded PAE values which varied with antibiotic concentrations up to 4–10×MIC. At higher concentrations, a plateau was reached.