In vitro activity of meropenem compared to nine other antimicrobial agents: importance of its stability when used in agar dilution systems.

Abstract

The antibacterial activity of meropenem was tested against 426 clinical isolates representing a wide range of aerobic and anaerobic species. The in vitro activity of meropenem was compared with that of iminpenem, ceftazidime, cefotaxime, ciprofloxacin, piperacillin and tobramycin against aerobic isolates, and also compared with that of imipenem, metronidazole, cefoxitin, clindamycin and piperacillin against the anaerobic isolates. Meropenem exhibited an extended spectrum of activity with low minimal inhibitory concentrations (MIC) against Gram negative aerobes, anaerobes, Gram positive anaerobes and most of the Gram positive aerobes. The MIC90 of meropenem against the Enterobacteriaceae ranged from 0.03 mg/l to 0.125 mg/l. Meropenem was very active against extended spectrum beta lactamase producing Klebsiella pneumoniae, most Acinetobacter species, Pseudomonas aeruginosa, Clostridium difficile (MIC90 of 1.0 mg/l), Clostridium perfringens and other Clostridium species. Even though imipenem exhibited better activity against the coagulase negative staphylococci, meropenem still had MIC's which were less than the break point (8.0 mg/l). The stability of meropenem in agar was determined indirectly by plotting the geometric mean MIC of control strains over a period of two weeks. Mean MIC of six control strains for meropenem was 0.05 mg/l and remained constant over 10 days, whereas the mean MIC of imipenem rose to 0.24 mg/l after two days and to 3.4 mg/l after 10 days. Meropenem was therefore far more stable in agar than imipenem. This has implications for laboratories using agar dilution as it requires less frequent plate preparation and decreased labour costs.