Meropenem and imipenem are the most active and potent ß-lactams against gram-negative bacteria and the only carbapenems commercially available in Brazil, USA, and Europe. Meropenem has higher in vitro activity against gram-negative bacteria, while imipenem has slightly higher in vitro activity against gram-positive. The objectives of this study are to compare the in vitro activities of these carbapenems and to evaluate the necessity of susceptibility testing both compounds at the routine of the microbiology laboratory. The broth microdilution results of 2,144 gram-negative bacilli were analyzed. Against Enterobacteriaceae meropenem was at least eight-fold more potent than imipenem. Against Pseudomonas aeruginosa meropenem (MIC50, 1mug/ml) was also more potent than imipenem (MIC50, 2mug/ml), and against Acinetobacter baumannii both carbapenems presented similar in vitro activities (MIC50, 1mug/ml for both). Only 2.7% of the isolates presented discrepant category results between the carbapenems; i.e. susceptible to one and resistant to the other. Forty-six isolates (2.14%) were susceptible to meropenem and resistant to imipenem; while only 12 isolates (0.55%) were susceptible to imipenem and resistant to meropenem. The vast majority of discordant results (91.4%) occur among non-fermentative bacilli (NF-BGN). Five discordant results were detected among 1,350 Enterobacteriaceae evaluated (0.37%); while 53 discrepant results were detected among 794 NF-BGN (6.64%). Isolates showing susceptibility to meropenem and resistance to imipenem account for 80% of the discrepant results. The results of this study indicate that the microbiology laboratory may susceptibility test only one of the carbapenems for Enterobacteriaceae and use the same category result for the other one. It is important that the results for both carbapenems are sent to the physicians in order for them to be able to choose the most appropriated for the case. On the other hand, for NF-BGN the laboratory should susceptibility test both carbapenems.
Antimicrobial activity; Carbapenems; Meropenem; Imipenem
