Effect of combined colistin and meropenem against meropenem resistant Acinetobacter baumannii and Pseudomonas aeruginosa by checkerboard method: A cross sectional analytical study.

Meropenem-resistant Acinetobacter baumannii and Pseudomonas aeruginosa are the two most common nosocomial pathogens causing ventilator-associated pneumonia. To combat this resistance, different combinations of antibiotics have been evaluated for their efficacy in laboratories as well as in clinical situations.The aim of the study was to investigate the effect of combined colistin and meropenem against meropenem-resistant isolates of A. baumannii and P. aeruginosa by checkerboard method.Fifty meropenem-resistant isolates of A. baumannii (n = 25) and P. aeruginosa (n = 25) from endotracheal aspirates were studied. The MIC of colistin and meropenem was found using the microbroth dilution method. The fractional inhibitory concentration was calculated for the combination of antibiotics by checkerboard assay and the antibiotic interactions were assessed. Fisher's exact test was carried out for statistical comparison of categorical variables.A synergistic effect between colistin and meropenem was observed in 18/25 (72%) and 6/25 (24%) isolates of Acinetobacter baumannnii and P. Aeruginosa, respectively, with fractional inhibitory concentration indices of ≤0.5. None of the tested isolates exhibited antagonism.Our results showed that combinations of colistin and meropenem are associated with improvement in minimum inhibitory concentration and may be a promising strategy in treating meropenem-resistant A. baumannii respiratory tract infections.