In vitro activity of colistin combined with meropenem against multidrug resistant Klebsiella pneumoniae

Treatment of nosocomial infections caused by multidrug resistant Gram negative bacilli is the major hurdle faced by clinicians across the world. As new antimicrobial drugs are not expected in the near future, there is a compelling need to maximize the potential of the available antimicrobial agents as combination therapy. Colistin is currently being used as the reserve drug in the treatment of multidrug and pan-drug resistant gram-negative bacterial infections. Literature suggests that colistin when used as combination therapy with a synergistic drug is superior to monotherapy.This study intends to detect the in vitro synergy between colistin and meropenem thereby to determine whether the combination would be therapeutically useful. 35 isolates of multidrug resistant Klebsiella pneumoniae were included in the study. The minimum inhibitory concentration of colistin and meropenem were determined for all the study isolates using E-test strips. The presence of in vitro synergy between colistin and meropenem was detected by the fixed ratio method using E-test strips. The drug combination was found to act synergistically in 29 of the tested isolates. Antagonism was noted in 11 while the remaining 60 neither showed synergy nor antagonism. This is a pilot study conducted with a small sample size. Though the results do not favor the drug combination, more number of samples should be tested to reiterate the findings. Also, the performance of the fixed ratio method of in vitro synergy testing should be validated with the gold standard time-kill assay