In vitro Synergistic Effect of Colistin in Combination with Meropenem and Rifampin against Carbapenem-Resistant E. coli and K. pneumoniae

Antibiotic resistance poses a real threat to treat infections caused by pathogenic bacteria [1]. As there are increasing numbers of reports on bacterial resistance to antibiotics especially during monotherapy, there is a renewed interest towards combination therapy [2]. Synergistic effects of various drug combinations are under study and some combinations are prescribed clinically [3,4]. The problems with existing synergy tests are; there are no exact gold-standard methods to perform the test in clinical laboratories, the available reference tests such as checkerboard and time-kill analysis are time-consuming, requires predetermined minimal inhibitory concentration (MIC) values and interpretation of results are difficult [5-7]. Combination therapy is essential at least for the treatment of patients to whom monotherapy fails. Carbapenem and colistin are considered as an antibiotic of last resort and rifampin is frequently used in combination with other antibiotics to increase its efficiency. In order to make combination therapy more precise, an in vitro synergy test methods should be more rapid and clinically dependable [5]. In this study, two different synergy tests (checkerboard and plate synergy method) were compared using the combination of colistin-meropenem and colistin-rifampin.