Synergistic Activity of Ceragenins Against Carbapenem-Resistant Acinetobacter baumannii Strains in Both Checkerboard and Dynamic Time-Kill Assays

Acinetobacter baumannii is an emerging opportunistic pathogen that primarily infects critically ill patients in nosocomial settings and there is a need for identifying new alternative therapeutic agents against these organisms. Ceragenins are non-peptide, membrane-active agents that mimic the antimicrobial properties of antimicrobial peptides (AMPs) and affect the membrane permeability of microorganisms. The in vitro activities of CSA-8, CSA-13, CSA-44, CSA-131, CSA-138 either alone or in combination with colistin (sulphate) were determined against 25 carbapenem-resistant A. baumannii strains. Minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of selected ceragenins and colistin against these isolates were measured by in vitro microbroth dilution techniques. Checkerboard techniques and time-kill assays were performed to determine the activities of combinations. The MIC(50) values (mg/L) of CSA-8, CSA-13, CSA-44, CSA-131, CSA-138 and colistin were 32, 4, 8, 2, 4 and 0.5, respectively. The MIC(90) (mg/L) of CSA-8, CSA-13, CSA-44, CSA-131, CSA-138 and colistin were 128, 8, 16, 8, 16 and 16, respectively. At 6 h, 1×MIC and 2×MIC of CSA-13 were bactericidal. CSA-13 + colistin combination displayed synergistic interaction. Antagonism between antimicrobials was not observed. According to the results, CSA-13 and CSA-131 can be good alternatives for infections caused by carbapenem-resistant A. baumannii.