Colistin does not potentiate ceftazidime-avibactam killing of carbapenem-resistant enterobacteriaceae in vitro or suppress emergence of ceftazidime-avibactam resistance

Abstract

We tested ceftazidime-avibactam and colistin against 24 carbapenem-resistant Enterobacteriaceae (CRE) isolates by time-kill studies. Ceftazidime-avibactam at 0.25, 1, and 4 the MIC was bactericidal against 8%, 21%, and 88% of the isolates, respectively. Colistin (2 g/ml) was bactericidal against 83% (12 h) and 42% (24 h) of the isolates. In combination, synergy and antagonism were identified against 13% and 46% of the isolates, respectively. The combination did not suppress ceftazidime-avibactam resistance. Colistin plus ceftazidime-avibactam did not provide a benefit over ceftazidime-avibactam against most CRE isolates.