In vitro activity of colistin sulfate against Enterobacteriaceae producing extended-spectrum β-lactamases

The widespread multidrug-resistant Enterobacteriaceae pose a serious therapeutic challenge. Colistin and tigecycline are potential antimicrobial agents for treating infections caused by extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae. We evaluated the in-vitro activity of colistin sulfate against 253 ESBL producers isolated from patients admitted to a medical center in southern Taiwan (Escherichia coli, n = 82; Klebsiella pneumoniae, n = 102; Enterobacter cloacae, n = 34; and Serratia marcescens, n = 35). Colistin showed promising in-vitro activity against E. coli, K. pneumoniae, and E. cloacae, but not S. marcescens. One ESBL-producing K. pneumoniae strain with resistance to carbapenems (ertapenem, imipenem, and meropenem) was selected for time-killing studies. A combination of colistin and tigecycline showed synergism, but there was an inoculum effect. In conclusion, colistin was active against most ESBL-producing Enterobacteriaceae, and a combination of colistin with tigecycline was synergistic against some highly resistant strains, even those with carbapenem resistance.