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 beta-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. Copyright (C) 2013, Taiwan Society of Microbiology. Published by Elsevier Taiwan LLC. All rights reserved.