Phenotypic detection of plasmid-mediated colistin resistance in Enterobacteriaceae

Abstract

The aim of this work was to evaluate an easy-to-perform assay basedupon inhibition of mobile colistin resistance (MCR) activity by EDTA. We included 92nonrelated isolates of Enterobacteriaceae (74 Escherichia coli, 17 Klebsiella pneumoniae,and 1 Serratia marcescens). Our proposed method is based on a modificationof the colistin agar-spot screening test (CAST), a plate containing 3 g/ml colistin,by adding an extra plate of colistin agar-spot supplemented with EDTA (eCAST).Bacterial growth was evaluated after 24 h of incubation at 35°C. All the colistinresistantisolates showed development on the CAST plates. Colistin-resistant K. pneumoniaewithout mcr-1 and S. marcescens also grew on the eCAST plates. In contrast,colistin-resistant MCR-producing E. coli was not able to grow in eCAST plates. Thecombined CAST/eCAST test could provide a simple and easy-to-perform method todifferentiate MCR-producing Enterobacteriaceae from those in which colistin resistanceis mediated by chromosomal mechanisms.