Colistin-Resistant mcr-1-Positive Pathogenic Escherichia coli in Swine, Japan, 2007-2014.

Abstract

To the Editor: Colistin is an old-generation antimicrobial agent; however, because it is one of the few agents that remain effective against multidrug-resistant gram-negative bacteria (e.g., carbapenem-resistant Pseudomonas aeruginosa and Enterobacteriaceae), its clinical usefulness is being increasingly recognized (1). Previous reports have described the mechanisms of colistin resistance (2) as being chromosomally mediated and not associated with horizontal gene transfer. However, from 2011 through 2014, a plasmid-encoded colistin-resistance gene, mcr-1, was identified in colistin-resistant Escherichia coli isolated in China, particularly from animals. Specifically, mcr-1–positive isolates were found in 21% of healthy swine at slaughter, 15% of marketed pork and chicken meat, and 1% of hospitalized human patients (3). A study of E. coli isolated from healthy cattle, swine, and chickens in Japan during 2000–2014 found only 2 (0.02%) of 9,308 isolates positive for mcr-1 (4). We report the rates at which mcr-1 was detected in our stored collection of E. coli isolates from diseased swine (swine with diarrhea or edema disease), hereafter referred to as swine-pathogenic E. coli.