Frequency of MCR-1-mediated colistin resistance among Escherichia coli clinical isolates obtained from patients in Canadian hospitals (CANWARD 2008-2015).

BACKGROUND: Colistin is often used as an antimicrobial of last resort for the treatment of infections caused by multidrug-resistant gram-negative bacilli. In 2015, plasmid-mediated colistin resistance in Escherichia coli due to MCR-1 was described. The purpose of this study was to evaluate the frequency of colistin resistance among E. coli clinical isolates obtained from patients in Canadian hospitals as part of the Canadian Ward Surveillance Study (CANWARD) and to determine how often the mcr-1 gene is detected among the colistin-resistant subset.
METHODS: From January 2008 to December 2015 (excluding 2011), 10 to 15 sentinel hospitals submitted consecutive clinical isolates (1 per patient per infection site) from blood (100-240), respiratory (100-150), urine (25-100) and wound (25-100) infections. We performed susceptibility testing using Clinical and Laboratory Standards Institute broth microdilution methods. Isolates that showed resistance to colistin as defined by European Committee on Antimicrobial Susceptibility Testing breakpoints (minimum inhibitory concentration ≥ 4 µg/mL) were evaluated for the mcr-1 gene by polymerase chain reaction.
RESULTS: In total, 5571 E. coli clinical isolates were obtained over the study years. Twelve isolates (0.2%) were resistant to colistin. The proportion of colistin-resistant isolates varied from 0.0% to 0.5% depending on the study year, and there was no clear trend toward increasing resistance over time. Typically the colistin-resistant isolates remained susceptible to antimicrobials from several other classes. Two colistin-resistant isolates (0.04%) were found to harbour the mcr-1 gene.
INTERPRETATION: The results suggest that colistin resistance among E. coli human clinical isolates, including resistance mediated by the mcr-1 gene, remains rare in Canada.