Dissemination of the mcr-1 colistin resistance gene among pigs: An experimental study

Colistin resistance in Enterobacteriaceae is a public health problem. The present study was designed to evaluate the dissemination of a colistin-resistant Escherichia coli strain and its resistance gene, mcr-1, between orally inoculated pigs and their contacts. A non-inoculated control group, one low-dose and one high-dose group-both including two pens of two inoculated and three contact pigs-were raised in separate rooms. After inoculation of a colistin- and rifampicin-resistant E. coli suspension (2.5 × 10⁵ CFU/pig for the low-dose group and 2.5 × 10⁸ CFU/pig for the high-dose group), feces from inoculated and non-inoculated contact pigs were collected and inoculated on colistin- and rifampicin-supplemented media directly or after enrichment in rifampicin-supplemented media, then the isolates were characterized. PCR was used to detect the mcr-1 gene in lysates from feces cultivated in colistin-supplemented broth and DNA prepared from feces. Results showed that the low-dose inoculum was probably insufficient to obtain durable colonization, but could lead to the temporary presence of mcr-1-positive E. coli strains. The high-dose inoculum resulted in durable colonization of both inoculated and contact animals. In all groups, the mcr-1 gene was also detected in rifampicin-susceptible strains, suggesting its transfer to several commensal strains. A comparison of detection methods showed that more positive samples were obtained with cultures in rifampicin-supplemented media and suggests that current methods to evaluate the prevalence of colistin resistance in fecal samples suffer from poor sensitivity.