Plasmid Mediated Colistin Resistance in Food Animal Intestinal Contents Detected by Selective Enrichment

Introduction: Colistin (polymyxin E) is a cationic polypeptide antibiotic that has broad-spectrum activity against Gram negative bacteria. It is classified as critically important in human medicine for treating hard-to-treat multi-drug resistant infections. Recently, a plasmid-mediated colistin resistance gene (mcr-1) was discovered in China followed by global reports of mcr-1 genes in bacterial isolates from humans and livestock sources; however, information on prevalence of mcr-1 in the U.S. was lacking. The presence of colistin resistance on a plasmid greatly increases the likelihood that mcr-1 will be horizontally transferred among a broad spectrum of bacteria, including pathogens. Dissemination of mcr-1 can potentially limit the lifespan of this critically important antibiotic.

Purpose: This study was conducted to determine the prevalence of plasmid-mediated colistin resistance in members of Enterobacteriaceae associated with U.S. food animal production.

Methods:  Cecal (intestinal content) samples (n=2003) from swine, turkey, chicken, and beef cattle were selectively enriched in buffered peptone water with colistin (2 ug/mL) with overnight incubations and plated onto MacConkey’s agar with colistin (2 ug/mL). Isolates were screened by PCR for the presence of the mcr-1 gene.

Results: Two Escherichia coli isolates from two samples of swine intestinal contents were positive for the mcr-1 gene, putting the overall prevalence of plasmid mediated colistin resistance in food animals at slaughter in the U.S. at 0.1%. One E. coli isolate was resistant to four other drugs (ampicillin, streptomycin, sulfisoxazole, and tetracycline) and the second isolate was only resistant to colistin. Both the strains of E. coli with the mcr-1 gene are not commonly associated with human disease.

Significance: This study demonstrated that plasmid-mediated colistin resistance is rare in animals at slaughter in the U.S. and suggested that the potential for widespread dissemination of the mcr-1 gene in this country, via food animal production, may be limited.