Screening for fecal carriage of MCR-producing Enterobacteriaceae in healthy humans and primary care patients

BACKGROUND: The extent of the occurrence of the plasmid-encoded colistin resistance genes mcr-1 and mcr-2 among humans is currently sparsely studied in Western Europe. OBJECTIVES: To determine the occurrence of MCR-producing Enterobacteriaceae in fecal samples of healthy humans with high occupational exposure to food and primary care patients in Switzerland. METHODS: Stool samples from 1091 healthy individuals and fecal swabs from 53 primary care patients were screened for polymyxin-resistant Enterobacteriaceae using LB agar containing 4 mg/L colistin. Minimal inhibitory concentrations (MICs) of colistin were determined for non-intrinsic colistin-resistant isolates. Isolates were screened by PCR for the presence of mcr-1 and mcr-2 genes. RESULTS: The fecal carriage rate of colistin resistant (MIC value >2 mg/l) Enterobacteriaceae was 1.5% for healthy people and 3.8% for primary care patients. Isolates included Hafnia alvei (n = 9), Escherichia coli (n = 3), Enterobacter cloacae (n = 4), Klebsiella pneumoniae (n = 1) and Raoultella ornithinolytica (n = 1). None of the isolates harbored the mcr-1 or mcr-2 genes. CONCLUSIONS: There is no evidence for the presence of MCR-producers in the fecal flora of healthy people or primary care patients. Therefore, the risk of transfer of mcr genes from animals, food or the environment to humans is likely to be low in Switzerland.