Polymyxins are cationic polypeptide antibiotics that interact with the lipopolysaccharides (LPS) and phospholipids in the outer membrane of Gram-negative bacteria [
1]. Due to the lack of novel antimicrobials, polymyxin E (colistin), once avoided because of its nephro- and neurotoxicity, has become a last-resort antimicrobial to treat life-threatening infections due to multidrug resistant (MDR) Gram-negative bacteria [
2]. However, in the agricultural sector, polymyxins are applied regularly for the treatment of gastrointestinal infections in livestock and their increased use may have promoted the emergence of colistin-resistant bacteria [
3]. Acquired resistance to polymyxins in
Enterobacteriaceae is mainly related to mutations or truncations in the genes encoding the PmrA/PmrB and PhoP/PhoQ two component systems (TCS), or to the expression of acquired
mcr-1 or
mcr-2 genes, which are plasmid-located [
4,
5]. In both cases, resistance arises through the modification of lipid A component of the outer membrane by 4-amino-4-deoxy-L-arabinose (L-Ara4N) or phosphoethanolamine (PEtN) [
1]. Plasmid-mediated colistin resistance has recently been acknowledged as a major threat to public health [
4]. There is evidence that
mcr-1 harboring
Enterobacteriaceae have been occurring globally in food-producing animals, in food and in humans for several years, often associated with other resistance genes including extended-spectrum ß-lactamases (ESBL) and carbapenemases [
6]. Further, the recently identified
mcr-2 gene, which shares 76.7% nucleotide sequence homology with
mcr-1, has been found to be more prevalent than
mcr-1 in porcine colistin resistant
Escherichia (
E.) coli isolates in Belgium [
5].
Hence, there is need for continuous surveillance of colistin resistance in
Enterobacteriaceae in order to reduce the risk to human health. The intestinal microbiota forms a major reservoir of antibiotic resistant bacteria in humans [
7], therefore asymptomatic carriage of MCR producers must be taken into account in prevention and control efforts. A recent study showed that 10% of travelers returning from India were fecal carriers of colistin resistant
Enterobacteriaceae, and had probably acquired such strains via the food chain [
8]. This study was conducted in order to (i) assess the occurrence of colistin-resistant
Enterobacteriaceae in the fecal flora of healthy people with high occupational exposure to food and of primary care patients in Switzerland during the period of June to October 2016, and (ii) determine whether any of the resistant isolates harbored
mcr-1 or
mcr-2. Because Switzerland is situated at the geographical center of Europe and represents a socioeconomic and demographic intersection of the surrounding countries, this country is ideal for observing temporal-spatial trends in the occurrence of antibiotic resistance in the population of central Europe.