Subsequent to its discovery, several studies have revealed a global distribution of mcr-1, with an overall higher prevalence among Escherichia coli and Salmonella enterica, and occasional occurrence in other enterobacterial species. Most mcr-1-positive strains were of animal origin, and farm animals were identified as the principal reservoir of mcr-1 genes [1,4]. Investigation of archival strains dated the presence of mcr-1 back to at least the 1980s . As with other resistance genes, minor allelic variants of mcr-1 have been detected . More recently, additional transferable mcr genes (mcr-2, mcr-3, mcr-4, mcr-5, mcr-6, mcr-7 and mcr-8) have been reported, for which the global epidemiology remains to be clarified [7–13].
In South America, mcr-1 genes have been reported from several countries in isolates from humans, animals and food [14–27]. Recently, the Pan American Health Organisation (PAHO) section of the World Health Organization (WHO) recommended to implement and strengthen surveillance and epidemiological investigation of plasmid-mediated transferable colistin resistance in its Member States . In Bolivia, mcr-1 has thus far been reported in a braakii that was isolated from a ready-to-eat food sample , as well as in a few clinical isolates of E. coli referred from various departments to the National Institute of Health Laboratories (INLASA) (data not shown).
During the last two decades we carried out several surveillance studies in the Bolivian Chaco region, documenting a high prevalence of resistance to old and more recent antibiotics in commensal and pathogenic bacteria from humans [21,28–32].
In 2016, a new surveillance study was carried out in a population of healthy school children from several rural communities in this region to investigate the prevalence of intestinal parasites and the carriage of antibiotic-resistant bacteria. Here we report about an unexpected and high rate of faecal carriage of mcr-1-positive Enterobacterales in this population.