High prevalence of carriage of mcr-1-positive enteric bacteria among healthy children from rural communities in the Chaco region, Bolivia, September to October 2016

BackgroundThe <i>mcr-1</i> gene is a transferable resistance determinant against colistin, a last-resort antimicrobial for infections caused by multi-resistant Gram-negatives.AimTo study carriage of antibiotic-resistant bacteria in healthy school children as part of a helminth control and antimicrobial resistance survey in the Bolivian Chaco region.MethodsFrom September to October 2016 we collected faecal samples from healthy children in eight rural villages. Samples were screened for <i>mcr-1</i>- and <i>mcr-2</i> genes. Antimicrobial susceptibility testing was performed, and a subset of 18 isolates representative of individuals from different villages was analysed by whole genome sequencing (WGS).ResultsWe included 337 children (mean age: 9.2 years, range: 7-11; 53% females). The proportion of <i>mcr-1</i> carriers was high (38.3%) and present in all villages; only four children had previous antibiotic exposure. One or more <i>mcr-1</i>-positive isolates were recovered from 129 positive samples, yielding a total of 173 isolates (171 <i>Escherichia coli</i>, 1 <i>Citrobacter europaeus</i>, 1 <i>Enterobacter hormaechei</i>). No <i>mcr-2</i> was detected. Co-resistance to other antimicrobials varied in <i>mcr</i>-positive <i>E. coli</i>. All 171 isolates were susceptible to carbapenems and tigecycline; 41 (24.0%) were extended-spectrum β-lactamase producers and most of them (37/41) carried <i>bla</i>_CTX-M-type genes. WGS revealed heterogeneity of clonal lineages and <i>mcr</i>-genetic supports.ConclusionThis high prevalence of <i>mcr-1</i>-like carriage, in absence of professional exposure, is unexpected. Its extent at the national level should be investigated with priority. Possible causes should be studied; they may include unrestricted use of colistin in veterinary medicine and animal breeding, and importation of <i>mcr-1</i>-positive bacteria via food and animals.