Population Structure and Resistance Genes in Antibiotic-Resistant Bacteria from a Remote Community with Minimal Antibiotic Exposure

Abstract

In a previous study, we detected unexpectedly high levels of acquired antibiotic resistance in commensal Escherichia coli isolates from a remote Guaraní Indian (Bolivia) community with very low levels of antibiotic exposure and limited exchanges with the exterior. Here we analyzed the structure of the resistant E. coli population from that community and the resistance mechanisms. The E. coli population (113 isolates from 72 inhabitants) showed a high degree of genetic heterogeneity, as evidenced by phylogenetic grouping (77% group A, 10% group B1, 8% group D, 5% group B2) and genotyping by randomly amplified polymorphic DNA (RAPD) analysis (44 different RAPD types). The acquired resistance genes were always of the same types as those found in antibiotic-exposed settings [bla TEM , bla PSE-1 , catI, cmlA6, tet(A), tet(B), dfrA1, dfrA7, dfrA8, dfrA17, sul1, sul2, aphA1, aadA1, aadA2, aadA5, aadB, and sat-1]. Class 1 and class 2 integrons were found in 12% and 4% of the isolates, respectively, and harbored arrays of gene cassettes similar to those already described. The cotransferability of multiple-resistance traits was observed from selected isolates and was found to be associated with resistance conjugative plasmids of the F, P, and N types. Overall, these data suggest that the resistance observed in this remote community is likely the consequence of the dissemination of resistant bacteria and resistance genes from antibiotic-exposed settings (rather than of an independent in situ selection) which involved both the clonal expansion of resistant strains and the horizontal transfer/recombination of mobile genetic elements harboring resistance genes.The notion that the global dissemination of microbial drug resistance observed in the antibiotic era is related to the selective pressure generated by the use of antibiotics in clinical and veterinary practices, animal husbandry, and agriculture is supported by studies that have clearly correlated the emergence and dissemination of resistance with the use of antibiotics (1,10,19) and by the absence of acquired resistance in clinical isolates from the preantibiotic era (13,14). Surprisingly, however, antibiotic-resistant bacteria have also recently been detected in humans and wild animals living in remote areas where antibiotic exposure has been absent or minimal (2,9,11,22,30,32), raising a question about the mechanisms of resistance spread in those settings. To the best of our knowledge, the most isolated human context thus far investigated is represented by a very remote community of Guaraní Indians in the Bolivian Chaco, where we detected high levels of acquired antibiotic resistance in commensal Escherichia coli isolates (2). In that community, exchanges with inhabitants of other areas were very limited; antibiotic exposure at the time of the study had been minimal; and locally collected rainwater was the only water source, ruling out the possibility of sustained contamination of drinking water from the exterior (2).In this work we have analyzed the population struc...

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