Conjugative transfer of multi-drug resistance genetic elements from environmental water borne bacteria to Escherichia coli

Watersheds contaminated with municipal, hospital and agricultural residues have been recognized as reservoirs for bacteria carrying antibiotic resistance genes (ARGs). Still, there is limited information about the ability of complex bacterial communities from contaminated environments to transfer antibiotic resistance determinants toimportant waterborne pathogens such as Escherichia coli.. The objective of this study was to determine the potential of environmental bacterial communities from the highly contaminated La Paz River in Bolivia, to transfer multi-drug resistance to an E. coli lab strain used as recipient. Additionally, we tested ZnSO4 and CuSO4 at sub-inhibitory concentrations as stressors and analyzed transfer frequencies (TFs), diversity, richness and acquired unique phenotypic multi-drug resistance profiles (MDRPs). The investigated bacterial communities where collected from surface water at an urban site close to a hospital and a site near an agricultural area where the water is used for irrigation. At both sites, high transfer frequencies of antibiotic resistance plasmids were identified. We found a significant difference in number of transconjugants and TFs between the urban and agricultural sites. Whole genome sequencing revealed that four putative plasmids were predominant among the transconjugants, carrying a wide range of antibiotic resistance genes, largely explaining their acquired phenotypic resistance patterns. Addition of ZnSO4 and CuSO4 at sub-inhibitory concentrations slightly increased the diversity of acquired MDRPs from the agricultural site, but had no effect on the transfer rate. Metal resistance genes were absent from the great majority of transconjugants, together suggesting a minor role, if any, of metals in the spread of multidrug-resistant plasmids at the investigated sites.