Selection for antibiotic resistance in complex microbial communities

Abstract

Measureable concentrations of antibiotics are released into the environment from anthropogenic sources. Environmental risk assessment investigates the risks these concentrations pose to aquatic life but does not determine whether selection for resistance is occurring. Recent studies suggest environmental concentrations of tetracycline, ciprofloxacin and cefotaxime may be able to select for resistance in complex microbial communities. The aims of research presented in this thesis were to determine whether selection for resistance occurs at environmentally relevant concentrations of macrolide antibiotics; to understand how mixtures of antibiotics affect selective endpoints; to understand the effect of temperature on selective endpoints, and, finally, to compare the methods used with previously published methods. Selective endpoints of macrolide antibiotics were found to be 1,000 µg/L of azithromycin and erythromycin and 750 µg/L of clarithromycin which is significantly higher than current environmental concentrations. Mixing of antibiotics produces at least an additive, if not a synergistic, effect. The selective endpoints of sulfamethoxazole and trimethoprim decrease over 11 times and by a half, respectively, when used in combination. The selective endpoint of macrolides is reduced by a third when they are found in combination. Selective endpoints of individual genes are affected by temperature but, as only preliminary data has been produced, overall effect concentrations have not been determined across the entire experimental resistome. Alternative gene targets associated with taxa favoured at low temperatures may be under selection. Finally, the phenomenon of increased persistence has been described and the minimal increased persistence concentration has been defined for the first time. Data presented here can be used by policy makers in environmental risk assessments, in conjunction with other ecotoxicological endpoints to determine safe release levels of antibiotic residues in wastewater. This will help minimise selection for antibiotic resistance in the environment and, therefore, exposure of humans to resistant bacteria through environmental transmission.