Life in an unsuspected antibiotics world: River biofilms

Waterborne bacteria that naturally live in biofilms are continuously exposed to pharmaceutical residues, regu-larly released into the freshwater environment. At the source level, the discharge of antibiotics into rivers has already been repeatedly linked to the development of antimicrobial resistance. But what about biofilms away from the discharge point? Two rivers, with sites subject to dispersed contamination of medium intensity, were studied as typical representatives of high-and middle-income countries. The biofilms developed on rocks indigenous to rivers are perfectly representative of environmental exposure. Our results show that away from the hotspots, the amount of antibiotics in the biofilms studied favours the maintenance and enrichment of existing resistant strains as well as the selection of new resistant mutants, and these favourable conditions remain over a period of time. Thus, in this type of river, the environmental risk of selection pressure is not only present downstream of urbanized areas but is also possible upstream and far downstream of wastewater treatment plant discharges. Despite this, correlation analysis found no strong positive correlation between antibiotic concen-trations and the abundance of measured integrons and their corresponding resistance genes. Nevertheless, this work highlights the need to consider the risks of antibiotics beyond hotspots as well.

Automated summary

Waterborne bacteria in biofilms are continuously exposed to pharmaceutical residues in the freshwater environment. The amount of antibiotics in the biofilms away from discharge points favors the maintenance and enrichment of resistant strains and the selection of new mutants over time. This study highlights the presence of environmental selection pressure in rivers beyond urban areas and the need to consider antibiotic risks beyond hotspots.