Life in an unsuspected antibiotics world: River biofilms.

Waterborne bacteria that naturally live in biofilms are continuously exposed to pharmaceutical residues, regularly 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 concentrations 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.

River Biofilms Microbiome and Resistome Responses to Wastewater Treatment Plant Effluents Containing Antibiotics.

Continuous exposure to low concentrations of antibiotics (sub-minimal inhibitory concentration: sub-MIC) is thought to lead to the development of antimicrobial resistance (AMR) in the environmental microbiota. However, the relationship between antibiotic exposure and resistance selection in environmental bacterial communities is still poorly understood and unproven. Therefore, we measured the concentration of twenty antibiotics, resistome quality, and analyzed the taxonomic composition of microorganisms in river biofilms collected upstream (UPS) and downstream (DWS) (at the point of discharge) from the wastewater treatment plant (WWTP) of Poitiers (France). The results of statistical analysis showed that the antibiotic content, resistome, and microbiome composition in biofilms collected UPS were statistically different from that collected DWS. According to Procrustes analysis, microbial community composition and antibiotics content may be determinants of antibiotic resistance genes (ARGs) composition in samples collected DWS. However, network analysis showed that the occurrence and concentration of antibiotics measured in biofilms did not correlate with the occurrence and abundance of antibiotic resistance genes and mobile genetic elements. In addition, network analysis suggested patterns of co-occurrence between several ARGs and three classes of bacteria/algae: Bacteroidetes incertae sedis, Cyanobacteria/Chloroplast, and Nitrospira, in biofilm collected UPS. The absence of a direct effect of antibiotics on the selection of resistance genes in the collected samples suggests that the emergence of antibiotic resistance is probably not only due to the presence of antibiotics but is a more complex process involving the cumulative effect of the interaction between the bacterial communities (biotic) and the abiotic matrix. Nevertheless, this study confirms that WWTP is an important reservoir of various ARGs, and additional efforts and legislation with clearly defined concentration limits for antibiotics and resistance determinants in WWTP effluents are needed to prevent their spread and persistence in the environment.

Roof runoff contamination: Establishing material-pollutant relationships and material benchmarking based on laboratory leaching tests.

Because roofs represent a major part of the urban impervious surface, it is hypothesized that roof runoff is an important source of urban stormwater contamination. However, the contribution of different roofing materials to this contamination has only been examined to a limited extent. In this study, a resource and time efficient methodology, which uses some of the principles of a standardized leaching test (CEN/TS16637-2), was developed to identify material-pollutant relationships for sixteen commonly used roofing materials (EPDM, PVC, TPO, EVA, PU and bitumen membranes). Metals were detected in concentrations ranging from several µg/L in the leachate of synthetic materials up to 2.5 mg/L for Zn in the leachate of EPDM materials. Cd and Cr were not detected in any of the leachates. Furthermore, polycyclic aromatic hydrocarbons were detected in most leachates, with phenanthrene and naphthalene being most frequently detected in concentrations up to 4.5 µg/L for naphthalene. Further insights on organic pollutants' leaching from the tested materials were obtained by a non-target GC-MS screening of the leachates. Several commonly used additives such as flame retardants and light stabilizers were detected. Although no information on long-term leaching and material behavior under outdoor conditions could be obtained by the developed methodology, the laboratory test results could be used to benchmark the materials for their potential impact on roof runoff quality by the calculation of material indexes (which summarize the material-pollutant relationships). EPDM and PU roofing materials were identified as the materials having the highest potential to affect roof runoff quality.