Biomarkers for monitoring antibiotic resistance in aquatic environments

The occurrence of antimicrobial resistance raises concerns as a human health threat that can be propagated through the environment. Wastewater discharge into the environment is an important source for antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs). Sewage collection and urban wastewater treatment plants (UWTPs) are major barriers that reduce environmental contamination by sewage-derived pathogens and nutrients. However, the continuous discharge of ARB and ARGs through wastewater, including when well-functioning UWTPs are available, is unavoidable. Regular and integrated antibiotic resistance monitoring in both wastewater and receiving water bodies would contribute to improve control measures. However, monitoring processes are not harmonized being the choice of suitable biomarkers a first limitation. In this study, we tested 10 selected potential antibiotic resistance biomarkers, which have been described has being associated to humans, and rare in clean environments - intI1, sul1, ermB, ermF, aph(3’’)-Ib, qacEΔ1, uidA, mefC, tetX and crAssphage. The public database MGnify (https://www.ebi.ac.uk/metagenomics/; hosted by EMBL-EBI), was screened using the filters corresponding to origin - human gut, wastewater, sewage, and fresh water. These biomarkers and the 16S rRNA gene were monitored by quantitative PCR (qPCR) tested in raw wastewater, activated sludge, treated wastewater and surface water (upstream and downstream the UWTP) samples, collected from different countries (Portugal, Czech Republic, Denmark, The Netherlands, and Israel).
The abundance of the 10 potential biomarkers decreased on average by up to 2.5 log-units gene copies/mL of sample from raw wastewater to surface water, due to treatment and/or dilution in surface water. A clustering analysis of samples based on biomarkers abundance, grouped the samples according to the (waste)water type. This classification was confirmed when 12 anonymous (waste)water samples were analysed in a blind test.
The tested biomarkers were observed to differentiate different types of sample, permitting the assessment of wastewater treatment efficiency or of impacts of UWTPs discharge or others in aquatic environments. The selection of suitable biomarkers that can typify different water sources and levels of ARG contamination, along with harmonized qPCR procedures, can facilitate regular and integrated legal requirements to antibiotic resistance monitoring in wastewater and related aquatic environments.