The overuse of antimicrobials in humans and animals is driving the emergence of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs), which spread between the One Health compartments: humans, animals, and the environment (water, air, soil). The extent to which surface waters play a role in the dissemination of ARB/ARGs is poorly understood, but recreational use and drinking water may result in human exposure. This is the first large scale survey of the prevalence of cefotaxime-resistant E. coli (CTX-EC) and ARGs in drinking water sources and final water in Scotland.

Drinking water sources with different catchments were selected to determine the effect of faecal pollution sources (human, livestock, agriculture, and wildlife) on CTX-EC and ARGs. Samples from 29 drinking water sources and three final waters (post-treatment) were analysed. Total E. coli and CTX-EC were enumerated and HTqPCR was used to quantify 56 ARGs.

CTX-EC were detected in seven drinking water sources (24%). In the majority (95%) of samples positive for CTX-EC, the proportion of CTX-EC to total E. coli was low (<2%). Logistic regression found significant associations between CTX-EC presence and two catchment variables: septic tanks and pastures. ARGs were present in 90% of drinking water sources and 44 ARGs from eight antibiotic classes were quantified; however, mean relative abundances of ARGs were low (3E-06 to 0.006 ARG copies per 16S rRNA gene). In final waters, all ARGs were below the limit of detection, and no E. coli or CTX-EC were recovered.

This study found that the overall AMR load in Scottish drinking water sources is low. CTX-EC concentrations were low, and their prevalence was around half of that found in coastal waters in Scotland. ARGs were widely detected in drinking water sources, but their relative abundances were low, and drinking water treatment processes effectively removed them.