Understanding variation of antimicrobial resistance genes in two agricultural catchments in Scotland

Antimicrobial resistance (AMR) is a global public health threat as it reduces the effectiveness of drug treatments and therefore our ability to combat infections. For the first time, this study used high temporal resolution data of antimicrobial resistance genes (ARG) in two livestock-dominated agricultural study catchments in south-west Scotland to understand ‘background’ ARG loadings in the absence of pollution point-sources such as sewage treatment works and health care facilities. Daily composite samples were collected twice a week over a 2-year period 2017–2019, resulting in 124 samples in Cessnock (22 km2) and 93 in the Mein (12 km2) catchment. We found a seasonal pattern in ARG relative abundance, with highest abundance in winter inversely related to the highest bacterial abundance in summer. This could reflect seasonal input from faecal pollution during winter high flows, indicated by a relationship between E. coli and terrestrial dissolved organic matter (DOM) sources. However, faecal indicator organisms (FIOs) ranked as less important predictors, so the seasonal pattern could be caused by microbial stress during cold periods and/or competition with susceptible bacteria during the warm periods. These natural fluctuations need to be considered when inferring potential drivers of AMR prevalence in surface waters. Therefore, in future monitoring, we recommend higher temporal resolution and longer-term monitoring (at least one year) to improve our understanding of natural seasonal variation in ARG abundance and evaluate effectiveness of mitigation measures (e.g., waste management).