A new study provides important insights into the spread of antibiotic resistance (ABR) in different river systems. The study, conducted by researchers from the Karl Landsteiner University of Health Sciences (KL Krems) and partner organizations, uses a comprehensive, harmonized methodology that has been applied for the first time to compare different rivers. The work is published in the Journal of Hazardous Materials.
By analysing samples from four Lower Austrian rivers at different times of the year, it was possible to compare resistance trends with unprecedented statistical certainty. At the same time, a baseline for ABR pollution in Lower Austrian rivers was established for the first time. The results show that human faecal pollution is a critical factor in the spread of resistance genes in the aquatic environment. However, the nature and extent of this pollution varies greatly between rivers, despite the generally low levels of pollution in the rivers studied. This highlights the need for targeted monitoring, for which the work of KL Krems now provides an effective methodological basis.
Surface waters play a crucial role in the spread of antibiotic resistance (ABR). Sewage treatment plants, agricultural run-off and other sources of pollution contribute significantly to the emergence of antibiotic-resistant bacteria. Despite growing awareness, previous studies have used different methodologies and have mostly focused on individual water systems. This makes it impossible to compare them. Researchers at KL Krems have now developed a harmonized and cross-river study design that allows data from different rivers to be compared.
Individual Rivers
"Our research clearly shows that the distribution of ABR in rivers varies greatly," says Prof. Alexander Kirschner, microbiologist at MedUni Vienna, Karl Landsteiner University Krems and Deputy Head of ICC Water & Health*. "This discovery was only possible because we applied a comprehensive, harmonized methodology that allows us to compare the patterns of ABR in flowing waters. In particular, we took into account the role of the water itself and of biofilms". In fact, the comparison has now shown for the first time that while pollution from human faeces generally has a strong influence, factors such as heavy metals, antibiotics, and river dynamics also correlate with ABR to varying degrees. Although possible causal relationships were not initially investigated.
This observation was based on data from four Austrian rivers (Danube, Kamp, Ybbs, Gölsen/Traisen) at five time points in one year. This spatial and temporal spread ensured that the interpretation of general ABR patterns was not biased by seasonal or site-specific characteristics. An extremely robust study design made the data comparable. More than 5,000 bacterial isolates were tested, over 100,000 ABR tests and more than 3,000 resistance gene analyses were performed. By combining classical cultivation techniques for the bacterium Escherichia coli (a clinically highly relevant indicator of ABR and faecal contamination) with quantitative analysis of resistance genes in the river microbiome, a uniquely detailed insight was gained. Combined with "smart" genetic faecal diagnostics, it was also possible to differentiate between human and/or agricultural sources as the cause of ABR inputs.
Volume Matters
The details showed that human faecal pollution is an important factor in the spread of ABR, especially in small, low-flow rivers. In these low-flow river systems, factors such as heavy metal contamination and antibiotic residues were also more strongly correlated than in the Danube. In an international comparison, however, the four Austrian rivers showed low to moderate ABR contamination, indicating comparatively good wastewater management and high antibiotic stewardship standards in the country.
Biofilms – Hotspots or Cold Cases?
Biofilms – slimy communities of microorganisms on the surfaces of rocks, which have been described as hotspots of ABR in some small-scale studies – showed very inconsistent patterns of ABR spread in this study, depending on the specific local situation. In general, however, the correlation with pollution parameters was rather weak compared to water.
"Our work shows that specific statements about individual resistances or resistance genes are problematic," says Prof. Andreas Farnleitner, head of the ICC at KL Krems and TU Vienna. "Trends vary from stream to stream, and small studies carry the risk of over-interpretation. By combining different methods, we have now created a powerful framework for more precise, large-scale comparisons. What's more, this is timely, as an EU regulation that will come into force in 2027 will require the monitoring of ABR in large wastewater treatment plants. The research conducted by KL Krems and ICC Water & Health together with the Medical Universities of Vienna and Graz and the University of Natural Resources and Applied Life Sciences Vienna at IFA Tulln thus provides policy makers and environmental authorities with the tools they need to respond effectively to current environmental challenges.