New research shows WWTPs are not always a breeding ground for antibiotic resistance

A global study led by researchers at the Centre for Antibiotic Resistance Research (CARe) in Gothenburg in Sweden shows that municipal wastewater is not always the breeding ground for antibiotic resistance that it is often assumed to be. By testing wastewater from 47 countries, the researchers found that in some samples resistant E. coli could take over, but in most wastewaters non-resistant E coli were more competitive. These results may adds to our understanding of when and where resistance is likely to develop and spread.

Municipal Wastewater Treatment Plants (WWTPs) contains large amounts of excreted antibiotics and has therefore long been suspected to be an environment where antibiotic-resistant bacteria can arise and spread. Now a study published in Nature Communications, led by a team from the University of Gothenburg, provides a more nuanced picture. By examining how untreated municipal wastewater from 47 countries affects the selection of resistant E. coli, the researchers show that although some samples can indeed drive resistance, most instead suppress it.

Disadvantages for resistant E.coli

“The most fascinating thing we found is the widespread disadvantages for resistant E. coli in wastewater from most countries. This suggests that municipal wastewater treatment plants are not always breeding grounds for resistance, as they are sometimes portrayed,” says Professor Joakim Larsson, senior author of the study and director of CARe, in a press release.

Driver for resistance in E.coli remains unidentified

The researchers also measured 22 different antibiotics and 20 antibacterial biocides in all samples. Although some antibiotics exceeded levels predicted to be able to select for resistance in certain bacteria, no individual substance could be identified as a clear driver of resistance in E. coli. The chemical profiles often correlated only weakly with the observed selection effects, which according to the researchers indicates that complex mixtures, or substances not included in the analyses, may influence which bacteria are favoured. “Resistance often comes at a cost for the bacterium,” explains Joakim Larsson. “If there is not enough antibiotic present, the susceptible bacteria often grow better. It may also be due to specific adaptations in certain E. coli to the wastewater environment.”

Antibiotic resistance dynamics are complex

The study confirmed its findings both through experiments with synthetic communities consisting of 340 different E. coli strains and through the use of natural microbial communities from wastewater. Both methods showed similar patterns of selection and deselection. Together, the results challenge common assumptions about municipal wastewater and highlight the complexity of resistance dynamics in real-world environments. The researchers emphasize that while some wastewater samples clearly can select for resistance, the broadly observed suppression of resistant bacteria may help reduce the risk that resistance develops and spreads in many contexts.

AI study on antibiotic resistance

Research in this field is currently intense. In April the Swedish mediaplatform Cirkulation reported on another study from CARe led by researchers from the Chalmers University of Technology, University of Gothenburg, and the Fraunhofer Chalmers Centre. The scientists used an AI model to gain new insights into where resistance emerges. That study, also published in Nature Communications, showed in which environments resistance genes are likely to be transferred between different bacteria, and what makes some bacteria more prone than others to exchange genes. “We see that bacteria found in humans and in wastewater treatment plants have a higher likelihood of becoming resistant through gene transfer,” one of the researchers said at the time.

This article was published first by the Swedish platform for water professionals cirkulation.se