Municipal wastewater contains a large range of excreted antibiotics and has therefore long been suspected to be a spawning ground for antibiotic-resistant bacteria. A new study in Nature Communications led by a team from the University of Gothenburg, provides a more nuanced picture.
By testing the potential of untreated municipal wastewater from 47 countries to select for resistant E. coli, the researchers show that while some samples indeed do so, most instead suppress them.
“What we found most intriguing is the widespread disadvantages for resistant E. coli in wastewaters from most countries,” says Professor Joakim Larsson, senior author of the study and director of CARe. “This suggests that municipal wastewater treatment plants may not always be breeding grounds for resistance, as sometimes perceived.”
To understand what drives these patterns, the researchers also measured 22 antibiotics and 20 antibacterial biocides in all samples. While some antibiotics exceeded thresholds predicted to select for resistance in certain bacteria, none stood out as a clear driver of resistance in E. coli. Chemical patterns often correlated only weakly with the observed selection outcomes, suggesting that complex mixtures—or unmeasured compounds—may influence which bacteria thrive.
Why, then, does resistance often come at a disadvantage in wastewater? “Resistance often comes with a cost to the bacterium,” Joakim Larsson explains. “If there is not enough antibiotic present, the sensitive ones often grow better. It might also have something to do with particular adaptations of certain E. coli lineages to the sewage environment.”
The study validated their findings using both synthetic communities of 340 diverse E. coli strains and by using natural wastewater microbial communities, showing similar patterns of both selection and deselection. Together, these results challenge common assumptions about municipal wastewater and highlight the complexity of resistance dynamics in real-world environments.
Researchers emphasize that although some wastewater clearly can select for resistance, the widely observed suppression of resistant strains may help reduce the risk of resistance evolution and transmission in many settings.
