Antibiotics continue to drive resistance to bacteria, even after they are broken down in wastewater treatment plants and discharged into rivers and seas, new research published on World Oceans Day has shown for the first time.

When we take antibiotics, around 90 per cent of the drugs passed through our bodies go into toilets and ends up in sewage. Previous research has shown that even in our waterways, the drugs may continue to drive bacteria to become resistant, and these bacteria can be ingested by river and sea swimmers and surfers. It contributes to the global issue of antibiotics failing to treat infection, a growing problem already a factor in five million deaths a year globally.

Now, for the first time, a study by the Centre’s microbiology team and The University of Queensland has looked at antibiotic transformation products – the metabolites which form when antibiotics break down in the environment, including in wastewater treatment plants.

Published in Nature Water, the study found that some of these metabolites drive resistance just as much as the antibiotic from which they originate. In some cases, these transformation products triggered resistance at the same concentration as the original antibiotics. This suggests that even after antibiotics break down, their by-products can still retain the ability to promote antibiotic resistance in bacteria.

Researchers are now calling for new risk assessments to account for the impact of metabolites, and identify treatment processes which could stop this resistance developing further. Pooja Lakhey, of The University of Queensland, who co-led the paper, said: “Antibiotic resistance is a major global health threat, labelled the ‘silent pandemic’ and already contributing to around five million deaths a year. We urgently need to address all avenues to reduce the issue of bacteria developing resistance to the drugs we use to treat them.

“Wastewater treatment is widely considered to reduce antibiotic concentrations and biological activity, although not necessarily eliminate them. Our research shows that wastewater treatments plants can act as hidden reservoirs of bioactivity, with both antibiotics and metabolites contributing to driving resistance.”

In the current study, lab experiments showed that bacterial communities in wastewater samples collected from both Queensland and Cornwall developed resistance when they were exposed to metabolites from three different classes of antibiotics.

Previous research by The University of Queensland measured the concentration of around 100 antibiotics and metabolites in 50 wastewater treatment plants across Australia. They found high concentrations of some of these metabolites in water systems. Some treatment plants were removing higher levels of the compounds than others, , indicating that wastewater treatment plants may provide solutions to the problem.

Dr Aimee Murray, senior lecturer of microbiology at the European Centre for Environment & Human Health, said: “We urgently need to reduce the number of resistant bacteria in our waterways, so that there’s less exposure and risk for people swimming in our rivers and seas. We are recommending an approach to risk assessment which takes into account the impact of both the original chemical, such as an antibiotic, and the component parts into which it breaks down where these continue to drive resistance. We need to move towards treatment processes which can reduce the danger of all these elements in driving resistance.”

Dr Jake O’Brien from the University of Queensland, said antibiotics and their breakdown products were not commonly tested in wastewater or environmental monitoring programs, “water utilities are not the reason for antimicrobial resistance but unfortunately are stuck dealing with whatever happens to get flushed,’’ he said.

“They’re pivotal for giving us samples and many are interested in trying to deal with problems once we’ve identified them. The real issues are that humans use too many antibiotics and their breakdown products continue to drive antibiotic resistance.’’

The study was funded by the UQ-Exeter Institute, a partnership between the universities of Exeter and Queensland and the Natural Environment Research Council. The study is titled ‘Antibiotic Transformation Products Exert Selective Pressure for Antimicrobial Resistance Comparable to Parent Compounds’, and is published in Nature Water here doi.org/10.1038/s44221-026-00663-4.