Hospital wastewater may create conditions that help antibiotic-resistant bacteria grow, turning sewer systems into potential hotspots for antimicrobial resistance, a study from Sweden suggests.
Antibiotics taken by patients can pass through the body and enter hospital sewers. Researchers say residues in the wastewater may encourage resistant bacteria to thrive.
Antibiotic resistance develops when bacteria can withstand drugs meant to kill them, making some infections more difficult to treat. The CDC’s 2026 Antimicrobial Resistance Threats Report estimates that the United States sees more than 9.3 million drug-resistant infections and over 39,000 related deaths each year.
To investigate whether hospital wastewater actively promotes resistance—not merely carries bacteria shed by patients—the researchers studied samples from 10 hospitals and eight municipal wastewater treatment plants.
They removed existing bacteria from the hospital wastewater, then exposed other bacteria to the filtered samples in laboratory tests. All 10 samples encouraged the growth of E. coli strains resistant to ciprofloxacin and cefotaxime, two antibiotics used to treat bacterial infections. None of the municipal wastewater samples had the same effect.
The findings suggest that substances in hospital wastewater can give resistant bacteria an advantage.
The team also placed plastic plates inside sewer pipes to observe bacteria growing on their surfaces. After 30 days, bacteria on the hospital sewer surfaces were about three times as likely to resist the two antibiotics as bacteria in the surrounding wastewater. The researchers did not see this pattern in municipal sewers.
Measurements and laboratory tests pointed to ciprofloxacin as a major contributor to the effect, with cefotaxime also playing a part.
Joakim Larsson, a professor of environmental pharmacology at the University of Gothenburg and director of its Centre for Antibiotic Resistance Research, said the results indicate that hospital sewers could provide a setting where bacteria that are difficult to treat can multiply. Understanding where resistance is encouraged, he said, may help guide efforts to reduce the risk.
The study has limitations. All the sampling took place in Sweden, and the researchers concentrated mainly on E. coli and a small selection of antibiotics. The results may not apply to other bacteria, drugs or locations.
The sewer experiment ran for about 30 days, so it is not clear what happens over longer periods. Nor did the study establish whether resistant bacteria growing in hospital sewers lead to more infections among patients or people in nearby communities.
Although the researchers did not directly connect wastewater to human illness, Larsson said people could potentially encounter resistant bacteria through contaminated water. During heavy rain, treatment plants can become overloaded and release untreated or partly treated wastewater containing bacteria. People exposed to that water may acquire the bacteria and pass them to others, he said. How much exposure is needed for bacteria to colonize the gut or cause an infection remains unknown.
The researchers outlined two possible ways to reduce the risk. One is to use alternative antibiotics that are less likely to pass through the body unchanged and encourage resistance in wastewater. Larsson cautioned, however, that patients’ treatment needs must come first, and equally effective alternatives may not always be available.
Another option is to treat hospital wastewater to remove antibiotics that may promote resistance, particularly fluoroquinolones and beta-lactams, as well as harmful bacteria. Larsson said treatment at municipal plants may be less effective because concentrations of antibiotics that favor resistant bacteria are mainly found in hospital sewers.
Technologies that remove antibiotics from wastewater already exist, but they can be expensive and require extra treatment. Larsson said a better understanding of what drives resistance could help researchers develop more targeted, and potentially less costly, solutions.
