Hospital wastewater may do more than carry antibiotic resistant bacteria from patients. A new Swedish study published in Nature Communications found that the wastewater itself can create conditions favoring resistant strains, raising concern that hospital sewer systems could become hotspots for antimicrobial resistance.

Antibiotics taken by patients can pass through their bodies and enter hospital sewers. Once there, the drugs may create an environment that helps bacteria capable of surviving antibiotic treatment grow more successfully.

Antibiotic resistance occurs when bacteria develop the ability to survive drugs intended to kill them, making some infections harder to treat. More than 9.3 million drug resistant infections occur in the U.S. each year, causing more than 39,000 deaths, according to the CDC's 2026 Antimicrobial Resistance Threats Report.

Researchers examined wastewater from 10 hospitals and eight municipal wastewater treatment plants. Their goal was to determine whether hospital wastewater actively promoted resistant bacteria rather than merely carrying bacteria that had already developed resistance inside patients.

The team first filtered the hospital wastewater to remove bacteria already present. Researchers then exposed other bacteria to the filtered water during laboratory experiments, allowing them to examine whether substances remaining in the wastewater affected bacterial growth.

All 10 hospital wastewater samples significantly favored the growth of E. coli strains resistant to ciprofloxacin and cefotaxime. Both drugs are antibiotics used to treat bacterial infections, while none of the eight municipal wastewater samples produced the same effect.

The results suggested that something within hospital wastewater was helping resistant bacteria thrive. The stark difference between the hospital and municipal samples also directed researchers toward conditions specific to wastewater produced by hospitals.

Researchers next placed plastic plates inside sewer pipes to examine bacteria growing on the surfaces. After 30 days, bacteria on surfaces inside hospital pipes were about three times as likely to resist the two antibiotics as bacteria found in the surrounding wastewater.

That pattern did not appear in municipal sewers. Researchers also measured antibiotic levels in wastewater and tested whether the same concentrations promoted resistant bacteria during laboratory experiments.

Ciprofloxacin emerged as a major contributor, while cefotaxime also played a role. Joakim Larsson, professor of environmental pharmacology at the University of Gothenburg and Director of Centre for Antibiotic Resistance Research, described the finding as a warning.

"The discovery [suggests] that hospital sewers could be a spawning ground for bacteria that are difficult to treat with antibiotics," Larsson told Fox News Digital. He added, "The patients’ need for antibiotics must not be compromised."

Larsson said identifying where resistant bacteria are promoted, and where they are less likely to develop, is important when deciding how to direct resources intended to limit antibiotic resistance. The findings, however, came with several limitations.

All sampling locations were in Sweden, and researchers primarily examined E. coli and a limited group of antibiotics. The results therefore may not apply to every type of bacteria or to hospital and municipal wastewater systems in other locations.

The sewer experiment lasted approximately 30 days, leaving longer term effects unclear. The study also did not establish whether resistant bacteria growing in hospital sewers cause additional infections among patients or people in surrounding communities.

Although researchers did not directly link hospital wastewater to human infections, Larsson said resistant bacteria could potentially reach people through contaminated water. "When it rains heavily, treatment plants are typically overloaded, and release untreated or partially untreated wastewater with lots of bacteria," he told Fox News Digital.

"If people get in contact with such waters, they can acquire those bacteria and then continue spreading them from person to person," Larsson said. "One thing that remains uncertain is how much you need to be exposed for resistant bacteria to colonize your gut or infect you."

Researchers identified two possible approaches for reducing antibiotic resistance in hospital sewers. One would involve using alternative antibiotics that are less likely to pass through patients unchanged and encourage resistant bacteria in wastewater, although equally effective alternatives may not always be available.

"But – and it is an important but – the patients’ need for antibiotics must not be compromised," Larsson said. Patients’ medical needs must remain the priority when doctors choose treatments.

The second approach would treat hospital wastewater to remove antibiotics that promote resistance, particularly fluoroquinolones and beta-lactams, while also removing harmful bacteria. Larsson said treatment at municipal plants would probably be less effective because antibiotic concentrations favoring resistance are primarily found in hospital sewers.

Technologies capable of removing antibiotics from wastewater already exist, but Larsson noted that they can be costly and require additional treatment. "When we know more about the drivers, we think it might be possible to develop technical solutions that are targeted and hopefully less expensive," he said.