Beware the hospital sink drain. A new study out this week shows that these drains can be a common source of scary, drug-resistant pathogens, even after deep cleaning.

Researchers at the University of the Balearic Islands in Spain led the study, published Friday in Frontiers in Microbiology. They analyzed bacteria collected from drains around a single, well-maintained hospital and unearthed a rich diversity of troublesome microbes, including superbugs that can resist several antibiotics at once. The findings suggest that stemming the spread of these bacteria within hospitals is even harder than expected, the researchers say.

Antibiotic resistance is one of the most pressing public health issues of our time. It’s already estimated that these infections kill at least 1.27 million people worldwide annually, including 35,000 Americans a year. And studies have projected that drug-resistant bacteria will eventually match or even surpass the current annual death toll of cancer (roughly 10 million deaths a year) in the next few decades.

Hospitals and other health care facilities are common breeding grounds of antibiotic resistance for several reasons. Antimicrobials are widely used in these places, directly fueling resistance. Patients are also in poorer health than usual, making it easier for infections to prosper and spread like wildfire between people. Given these known risks, hospitals and their employees routinely take steps to mitigate antibiotic resistance, which often includes the deep cleaning of areas where bacteria can linger, such as plumbing systems and sink drains.

The researchers wanted to test how effective these protective measures truly are. They analyzed sink drains from a single university hospital on the island of Majorca that was built in 2001. According to the researchers, the hospital routinely cleans its sinks and drains with bleach, further disinfects them with pressurized steam and chemicals every two weeks, and treats its piping with chlorine once a year.

From February 2022 to February 2023, they collected (using cotton swabs) and genetically sequenced bacterial samples from six sink drains in five wards: two from ICU units, one each for hematology, short stays, and general medicine; and a microbiology lab.

All told, they identified 67 different bacterial species from the drains. While there were some differences between the wards, all the drains contained a wide range of bacteria, including the newer ICU unit that came into operation the previous year. The researchers also isolated strains of bacteria commonly associated with serious hospital-acquired infections, including Pseudomonas aeruginosa, Klebsiella pneumoniae, and Staphylococcus aureus. And though there were ebbs and flows in the kinds of bacteria found in these drains throughout the year, the cleaning procedures used by the hospital didn’t seem to have much of an effect on their overall diversity.

“Here we show that hospital sink drains host bacterial populations that change over time, despite impeccable cleaning protocols in the particular hospital we looked at,” said senior researcher Margarita Gomila, a professor at the University of the Balearic Islands, in a statement from Frontiers, publishers of the study. “The bacteria we found may originate from many sources, from patients, medical personnel, and even the environment surrounding the hospital. Once established in sink drains, they can spread outwards, posing significant risks to immunocompromised patients above all.”

The researchers further found that most samples contained at least some bacteria that were resistant to multiple antibiotics, including strains of P. aeruginosa and K. pneumoniae. In a few bacteria, they also identified a worrying gene that promotes resistance to carbapenems, a newer class of antibiotics often used as a last resort treatment for infections that have stopped responding to other drugs.

Obviously, the researchers aren’t saying that hospitals should stop cleaning their sink drains. But given how well these bacteria are thriving even in the face of deep cleaning, it’s clear that more has to be done to stop or at least slow down the proliferation of superbugs within these risky environments.

“Cleaning protocols are important and should be frequently applied, especially in wards that are kept separate precisely to slow the spread of potentially harmful bacteria. But to get to the bottom of the problem, it’s essential to study the source of these bacteria and their routes of transmission,” said lead researcher José Laço, a PhD student in Gomila’s laboratory, in a statement.