For years, researchers have tracked the source of hospital outbreaks back to superbug-splashing sinks. For instance, researchers found that an outbreak in a Canadian hospital that spanned the years 2004 to 2006 was caused by multidrug-resistant Pseudomonas aeruginosa breeding and splashing out of the drains of hand-wash sinks in patient areas. Thirty-six patients were infected in the outbreak, 12 of whom died of their sink-spawned infection. Investigators found that with the water running, the sinks could launch deadly germs at least a meter away.

Despite the discoveries, researchers have puzzled over how sinks become superbug spreaders—and how to keep them from doing it. In the case of the Canadian hospital outbreak, no amount of cleaning or disinfectants fixed the problem. The hospital only ended the outbreak by renovating the sinks so they weren’t so splashy.

Now, with a new study published in Applied and Environmental Microbiology, researchers may finally have an answer to superbugs’ sink-dwelling skills: they survive in P-traps and can quickly climb pipes. More specifically, researchers at the University of Virginia found that bacteria can happily colonize a sink’s P-trap and then sneak back up the pipe and into the drain by forming a protective, creeping film, called a biofilm, on the plumbing. Once they get to the drain, they only need a burst of water to scatter up into the sink and surrounding, touchable surfaces.

In their experiments, which used sinks modeled after the ones commonly found in UVA’s medical center, the researchers found that bacteria could launch up to about three-quarters of a meter.

“This work helps to more clearly define the mechanism and risk of transmission from a wastewater source to hospitalized patients in a world with increasingly antibiotic resistant bacteria, which can thrive in wastewater environments and cause infections in vulnerable patients,” the authors, led by molecular epidemiologist Amy Mathers, concluded.

For the experiment, the researchers engineered a mock sink station—five hospital-like sinks set side-by-side, but separated by 0.6-meter-high Plexiglas barriers. They sterilized all of the individual plumbing pieces, sinks, and surrounding area. Earlier work had found that bacteria could dwell in P-traps and mysteriously reappear in sinks. So, the researchers tried inoculating a sterilized P-trap (on sink five) with E. coli that had been engineered to glow green. With the fluorescent tag, the researchers could easily identify them as original P-trap dwellers if they moved around in the sink setup.

With just water, the E. coli stuck around in the P-trap but didn’t move up. But, more than just water goes down hospital sinks—bodily fluids, discarded beverages, feeding supplements, and other fluids get poured down, too. To mimic this, the researchers added some fluid nutrients on a daily basis over seven days.

With the food, the bacteria thrived. They formed a biofilm in the P-trap and started moving—2.5 centimeters upward each day. By day seven, they had made it to the drain.

Next, the researchers set up petri dishes full of nutrient-laden agar around the sinks. Any pipe-climbing germs that got scattered and landed on a plate would grow on the nutrients and form visible colonies. The researchers found that the green-glowing E. coli spattered up to .76 meters away from the drain and onto touchable surfaces.

But, the bacteria didn’t just move up—they moved down, too. The researchers found that after the seven-day experiment, the germs had spread to neighboring sinks. When bacteria levels were low, they spread to sinks two and three. With high levels of bacteria, all but sink one became inhabited.

Though plumbing and sink designs vary—as do the contents of what we pour down them—the researchers concluded that P-traps could be problematic to public health.

“The retained water in a sink P-trap is present to provide a water barrier to prevent off-gassing of sewer smell,” the researchers write, “but it may inadvertently provide favorable conditions for pathogenic and opportunistic antibiotic-resistant microorganisms to survive and develop resilient biofilms.”

Applied and Environmental Microbiology, 2017. DOI: 10.1128/AEM.03327-16 (About DOIs).