A new study suggests many so-called hospital-acquired infections actually arrive with the patient.

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The most dangerous germs in the hospital may be those you bring with you

Hospitals may look squeaky clean, but microbiologists know better. Staphylococcus aureus sits on the doctor’s pager, Corynebacterium striatum lives on the sink faucet, and Enterococcus faecalis hangs out on the bedrail—all threatening the health of patients. But a new paper suggests that the most dangerous bacteria aren’t the ones you encounter in the hospital, but those you bring in yourself.

“People have suspected that hospital-associated infection pathogens might be coming in with patients, but this is the first environmental verification of that,” says Jordan Peccia, an environmental engineer at Yale University who studies disease transmission in buildings and was not involved with the study.

Tracking roving microbes is difficult if you don’t know where they came from in the first place. So in the new study, a team led by microbial ecologist Jack Gilbert of the University of Chicago in Illinois embedded itself in a brand new hospital—the university’s Center for Care and Discovery, which opened in 2013. Months before the opening, the researchers talked extensively to the 10-story hospital’s architects, doctors, and nurses to map out where staff and patients would enter and exit, which restrooms and vending machines they would use, and where they might come into contact with other people. “It was remarkable to have access to this kind of environment as it was being built,” Gilbert says. “It took months and months to get administrative approval.” They also swabbed surfaces all over the hospital right before it opened, and found essentially a pristine environment.

That didn’t last long. During the first day of operation, there was a sharp uptick in the numbers of Corynebacterium, Staphylococcus, and Streptococcus on phones, countertops, and computer mice in nurse stations. These bacteria are responsible for infections like diphtheria, Methicillin-resistant Staphylococcus aureus, and pneumonia—diseases especially dangerous to people with weakened immune systems.

For the next 365 days, Gilbert and his colleagues swabbed work surfaces in many rooms, as well as the skin and clothing of doctors, nurses, hospital staff, and patients—a total of 6523 samples. “It was a bit awkward,” Gilbert recalls. “People would have just arrived for treatment, and we’d say, ‘Excuse me, can I swab your armpit, nose and hand?’”

The team compared patients’ microbial profile at their arrival and when they were discharged, and applied statistical methods to determine what had influenced their microbial makeup over the course of their stay. Bedrails jumped out as a significant potential reservoir of bacteria; the microbes living on them resembled those found on the patients more than those on any other sampled material did. That suggests bedrails are capable of picking up a person’s bacteria and harboring it until the bed’s next occupant comes along—unless the bed is well sterilized in between.

Of the 252 patients who participated in the study, about 20 came down with what experts would define as a hospital-acquired infection, the researchers report today in Science Translational Medicine. That’s close to stats from the Centers for Disease Control and Prevention, which has found that about one in 25 hospitalized patients will get such infections. But room swabs of the patient in the new study, as well as the nurses and doctors they interacted with, failed to turn up the bacteria responsible for their infections, suggesting they don’t appear to have gotten their microbes from the hospital at all, according to the team’s analysis. “The most likely explanation is the patients already had those bacteria when they were admitted,” Gilbert says.

That would be a big shift away from current thinking in much of the medical world, which considers hospitals to be harbors of potentially dangerous diseases. Instead, the study suggests hospital rooms’ microbes might not be any more dangerous than those in patients’ own homes. “If it’s true,” says Gilbert, “that’s going to be a paradigm shift in how we think of hospital-acquired infections.”

It’s nearly impossible to totally sterilize any room, he says, and most hospitals are already doing a pretty good job keeping things as disease-free as possible. Rather than focusing on ever more potent sterilization efforts, more resources should be spent treating people with probiotics and antibiotics before they enter a hospital when possible, he says.