Hospitals have strict hygiene and sanitation protocols to protect patients from bacteria that rarely sicken healthy people but can be deadly for vulnerable patients already hospitalized with serious illnesses. Nearly 100,000 people die every year in U.S. hospitals of infections they develop after being admitted. But despite intense infection-control efforts, new strains of bacteria keep on emerging, seemingly out of nowhere, to sicken people in hospitals worldwide.

Researchers at Washington University School of Medicine in St. Louis have found evidence pointing to an unexpected source of such bacteria: the hospitalized patients themselves. Studying mice, the researchers discovered that urinary tract infections (UTIs) can arise after sterile tubes, called catheters, are inserted into the urinary tract, even when no bacteria are detectable in the bladder beforehand. Such tubes are commonly used in hospitals to empty the bladders of people undergoing surgery. In the mice, inserting the tubes activated dormant Acinetobacter baumannii (A. baumannii)bacteria hidden in bladder cells, triggering them to emerge, multiply and cause UTIs, the researchers said.

The findings, published on Jan. 11 in Science Translational Medicine, suggest that screening patients for hidden reservoirs of dangerous bacteria could supplement infection-control efforts and help prevent deadly infections.

Once the infections had resolved and no bacteria were detected in the mice's urine for two months, the researchers inserted catheters into the mice's urinary tracts with a sterile technique. Within 24 hours, about half of the mice developed UTIs caused by the same strain of A. baumannii as the initial infection.

"The bacteria must have been there all along, hiding inside bladder cells until the catheter was introduced," Hultgren said. "Catheterization induces inflammation, and inflammation causes the reservoir to activate, and the infection blooms."

Since A. baumannii rarely causes symptoms in otherwise healthy people, many people who carry the bacteria may never know they're infected, the researchers said. As part of this study, the researchers searched the scientific literature and discovered that about 2% of healthy people carry A. baumannii in their urine.

"I wouldn't put much weight on the precise percentage, but I think we can say with certainty that some percentage of the population is walking around with A. baumannii," Feldman said. "As long as they're basically healthy, it doesn't cause any problems, but once they're hospitalized, it's a different matter. This changes how we think about infection control. We can start considering how to check if patients already have Acinetobacter before they receive certain types of treatment; how we can get rid of it; and if other bacteria that cause deadly outbreaks in hospitals, such as Klebsiella, hide in the body in the same way. That's what we're working on figuring out now."