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On the lam. Infectious bacteria may hide inside biofilm-like pods.

Invasion of the Bladder Bugs

The microbes responsible for urinary tract infections sometimes survive onslaughts by the immune system and antibiotics, reemerging to strike again. A new study reported in Science suggests one strategy that may make them so resilient: The bacteria band together inside mouse bladder cells in pods. These newfound pods resemble biofilms, masses of interacting bacteria that are difficult to wipe out.

About 8 million urinary tract infections are diagnosed each year in the United States, predominantly in women. They cause fever, frequent and painful urination, and can seed more dangerous kidney infections. Antibiotics usually knock out the infections, but in some unlucky women, they recur every few weeks or months.

To find out how Escherichia coli, which cause most of these infections, can survive, researchers led by Scott Hultgren of Washington University School of Medicine in St. Louis started simply: They infected mice, dissected out the bladder, then looked at the bladder's surface under a microscope. Uninfected bladders appeared smooth, but infected bladders had bumps all over them. “They looked like fried eggs,” Hultgren says. Inside, these pods were crowded with bacteria. The bacteria were coated with projections called pili and suspended in a network of fibers, characteristics also found in biofilms, slimy microbial communities that grow on pacemakers, catheters, and other medical devices. But unlike known biofilms, these were growing inside human cells.

The discovery is “very exciting and may cause us to change preexisting notions” about how bladder infections develop, says urologist David Uehling of the University of Wisconsin School of Medicine in Madison, but he cautions that researchers don't yet know whether such pods occur in human bladder cells. Internist Pradeep Singh of the University of Iowa College of Medicine in Iowa City, who studies lung biofilms that plague children with cystic fibrosis, adds that the results suggest that biofilms could be important in other infections thought to be caused by free-swimming bacteria, such as pneumonia and bloodstream infections, and perhaps in diseases such as tuberculosis in which bacteria grow inside human cells.