About half of American women experience bladder infections at least once during their lifetimes and as many as 10 percent suffer three to five infections per year. Antibiotics used to treat these infections may have little effect against bacteria that are hiding in other cells, said co-researcher Soman Abraham, Ph.D., assistant professor of pathology and of molecular microbiology. Now that physicians are on to E. coli's secret, they may be able to erase more bladder infections by using antibiotics that more thoroughly penetrate cells, he explained. E. coli usually lives harmlessly in the intestines, but female anatomy makes it easy for the bacterium to get swept into the urinary tract. Once there, the bugs use sticky, hair-like structures called pili to cling to the walls of the urethra and bladder.
Strong adhesives make good sense in an environment constantly flushed with urine.
To their great surprise, Baorto and colleagues found that the sticky appendages also can work like keys to open up macrophages, immune-system cells that are hearty consumers of bacteria. Using cultures of mouse macrophages, the researchers discovered that the pili latch on to a particular protein on the macrophage's surface called CD48. Once attached, the bug slides easily into the cell. After letting E. coli invade the macrophages, the researchers bathed the cells in the antibiotic gentamicin. The bugs on the outside of the cells died quickly, but those inside the macrophages thrived throughout the four-day experiment.