Urinary tract infections are a major nuisance for women. Nearly half of all women will suffer from one at least once in their lifetime, and for half of those women, UTIs will be a recurring nightmare. The infections are usually treated with antibiotics, but each time a UTI returns it is even more likely the infection will recur yet again.

Scientists had been pretty certain that the blame for UTIs rested entirely with the bacteria E. coli, and when E. coli was re-introduced into the urinary tract, a UTI recurred. But the latest research, out of Washington University, is pointing to a new culprit.

The vaginal bacterium Gardnerella vaginalis, scientists learned, triggers E. coli already present in your bladder to create another UTI. What's more, they discovered that G. vaginalis may be a contributor to a potentially deadly kidney infection.

Kidney infections are generally rare in women – only 1 percent of women with bladder infections go on to get one. But when one does hit, it hits hard, usually involving fever, nausea, back pain, and vomiting.

“We found that a particular vaginal bacterium, Gardnerella vaginalis, did not cause infection during exposure to the urinary tract, but it damaged the cells on the surface of the bladder and caused E. coli from a previous UTI to start multiplying, leading to another bout of disease,” said the study’s senior author, Amanda Lewis, PhD.

The scientists had been studying bacterial vaginosis, which is caused by an overgrowth of harmful bacteria, that results in vaginal odor and discharge. This is a common condition with UTIs.

Next Steps? New clinical studies are needed to inform doctors treating women for UTIs and the rare kidney infections to check whether bacterial vaginosis may put some women at greater risk for this severe form of UTI. Both UTIs and bacterial vaginosis may be treated with antibiotics of various kinds, but standard UTI antibiotics will not cure a woman of G. vaginalis.

“A lot of women swear that every time they have sex they get a UTI, and obviously that’s a huge burden,” Lewis said. “We don’t doubt that re-infection with E. coli is partly responsible, but we think we’ve found another pretty compelling reason why the connection between sexual activity and recurrent UTI might exist: Vaginal bacteria like G. vaginalis are moved into the urinary tract during sex.”

The research has been published in PLOS Pathogens.