Despite recurrent urinary tract infections, like cystitis, affecting many women worldwide, it is not entirely clear what triggers them. In a new research, scientists have shown that the exposure of the bladder to a specific vaginal bacteria could be to blame.

Urinary tract infections (UTIs) are characterised by symptoms like a need to pee more often than usual, pain when peeing or a general feeling of being unwell or tired.

Scientists think that many recurrent UTIs occur when dormant E. coli present in the lining of the bladder are reactivated. The bacteria is thought to be involved in recurrent UTIs in about 80% of women.

Research in animals has suggested that during a first infection, E. coli invade bladder tissue and are not entirely killed by antibiotics. When E. coli stop being dormant, they can go on to cause other UTIs.

But the question of how E. coli can be reactivated remains. The authors of the study now published in PLOS Pathogens believe they have found a potential explanation.

Sex and UTIs

The team from Washington University School of Medicine hypothesised that some vaginal bacteria are mechanically transferred to the urinary tract during sex, which damages bladder tissue and leads to a reactivation of E. coli infection.

The scientists raised mice with E. coli reservoirs in their bladders and exposed their urinary tracts to two vaginal bacteria species commonly found in women.

The first, known as Lactobacillus crispatus, is found in high proportions in the healthy vagina and had no effects on E.coli. It was not linked to the emergence of UTIs.

However, the second bacteria called Gardnerella vaginalis damaged the cells lining the bladder and activated the dormant E. coli. The mice went on to develop UTIs.

The results indicate that bladder exposure to Gardnerella vaginalis, which are brought into the urinary tract and bladder during sexual intercourse, may be a plausible trigger of recurrent UTIs.

"One of the reasons these findings are so exciting is that we hear a lot of women swear they get UTIs every time they have sex, and here we show how bringing vaginal bacteria into the bladder may contribute to the emergence of these infections", lead author Amanda Lewis told IBTimes UK.

Women with bacterial vaginosis

It is not yet clear how these findings in mice may be applicable to humans. Nevertheless, scientists know that a certain population of women is more likely to have higher numbers of Gardnerella vaginalis – women who suffer from bacterial vaginosis.

This common, yet poorly understood condition is associated with a higher risk of reproductive complications (such as infections in the womb) and of getting some sexually transmitted infections.

These patients are also more likely to develop UTIs and by identifying the role of Gardnerella vaginalis, this study provides a possible explanation for why that is.

More research will be needed, but the study's authors hope that they can convince clinicians to collaborate with them and to test their findings in humans. One idea would be to test whether treating women for bacterial vaginosis reduces the number of UTIs they get.