The model also revealed a second factor. Prolonged exposure to highly concentrated urine weakened the connections between bladder cells, making the tissue more vulnerable to bacterial invasion and reducing the effectiveness of antibiotic treatment.

A separate U.K. study, using a different 3D human bladder microtissue model, produced a similar finding. Nitrofurantoin — one of the first-choice antibiotics for uncomplicated lower UTIs — performed strongly in conventional laboratory tests but failed to eradicate the bacteria embedded in the bladder tissue model.

These are laboratory models, not clinical trials, and they do not show that every recurrent infection stems from hidden bacteria. But they reveal a plausible mechanism, and they suggest that bacteria detected in a urine sample may not always give the full picture of what is happening inside the bladder tissue itself.

Why women are hit hardest

Anatomy is a major reason women suffer more UTIs than men. A woman's urethra is much shorter than a man's, so bacteria have a shorter distance to travel before reaching the bladder. The urethral opening also sits close to the anus, where gut bacteria live.

Sexual activity and the use of spermicides are established risk factors for recurrent infection in premenopausal women.

Risk rises again around and after menopause. Falling estrogen changes the tissues of the vagina and urinary tract and is associated with a reduction in protective Lactobacillus bacteria, creating conditions in which UTI-causing bacteria can become more established.

What actually helps prevent recurrence

No single approach works for everyone, but several options have evidence behind them.

Vaginal estrogen. During perimenopause and after menopause, the National Institute for Health and Care Excellence (NICE), which produces health guidance for England, recommends considering vaginal estrogen for recurrent UTIs when behavioral and hygiene measures haven't helped or aren't suitable. It has some of the strongest evidence among non-antibiotic approaches.

Drinking more water. This helps people who don't normally drink much. In a trial of 140 premenopausal women who drank less than 1.5 liters (51 fluid ounces) of fluid a day, those asked to add another 1.5 liters averaged 1.7 bladder infections over a year, compared with 3.2 among those who kept their usual intake.

Cranberry products. These may reduce recurrence for some women, although evidence varies between studies and formulations. The European Association of Urology advises patients that the evidence remains contradictory and generally low quality.

D-mannose. This sugar, sold as a dietary supplement, is thought to make it harder for some E. coli to stick to the bladder lining. It looked promising in smaller studies but failed to prevent infections in a much larger U.K. randomized trial involving 598 women.

Preventive treatments. For people with recurrent infections, NICE says doctors can consider preventive options after any current infection has been treated. These include methenamine hippurate, a non-antibiotic urinary antiseptic, or preventive antibiotics. The choice depends on how frequent and severe the infections are, previous urine test results, previous antibiotic use, and whether measures such as vaginal estrogen or a single post-trigger antibiotic dose have helped.

Long-term antibiotic use also needs to be weighed against side effects and antimicrobial resistance, which the World Health Organization describes as a growing threat to global health.

A new view of an old infection

The mini-bladder findings recast a UTI as a disease shaped by interactions between bacteria, bladder tissue, the immune system, and the chemical and physical conditions inside the bladder.

Researchers in the U.K. bladder-model study also tested whether bacteria-killing viruses called bacteriophages, or phages, could target bacteria persisting in bladder tissue. Phages reduced clusters of bacteria inside bladder cells, but adding them to nitrofurantoin did not consistently clear more bacteria from the tissue than the antibiotic alone.

For someone whose infection keeps coming back, one possible explanation now has experimental support: a small population of bacteria survived the original treatment and simply started growing again.