Why Do Some UTIs Keep Coming Back? Understanding Biofilms and Bacterial Persistence
Understanding the Hidden Challenge Behind Recurrent UTIs
The symptoms are gone. The antibiotics worked. Life returns to normal. Then, a few weeks later, the familiar burning, urgency, and discomfort return.
For many women, the most frustrating part of a urinary tract infection is not the first infection, it's the second, third, or fourth one that follows.
Why does an infection seem to disappear, only to come back again?
Researchers now know that recurrent UTIs are often more complex than simply coming into contact with bacteria a second time. In some cases, bacteria may persist within the urinary tract using sophisticated survival strategies that help them persist over time.
One important area of recurrent UTI research is the role of biofilms, communities of bacteria that can attach to surfaces and create protective environments which may help them persist over time.
So, what exactly are biofilms, and what role might they play in recurrent UTIs?
How Biofilms May Contribute to Recurrent UTIs
A simplified illustration of how bacteria may attach to the bladder lining, form protective biofilms, and persist within the urinary tract. Biofilms are one of several mechanisms being studied in recurrent UTI research.
What Is a Biofilm?
A biofilm is a community of bacteria that attaches to a surface and surrounds itself with a protective layer.
Once bacteria attach to the bladder wall, they can begin producing a sticky matrix that helps them survive and remain in place.
This protective layer can help bacteria remain attached, reduce their exposure to the body's natural defenses, and make them more difficult to eliminate.
In the urinary tract, biofilms may act as a reservoir for bacteria, allowing them to persist and potentially contribute to recurrent infections.¹
Why Biofilms Matter in Recurrent UTIs
Most urinary tract infections begin when Escherichia coli (E. coli) bacteria travel from the intestinal tract to the urinary tract and attach to the bladder lining.¹
Biofilms generally develop through a series of stages in which bacteria attach, multiply, build a protective matrix, and eventually release bacteria that spread elsewhere in the urinary tract. This process may help explain why symptoms sometimes disappear temporarily but later return.
Researchers have also discovered that some uropathogenic E. coli can use additional survival strategies beyond biofilm formation. In particular, bacteria may temporarily reside inside bladder cells, forming intracellular bacterial communities that share many characteristics with biofilms. These protected bacterial reservoirs may help explain why some infections return despite treatment and remain an active area of UTI research.⁵
Recurrent UTIs are complex and can have several contributing factors, including anatomy, hormonal changes, immune responses, bacterial behavior, and individual susceptibility. Biofilms are not the only explanation, but they are one important mechanism that may help explain bacterial persistence.
It All Starts with Adhesion
Before a biofilm can form, bacteria must first attach to the urinary tract lining.
To do this, E. coli use tiny hair-like structures called pili or fimbriae that help them bind to urinary tract cells.¹
Without successful attachment, bacteria are less able to colonize the urinary tract, biofilms cannot develop, and bacteria are more easily removed through normal urinary flow.
Because bacterial adhesion is the first step in biofilm formation, researchers have investigated whether reducing bacterial attachment could help limit the opportunity for colonization and biofilm development.¹
Targeting Bacterial Adhesion: What Cranberry Research Has Revealed
Researchers have identified naturally occurring compounds in cranberry called A-type proanthocyanidins (PACs) that have been studied for their effects on uropathogenic E. coli.²
Unlike antibiotics, these compounds do not kill bacteria. Instead, research suggests they may influence bacterial adhesion, helping reduce the ability of bacteria to attach to urinary tract tissues.²
In an earlier multicenter, randomized, double-blind study, Howell et al. demonstrated measurable urinary Anti-Adhesion Activity (AAA) against uropathogenic E. coli following consumption of cranberry powder standardized for PAC content.³
Additionally, a study comparing cranberry dietary supplements, Howell et al. found that a cranberry juice-derived extract rich in soluble A-type PACs produced substantially greater urinary Anti-Adhesion Activity (AAA) than a product containing primarily insoluble PACs.⁴
Soluble PACs are naturally found in the juice portion of the cranberry, whereas insoluble PACs remain bound to the plant material left behind after juice extraction, often referred to as press cake or pomace. Whole-berry ingredients and ingredients derived from press cake therefore contain a greater proportion of insoluble PACs, while juice-derived extracts are richer in soluble PACs.
These findings suggest that not all cranberry ingredients behave in the same way. Products containing similar amounts of total PACs may not necessarily produce equivalent urinary Anti-Adhesion Activity, and the source, composition, and solubility of PACs may be important determinants of bioactivity.⁴
Additional laboratory studies have shown that cranberry exposure may alter bacterial surface properties and reduce biofilm formation by uropathogenic E. coli, suggesting that cranberry compounds may influence some of the earliest stages of bacterial colonization.⁶
After all, the fewer opportunities bacteria have to attach, the fewer opportunities they have to settle in, persist, and potentially contribute to recurrent urinary tract infections.
FAQs
What is a biofilm in a UTI?
A biofilm is a community of bacteria surrounded by a protective layer that allows them to attach to the bladder lining. Researchers believe biofilms may help bacteria persist in the urinary tract by making them harder to eliminate, making them one possible contributor to recurrent urinary tract infections.
Why do some UTIs keep coming back?
Recurrent UTIs can have several causes. In some people, bacteria may persist in the urinary tract through survival strategies such as biofilm formation or intracellular bacterial communities. Other factors, including anatomy, hormonal changes, immune responses, catheter use, and individual susceptibility, may also increase the risk of recurrence.
Do all cranberry supplements work the same way for urinary tract health?
No. Cranberry supplements can differ in their source, PAC composition, and solubility, which may affect their biological activity. Research suggests that cranberry juice-derived extract rich in soluble A-type PACs produce greater urinary anti-adhesion activity than products containing mostly insoluble PACs derived from whole cranberry fruit or press cake (the material left after juice extraction). As a result, not all cranberry supplements are expected to perform the same way.
References
- Flores-Mireles AL, Walker JN, Caparon M, Hultgren SJ. Urinary tract infections: epidemiology, mechanisms of infection and treatment options. Nature Reviews Microbiology. 2015;13(5):269-284. doi:10.1038/nrmicro3432
- Howell AB, Reed JD, Krueger CG, Winterbottom R, Cunningham DG, Leahy M. A-type cranberry proanthocyanidins and uropathogenic bacterial anti-adhesion activity. Phytochemistry. 2005;66(18):2281-2291. doi:10.1016/j.phytochem.2005.05.022
- Howell AB, Botto H, Combescure C, Blanc-Potard AB, Gausa L, Matsumoto T, Tenke P, Sotto A, Lavigne JP. Dosage effect on uropathogenic Escherichia coli anti-adhesion activity in urine following consumption of cranberry powder standardized for proanthocyanidin content: a multicentric randomized double blind study. BMC Infectious Diseases. 2010;10:94. doi:10.1186/1471-2334-10-94
- Howell AB, Dreyfus JF, Chughtai B. Differences in Urinary Bacterial Anti-Adhesion Activity after Intake of Cranberry Dietary Supplements with Soluble versus Insoluble Proanthocyanidins. Journal of Dietary Supplements. 2021. doi:10.1080/19390211.2021.1908480
- Mulvey MA, Schilling JD, Hultgren SJ. Establishment of a persistent Escherichia coli reservoir during the acute phase of a bladder infection. Infection and Immunity. 2001;69(7):4572-4579. doi:10.1128/IAI.69.7.4572-4579.2001
- Di Martino P, Agniel R, Gaillard JL, Denys P. Effects of Cranberry Juice on Uropathogenic Escherichia coli In Vitro Biofilm Formation. Journal of Chemotherapy. 2005;17(5):563-565. doi:10.1179/joc.2005.17.5.563
Disclaimer
This content is provided for educational and informational purposes only and is intended to summarize current scientific research related to urinary tract health. It is not intended to replace professional medical advice, diagnosis, or treatment. Always consult your physician or qualified healthcare provider with any questions you may have regarding a medical condition or before starting any new supplement or health regimen.
