"The d-mannose works when I get a bladder infection... but why did I get another one?"
Speaking with my patient last week reminded me how difficult it can be to break a string of bladder infections. She and I had spoken about a lot of things during our initial interview, and she was doing much better in general: her abdominal pain had completely resolved, on those days when she stuck with her gluten free diet, so the diarrhea wasn't part of the problem.
What I also knew about her is that she's an equestrian and sexually active: two activities (let's add cycling to the list) that often raise the risk of bladder infections.
During any activity where there is repeated, or sustained, or forceful pressure against the urethra (the opening of the bladder sitting at the front end of the vaginal opening), the repeated pressure can push any of the local bacteria toward the urethra. Bacteria live normally on every part of our skin, and perhaps even more around the folds outside the vagina, the anus, and the perineum (the area between vagina and anus).
Additionally, the repeated pressure can actually stretch open the urethra opening, allowing bacteria some partial access to the bladder.
Now, imagine what happens if you go to urinate after that activity, but you don't have much urine to pee out? The urethra opens fully, but the outgoing urine only partly feels the opening, leaving a ready pathway and location for invading bacteria to thrive.
Her eyes lit up, "Yes, that's it! I think I'm supposed to pee afterwards, but there's not always much there." I suggested that when that happens she should promptly drink two full glasses of water, and drink two more after her next urination as well.