The different causes of urinary tract infections and why

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There are many different causes of UTIs. From anecdotal, clinical, and survey data, sexual activity is likely the most common UTI trigger. However, it is far from the only cause. Each person is different. In this article, we go over the different UTI triggers that are consistently reported, and we put forth our best explanation of what is really happening.

All UTIs are caused by pathogenic bacteria in the urinary tract overgrowing and causing an infection, but how the bacteria get there can vary. On a high-level, anything that might introduce these bacteria to the urinary tract can cause a UTI.

UTIs can be split into 2 types:

Re-infections: UTIs caused by new bacteria that has entered the urinary tract.

Relapse infections: UTIs caused by old bacteria hidden in the urinary tract from a previous infection.

If you’re wondering what is causing your recurrent UTIs, it could be one or the other or both. In this article, we will primarily cover re-infections, or how bad bacteria get into the urinary tract in the first place and continue to be introduced but will also touch on relapse infections and link out to additional info when relevant. You can learn more about how initial infections become relapse infections here.

As mentioned above, sexual activity is the most commonly reported cause of UTIs, especially in younger women. During sexual activity, bacteria, most commonly e. coli, are transferred from the vagina, anus, or sexual partner to the urinary tract. Sexual activity physically moves or transplants bacteria to the urethra, where they can travel up and establish an infection.

The longer the time the bacteria have to travel up the urinary tract, the more likely an infection is. This is why peeing after sex is always a good idea. One of our best defenses against newly introduced bacteria is the physical act of urination, which flushes bacteria out. This is also a big reason why women are more likely to get UTIs than men. Since the length of the urethra from the opening to the bladder is much shorter in women, it is easier for bacteria to make their way up.

Vaginal health is a major component to UTIs from sexual activity as imbalances in the vaginal microbiome, like bacterial vaginosis (BV), increase risk of UTI by making the urinary tract more vulnerable to bacteria. You can read about this in more detail here.

As women enter menopause, UTI risk goes up. Many women who have not previously had issues with recurring UTIs suddenly do. During menopause, estrogen production declines. This leads to big changes in the vaginal microbiome and a declined in lactobacilli. Instead of being dominated by lactobacilli bacteria like that of younger women, the vaginal microbiome can become dominated by pathogenic bacteria, even e. coli. These bacteria are then more likely to travel to the urinary tract. While not necessarily a “cause” of UTIs, menopause, like periods, increases the risk of UTIs from all triggers. Menopausal and postmenopausal UTIs are still often triggered by certain activities, like sex and many other mentioned in this article. With more pathogenic bacteria around, the more likely these triggers are to cause UTIs. You can read about postmenopausal UTIs here.

Many women report getting UTIs cyclically immediately following their period. Like with sexual activity, the vaginal ecosystem plays an important role. Healthy vaginal pH is usually, between 4 and 5. The pH of blood is much higher, above 7. As tissue and blood travels from the uterus through the vagina, pH rises. Pathogenic bacteria thrive at a higher vaginal pH and are normally kept in check at a lower pH. Hormones are also changing as estrogen production dips prior to menstruation. This can lead to a temporary imbalance in the vagina microbiome (many women also report BV or yeast infections following menstruation). This can make the urinary tract more vulnerable to infection as bacteria travel from the vagina to the urethra.

It is also likely that the increased moisture in the area makes it easier for bacteria to grow and to move around. Bacteria can live on and travel through absorbent material used in feminine hygiene products like tampons and pads, as the fluid allows bacteria to move around. Research has shown that more frequent changing of feminine hygiene products during menstruation decreases the risk of UTIs. You can read more about menstruation and UTIs here.

Sweat and moisture allow bacteria to move around when exercising, which can transfer them to the urethra. If the clothes worn while exercising are not breathable, this typically increases moisture and makes bacterial travel more likely. Exercising and then not showering for an extended period of time will allow bacteria more time to travel before washing.

As mentioned above, our first line of defense against bacteria entering the urinary tract is physically flushing them out via urination. The more time (the longer we hold it) that we allow bacteria to travel up the urinary tract, the higher the risk of UTI. This is also why people with certain professions, like teachers, nurses, and more, who may not be able to use the bathroom as frequently as they’d like, experience higher rates of UTIs.

Anecdotally, people report that their UTIs seem to be related to periods of stress. This isn’t surprising because illnesses of many kinds are associated with stress, like times of high anxiety or lack of sleep. Stress comes in many forms, but periods of stress may weaken our immune system and therefore make us more vulnerable to infection.

Similar to when not urinating for long periods, taking airplane rides and sitting for a long time at once may increase UTI risk. Travel can also be stressful and tiring, which may also contribute to the association between traveling and UTIs. Typical hygiene routines may also be disrupted when traveling, which could lead to a higher likelihood of bacteria getting into the urinary tract. If you’re camping, for example, you’re likely not able to shower or bathe with the regularity you’re used to.

While there is no formal research recognizing taking a bath or swimming as risk factors, e. coli or other bacteria that washed off from the skin or urogenital area, or already in the water could move to the urinary tract when doing so. This could be why many women report UTIs following swimming or bathing. Many women who have noticed this pattern avoid taking baths because of it. In addition, following swimming, going a long period of time in a wet or damp bathing suit without letting the area breath and dry and without showering provides more opportunity and time for bacteria to travel to the urinary tract and establish an infection.

In elderly women, bacteriuria, or high levels of bacteria in the urine, is often present but with no symptoms. More than 50% of women over the age of 80 may have bacteriuria and therefore may test positive for a UTI in a urine culture. However, this is often asymptomatic, and if the individual is not experiencing symptoms, expert physicians recommend not treating with antibiotics. In this instance, bacterial populations are living in the bladder at all times, and while this is not an “infection”, it does leave the individual vulnerable to infection because changes in the makeup of this urinary microbiome could lead to a rise in pathogenic or “bad bacteria” and to an infection. Bacteriuria in the elderly should be monitored because if an infection does develop and is untreated it may lead to a kidney infection and sepsis, which can lead to death. UTI symptoms in the elderly can be difficult to decipher as they are different from those experienced by younger women and include additional symptoms like imbalance, forgetfulness, and dementia-like symptoms. You can read more on UTIs in the elderly here.

Catheter use is a known and well-established risk factor for UTIs. Bacteria can be introduced into the urethra during insertion of the catheter either in two ways: 1) if the catheter was not sterile and there are bacteria on the catheter itself or 2) if bacteria on the surrounding area or opening of the urethra attach to the catheter during insertion. For indwelling catheters, or catheters that remain in place for an extended period of time, the catheter itself becomes a surface for bacteria to adhere to and create protective biofilms on. Recurrent UTIs are a major issue for those using single-use or indwelling catheters. It is also much more common to see infections caused by bacteria other than e. coli.  

Surgeries to the bladder or surrounding organs like the gut can dramatically increase the risk of UTIs. If the surgery is reconstructive or for repairs to lacerations of the bladder this can leave permanent structural imperfections that can make bacterial invasion of these tissues much easier. Surgeries can also lead to the need for the use of catheters, which significantly increases the risk of UTIs.

While the basic mechanism of urinary tract infections is the same in men as it is in women, the cause is often different. UTIs are not common in men until they get older and they are typically associated with benign prostatic hyperplasia (BPH) or enlarged prostate. According to the National Institute of Diabetes and Digestive and Kidney Diseases at the NIH, “benign prostatic hyperplasia affects about 50 percent of men between the ages of 51 and 60 and up to 90 percent of men older than 80.” As the prostate, which sits around the urethra, enlarges, it constricts the urethra, making urination more difficult. This can lead to urinary retention or incomplete emptying of the bladder. Urinary retention increases the risk of UTI by allowing permanent pools of urine to collect in the bladder where bacteria can grow and cause UTIs. These bacteria often create biofilms in the bladder and can be persistent. In addition, given the close proximity of the tissue of the urinary tract and the prostate, the bacteria can travel to the prostate and cause an infection there. This is called bacterial prostatitis. Cyclical prostate and urinary tract infections in men with BPH can be common, especially if a catheter is used for complete emptying of the bladder.

31% of pregnant women will have either a symptomatic or an asymptomatic UTI while pregnant. UTIs in pregnancy are common because the anatomy of the urinary tract actually changes. Your growing uterus adds pressure onto your bladder, which can affect your ability to fully void. In addition, your progesterone and estrogen levels increase during pregnancy. Pregnancy also alters the makeup of your urine, reducing the acidity and increasing the amount of protein, hormones, and sugar in your urine. All of the above contribute to an increased likelihood of getting UTI in pregnancy. It’s recommended that all pregnant women receive a urinalysis and urine culture at 12 to 16 weeks or during the first prenatal visit to ensure there is not an infection.

Spencer is a Co-Founder of Uqora. Trained in biochemistry at UC Berkeley, Spencer uses his background to provide education on topics including UTIs, urinary tract health, and non-antibiotic UTI treatments.

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