“Pee after sex” is perhaps one of the most memorable pieces of advice I’ve picked up in conversations with female friends over the years. The theory is that peeing right after sex will help to flush out any bacteria that may have entered your body during sex and prevent them from infecting your urinary tract.

Urinary tract infections, or UTIs, commonly refers to infection of the urethra or bladder and affects mostly women. It’s estimated that roughly half of all women will experience a UTI at least once in their lifetime. Of these women, 25-40% suffer from repeated UTIs and must take antibiotics continuously to prevent a recurrence. The most common cause of UTIs is uropathogenic E. coli, or UPEC. UPEC can enter the body through the urethra and then move into the bladder. Left untreated, the bacteria can spread from the bladder to the kidneys and cause serious health complications.

One of the biggest risk factors for developing a UTI is sex. Each time a woman has sex, bacteria is introduced into her urogenital tract. Studies have shown that after sex, there is a 10 times increase in the amount of bacteria in a woman’s urine. In fact, many studies have found that the more frequently a woman has sex, the more likely she is to develop a UTI. But what we don’t know is whether the timing between these sexual escapades has any effect on the risk of developing a UTI.

Now, a new study from Dr. Scott Hultgren’s group at Washington University in St. Louis has shown that when it comes to sex and UTIs, timing does matter. The researchers found that there is a sensitive period after the first “bacteria insult” during which the likelihood of developing a severe and chronic infection increases dramatically.

To test their idea, the researchers developed a new mouse model that allowed them to carry out sequential infections and measure how the timing between infections affects the chances of developing a chronic infection. This new superinfection model is meant to mimic real life scenarios of frequent sex in a short period of time where bacteria are introduced into the female urinary tract in multiple, sequential events.

For the first round of infections, the researchers directly injected either a saline solution or a saline solution containing UPEC into the urinary tract of female mice. After the first infection, the researchers waited 1-2 hours, 6 hours, or 24 hours before infecting the mice with UPEC again. One week later, they measured the total amount of bacteria in the bladder. That measurement can be used to predict the likelihood of a chronic bladder infection. Mice that have more bacteria in their bladder at the one-week checkpoint are more likely to develop a chronic bladder infection. When the researchers compared the different groups of mice, they saw some pretty dramatic differences. In mice that first received a saline injection followed by UPEC, 30-60% were predicted to develop a chronic bladder infection, depending on the timing of the UPEC infection. In mice that were infected with UPEC twice within either a 1-2 or 6 hour window, the likelihood of developing a chronic bladder infection jumped to 80%. However, when the second UPEC infection occurred 24 hours after the first one, the likelihood dropped to 40%, roughly the same odds as when mice received a single UPEC infection.

Like humans, mice also differ in their susceptibility to UTIs. The first set of superinfection experiments used mice that are genetically more susceptible to UTIs. When the researchers repeated the same experiment in mice that are more resistant to UTIs, their results were, not surprisingly, different. In the resistant mice, the greatest likelihood of developing a chronic bladder infection happened when the second UPEC infection occurred 24 hours after the first. Unlike the susceptible mice, the risk reached a maximum of only 30% in the resistant mice.

The take-home message here is that when it comes to risk for a UTI, not all sex is equal. For a woman, having sex multiple times within a short period of time likely presents a greater risk for developing a recurrent UTI or bladder infection than having sex the same number of times over an evenly spaced, longer period of time. The length of the “sensitive” period varies among individuals and is influenced by many things, including genetics, immune status, and overall health. The researchers speculate that the initial exposure to bacteria may trigger an immune response that makes the body more susceptible to developing a chronic infection. However, it’s still not clear how this happens and whether this knowledge can be translated into better diagnostics and treatments. Until then, drink lots of water and remember to pee after sex.

Reference:
Schwartz, D., Conover, M., Hannan, T., & Hultgren, S. (2015). Uropathogenic Escherichia coli Superinfection Enhances the Severity of Mouse Bladder Infection PLoS Pathogens, 11 (1) DOI: 10.1371/journal.ppat.1004599