Urinary tract infections are common among women and lead to about four million visits to the doctor every year. UTIs produce a constellation of uncomfortable symptoms, from constant urgency and burning to more widespread infection. It has always been assumed that the presence of any bacteria in the urine was a sign of disease, but new research has refuted that claim, taking down the common idea that urine is sterile and changing the way doctors think about bacteria in the bladder.
How do we normally identify the presence of bacteria?
Before the invention of genetic technology, the main way to see if bacteria were present in a sample of urine was to put it on a dish and grow the bacteria. Once the bacteria replicated enough, a microscope could be used to identify the type of bacteria that was present and responsible for the symptoms. Growing bacteria in a dish requires filling that dish with food the bacteria can live on, which is called a growth medium. The trouble is, different kinds of bacteria need different things to live. In the same way that cows need a different diet than people, certain kinds of bacteria need certain nutrients in their environment to survive that others might not. That means that what bacteria you find growing in a sample of urine is very dependent on the growth medium you put in your growth dish.
Why did we think urine was sterile?
In the past, the only bacteria we’ve cared about are bacteria that are dangerous to health. That means that doctors have focused on finding growth media that allow the most dangerous bacteria to grow. But that has also meant that physicians could be missing other bacteria that are present in the bladder, but that can’t grow because the nutrients they need aren’t available. As researchers have started to understand more about the importance of healthy bacteria in human health, they’ve become more interested in where these bacteria are present and what kinds live where. The presumption that urine was sterile was based on the fact that no bacteria grew when normal urine was placed on these different growth media. But without looking for bacteria other than those known to cause disease, that idea was just an assumption.
How did these researchers find these new bacteria?
Gene sequencing techniques have gotten good enough and cheap enough that researchers can now sequence large numbers of genes at low cost in a short period of time. This has allowed researchers to build a big library of bacteria genomes along with the differences that make each of those bacteria unique. This team took urine samples from women and used sequencing technology to look for genetic signs of many more bacteria than those tested for using the traditional growth method. They found signs that bacteria are always present in urine, even in women who are perfectly healthy.
How did they interpret the presence of these bacteria?
The research adds to the idea that bacteria live on most surfaces of the human body and probably play a key role in health. In this case, the researchers found that certain strains of bacteria were associated with symptoms of a UTI even when traditional tests found no bacteria. This might indicate that there’s a delicate balance of bacteria that normally exist in the bladder and keep it healthy.
How does this research affect me?
The new test the team has developed isn’t available yet. But this expanded understanding of which bacteria cause disease may lead to new tests and new treatments for symptoms that would otherwise have gone untreated. That could help women get over these infections sooner and to stay infection-free for longer.