Urine is not sterile, and neither is the rest of you

Let’s say you find yourself lying at the bottom of a ravine with a dirt-filled gash in your leg. According to the Internet, the first thing you want to do is pee on your wound. After all, the common wisdom holds, urine is sterile.

Wrong again, Internet.

Urine is not sterile, even before it comes out of you and gets contaminated by your skin. Bacteria are present at low levels in the urine of healthy people not suffering from a urinary tract infection, Evann Hilt of Loyola University of Chicago reported May 18 at a conference of the American Society for Microbiology. Now, Hilt and her colleagues are figuring out what bacteria make up the normal bladder community and whether a change in that community might trigger urinary problems.

“Now that we know they’re there, the question is what are they doing?” Hilt says. Most likely, she says, “it’s like any other niche on our body. You have a good flora that keeps you healthy.”

It appears that the urban legend about urine being sterile has its roots in the 1950s, Hilt says, when epidemiologist Edward Kass was looking for a way to screen patients for urinary tract infections before surgery. Kass developed the midstream urine test (still used when you pee in a cup) and set a numerical cutoff for the number of bacteria in normal urine: not more than 100,000 colony-forming units (cell clusters on a culture dish) per milliliter of urine. A person tests “negative” for bacteria in their urine as long as the number of bacteria that grow in a lab dish containing the urine falls below this threshold. “It appears that the dogma that urine is sterile was an unintended consequence,” Hilt says.

Hilt and her colleagues used a more sensitive growth-culture technique to detect the low levels of bacteria in normal urine, reasoning that maybe some urinary bacteria don’t grow readily under the conditions of the standard test. Having already found bacterial genetic material in urine (as did another team), in their latest work they used catheters to collect urine directly from the bladders of 84 women, half of whom had overactive bladder syndrome, which causes patients to have to urinate frequently. They put samples of the urine in lab dishes and let the urine bacteria grow under friendlier conditions. More than 70 percent of the urine samples contained bacteria, including at least 33 types of bacteria (at the genus level) in normal urine. Women with overactive bladders had more types of bacteria in their urine (77 genera), including four species found only in overactive bladder patients.

This finding might provide hope for the 15 percent of women who suffer from overactive bladder; many aren’t helped by the standard therapy that treats the condition as purely a muscular problem.

Learning that urine is not sterile also changes the way we think about infection. It had generally been assumed that if there are bacteria in your urinary tract, you have an infection and that’s a bad thing. But if there is a normal community of bacteria, we may need to think about the bladder more in the way we have recently learned to think of the gut microbiome, in terms of “healthy” and “unhealthy” mixes of bacteria.

It’s not clear anymore what body parts are actually sterile. The placenta was long thought to be, but scientists have just learned that’s not true: They found bacteria on the baby’s side of the placenta. There’s also some evidence that babies are born with bacteria already in their guts, which must have gotten through the placenta.  

And what about brains? Surely the braincase is last bacteria-free bastion, protected by the blood-brain barrier.

Sadly, that’s not the case either. When I asked neuroscience writer Laura Sanders if our brains are sterile, she promptly said, “Oh, no. The brain’s full of all kinds of junk.” That includes viruses.

Last year, in fact, researchers reported finding soil bacteria in people’s brains. (Before making a dirty-mind joke, these were alpha-proteobacteria normally found in soil, but there’s no reason to think soil got into anyone’s brain.) The researchers were studying whether people with a compromised immune system from HIV/AIDS might be prone to brain infections. Instead, they found that all the brains they looked at contained bacteria, regardless of HIV status. No one knows how the bacteria get in there, or when. Could they be leftovers from fetal development? Lucky tricksters that make it through the blood-brain barrier? We don’t know, and just like for bladders, we don’t even know what to consider “normal” yet.   

So back to our original question: If urine isn’t sterile, does that mean you shouldn’t pee on a wound? Well, that was probably never a great idea anyway. If you don’t have clean water, you’re generally better off letting blood flow flush a wound, bathing it in infection-fighting white blood cells.

But if knowing there are bacteria in urine helps you talk a well-meaning friend out of peeing on you in an emergency, well, you’re welcome.  

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