The general point of having a shower is to get yourself clean. But our entire planet is effectively covered in microbes, so you'll get some of those raining down on your head, too.
That's why it's a good idea to find out what exactly lives in our plumbing, in case some of those germs turn out to be dangerous.
A team led by researchers from the University of Colorado (CU) Boulder looked into the microbial communities that live in our showerheads, and found some interesting differences between the US and Europe - along with potential lung disease pathogens. Yay?
"Most of those microbes are harmless, but a few are not," explains the senior author of the study, ecologist Noah Fierer from CU Boulder.
His team was especially interested in microbes of the genus Mycobacterium - a group of bacteria best known for its members that cause leprosy and tuberculosis.
Apart from those two nasties, the genus includes nearly 200 other species which are commonly found in the environment, including soil, dust, and our tap water. Together, these are referred to as non-tuberculous mycobacteria (NTM).
The 'myco' part of this bacterial genus name is a hint at fungi, and it actually holds a clue to one of their peculiar characteristics - these bacteria love growing colonies in a slimy mould-like fashion.
Which means that NTM could well be lurking in the slime you find in the grimiest areas of your bathroom, like the edges of the sink drain and, yes - the showerhead.
Most NTM aren't linked to any human diseases, so there's no need to freak out and buy an industrial-level sanitiser. But there is such a thing as NTM lung infection, and as part of their study, the researchers wanted to see whether showerheads specifically could harbour and distribute these pathogens.
"It's important to understand routes of mycobacterial exposure, especially in the household," said the lead author of the study, microbiologist Matt Gebert from CU Boulder.
"We can learn a lot from studying the biofilm that accumulates inside your showerhead, and the associate water chemistry."
To uncover such details on our slimy roommates, the team enlisted the help of citizen scientists from the United States and Europe. They ended up analysing the DNA of 656 biofilm samples from household showers, along with basic water chemistry data for each source.
Just as they suspected, there was no shortage of NTM in their results, although there turned out to be vast differences depending on the region where the samples where gathered, and the water sources.
"Our cultivation-independent analyses revealed that the genus Mycobacterium was consistently the most abundant genus of bacteria detected in residential showerheads," the researchers write in their study.
Mycobacteria were more common in US households as opposed to European ones, and the team hypothesised it could be due to differences in chemicals used to purify the water. In the US, chlorine is more common, but NTM tend to be resistant to this type of disinfectant.
Apart from geographical differences, households using well water had fewer mycobacteria in their water as opposed to homes that used municipal tap water.
And, curiously, metal showerheads had more NTM, while plastic ones were home to a more diverse microbiome, possibly due to chemicals present in plastic keeping the mycobacteria in check.
As we already noted, just because there are bacteria in your shower (and, also, literally everywhere), you should not panic.
But the team did discover that in US regions where NTM lung disease was most prevalent - places like Florida, New York, and parts of Southern California - the showerhead microbiomes were hosting more NTM, too.
For now, the team's results showed a correlational, not causation, but they're hoping to do future work to uncover why these potentially nasty lung germs hang around in some areas and not others.
"In terms of what's next, we're hoping to begin to explore, beyond identification and abundance, what is causing this striking geographic variation within the genus Mycobacterium," said Gebert.
The study was published in mBio.