The team analyzed DNA collected from 656 household showers in the United States and 13 countries in Europe.
Citizen scientists swabbed the inside of their showerheads with specialized kits, and mailed the 'biofilm' samples to Boulder.
By harnessing DNA sequencing technology, the researchers were able to identify which bacterial species that lived in showerhead slime, and how abundant they were.
When the researchers mapped out where potentially pathogenic mycobacteria thrived, the maps revealed 'hot spots' that roughly match regions where a nontuberculous mycobacterial (NTM) lung disease is most prevalent - parts of Southern California, Florida, and New York, highlighting the potentially important role of these showerhead bacteria in disease transmission.
Mycobacteria were far more abundant in showerheads receiving municipal tap water than in those receiving well water, as well as more abundant in U.S. households versus European.
These patterns are probably driven in part by differences in the use of chlorine disinfectants, the team reported this week in the American Society for Microbiology's journal mBio.
Mycobacteria tend to be somewhat resistant to the chlorine-based disinfectants used more heavily in the United States than in Europe - so in Europe, other bacterial species may be better able to thrive and outcompete the disease-causing strains.
Showerhead materials seemed to matter, too, the team say.
They found more mycobacteria in metal showerheads than in plastic onesplastic leaches some chemicals that support diverse bacterial communities, possibly preventing the mycobacteria from becoming too abundant.
"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, and what is potentially driving these 'hot spots',"said Gebert.
"And don't worry." he added.
"There is definitely no reason to fear showering."
