Every time you hop in the shower, you are in the company of millions of microorganisms, most of which probably won’t hurt you. 

However, researchers from the Cooperative Institute for Research in Environmental Sciences (CIRES) have identified Mycobacterium as the most abundant genus of bacteria growing in the slimy “biofilm” that lines the inside of showerheads. Some of those bacteria can cause lung disease in the immunocompromised. 

Matt Gebert, a CIRES researcher and lead author of the 2018 mBio study, and his colleagues analyzed DNA extracted from slime samples collected from hundreds of citizen scientists’ showerheads across the United States and Europe. 

They found that mycobacteria: are more prevalent in the United States than in Europe; thrive more in municipal tap water than in well water; are more abundant in metal showerheads than in plastic ones; and are more common in “hot spots” where certain types of lung disease caused by mycobacteria are also common—namely, parts of Southern California, Florida and New York.