Every time we take a shower, we breathe in a large number of aerosolized microorganisms dislodged from the biofilms found inside our showerheads. These showerhead-associated microbial communities can have important effects on human health. Most notably, showerheads can harbor large populations of Mycobacteria, a diverse genus of bacteria that includes both opportunistic pathogens as well as beneficial bacteria capable of modulating immune system function. Unfortunately, the basic ecology of showerhead-associated microbial communities remains understudied. In particular, we do not know how the diversity and abundances of Mycobacteria vary across the U.S. or in response to changes in household water chemistry. To address these knowledge gaps, we recruited >1200 volunteer households from across the United States to sample their showerhead biofilms and conduct basic chemical analyses of their household water. We then used high-throughput marker gene sequencing and cultivation-based analyses to characterize bacterial and micro-eukaryotic diversity in each of the collected samples.
Based on analyses of over 600 samples to date, our results highlight that there is substantial variability in the amounts and types of mycobacteria found in showerhead biofilms, with the relative abundances of mycobacteria in the showerhead biofilms ranging from 0% (no detectable mycobacteria) to >95% of 16S rRNA reads. Most of the detected mycobacterial strains are non-pathogenic, but a sub-set are either known pathogens or likely beneficial mycobacteria capable of reducing chronic inflammatory disorders. Furthermore, we detected strong geographic structure in mycobacterial abundances with a number of mycobacterial strains (including putative pathogens) having higher abundances in selected regions of the U.S. This geographic variation in mycobacterial abundances was often predictable from household water chemistry, with chlorine levels, and other water chemistry parameters consistently selecting for specific mycobacterial taxa. Lastly, our results indicate that certain mycobacteria appear to strongly co-occur with specific micro-eukaryotes, including some amoebae, highlighting a unique ecological strategy used by some mycobacteria to persist in showerhead biofilms. Together these results demonstrate the power of a ‘citizen science’-based approach to improve our understanding of those bacteria living with us in our homes that can have important impacts on our health and well-being.