Differences in the relative abundances of mycobacteria (as determined via 16S rRNA gene sequencing) across households in the United States on municipal versus well water (A), across showerheads constructed of different materials (B), and across households in the United States versus Europe (C). Differences in the relative abundances of mycobacteria (as determined via 16S rRNA gene sequencing) across households in the United States on municipal versus well water (A), across showerheads constructed of different materials (B), and across households in the United States versus Europe (C). Matthew J. Gebert et al. mBio 2018; doi:10.1128/mBio.01614-18