Ecological Analyses of Mycobacteria in Showerhead Biofilms and Their Relevance to Human Health

Abstract

Bacteria within the genus Mycobacterium can be abundant in showerheads, and the inhalation of aerosolized mycobacteria while showering has been implicated as a mode of transmission in nontuberculous mycobacterial (NTM) lung infections. Despite their importance, the diversity, distributions, and environmental predictors of showerhead-associated mycobacteria remain largely unresolved. To address these knowledge gaps, we worked with citizen scientists to collect showerhead biofilm samples and associated water chemistry data from 656 households located across the United States and Europe. Our cultivation-independent analyses revealed that the genus Mycobacterium was consistently the most abundant genus of bacteria detected in residential showerheads, and yet mycobacterial diversity and abundances were highly variable. Mycobacteria were far more abundant, on average, in showerheads receiving municipal water than in those receiving well water and in U.S. households than in European households, patterns that are likely driven by differences in the use of chlorine disinfectants. Moreover, we found that water source, water chemistry, and household location also influenced the prevalence of specific mycobacterial lineages detected in showerheads. We identified geographic regions within the United States where showerheads have particularly high abundances of potentially pathogenic lineages of mycobacteria, and these "hot spots" generally overlapped those regions where NTM lung disease is most prevalent. Together, these results emphasize the public health relevance of mycobacteria in showerhead biofilms. They further demonstrate that mycobacterial distributions in showerhead biofilms are often predictable from household location and water chemistry, knowledge that advances our understanding of NTM transmission dynamics and the development of strategies to reduce exposures to these emerging pathogens.IMPORTANCE Bacteria thrive in showerheads and throughout household water distribution systems. While most of these bacteria are innocuous, some are potential pathogens, including members of the genus Mycobacterium that can cause nontuberculous mycobacterial (NTM) lung infection, an increasing threat to public health. We found that showerheads in households across the United States and Europe often harbor abundant mycobacterial communities that vary in composition depending on geographic location, water chemistry, and water source, with households receiving water treated with chlorine disinfectants having particularly high abundances of certain mycobacteria. The regions in the United States where NTM lung infections are most common were the same regions where pathogenic mycobacteria were most prevalent in showerheads, highlighting the important role of showerheads in the transmission of NTM infections.

Keywords: Mycobacterium; NTM lung disease; nontuberculous mycobacterial infection; plumbing biofilms.

FIG 1

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).

FIG 2

Phylogenetic tree showing the mycobacterial diversity recovered from the cultivation-independent analyses (hsp65 gene sequencing). Included in the tree are the reference mycobacterial strains from Dai et al. (59). The colors indicate the 34 clades of Mycobacteria, with the labels indicating the taxonomic identity of each clade. The tree was rooted with a hsp65 sequence from Nocardia farcinica (DSM43665). The plot on the right shows percent occupancy of the top 25 mycobacterial clades, with occupancy assessed as the percentage of samples (among 656 in total) in which each clade was detected. Colors indicate unique mycobacterial clades, with the color scheme used in the tree matching the color scheme used in the associated plot of mycobacterial occupancy.

FIG 3

Abundances of the top 25 mycobacterial clades detected across homes in the United States on well versus municipal water (n = 520 and 86, respectively; top panel) and across homes in the United States versus Europe (municipal water only, n = 606 and 50, respectively; bottom panel). The y axes were split to better illustrate the differences among the less-abundant clades.

FIG 4

Differential abundances of each of four lineages of mycobacteria that include pathogens across the geographic clusters of showerhead biofilm samples. The different colors indicate the mean abundances of each mycobacterial lineage across each of the 23 geographic clusters identified (uncolored points are samples from households not included in any of these 23 clusters). For details on each cluster and the abundances of mycobacteria within each cluster, see Fig. S8.

FIG 5

Relationships between the summed relative abundances of three potentially pathogenic mycobacterial clades that were frequently detected in showerheads (M. abscessus, M. fortuitum, and MAC) and the reported prevalences of NTM disease across Medicare beneficiaries and cystic fibrosis patients (50, 70). The strength and significance of the correlations were tested using both Pearson correlations and distance-based linear models (DISTLM; output shown). Each point indicates a different U.S. state, and data were aggregated to the state level (using median abundances), as the disease prevalence data were available only at the state level of resolution. Only states with >10 showerhead samples were included in these analyses.