Prevalence of Showerhead Bacteria Correlates With Prevalence of NTM Lung Infections

A study published in the journal mBio¹ has found a correlation between regions where mycobacteria are most prevalent in showerhead biofilm and regions where nontuberous mycobacterial (NTM) lung infections are the most prevalent, providing more evidence about the important role played by showerheads in transmission of NTM lung infections in the United States and Europe.

Mycobacteria are abundant in the biofilms that coat the inside of showerheads as well as in water distribution systems,² and the inhalation of aerosolized mycobacteria while showering has been implicated as a mode of transmission in NTM lung infections. These infections are an increasing threat to public health in the United States and other developed nations because they are increasing in prevalence and are often difficult to treat.

The researchers collected showerhead biofilm samples and water chemistry data from 656 households across the United States and Europe from July 2016 to November 2016. The samples were collected by citizen scientists participating in the Showerhead Microbiome Project.

Participants were provided with a sampling kit containing a dual-tipped sterile Puritan CultureSwab, a water chemistry analysis kit, sterile gloves, and a brief questionnaire. All samples were collected by swabbing the interior of an unscrewed showerhead, with participants asked to swab the most commonly used showerhead in each household as close to the inside interface of the showerhead as possible. Participants provided home addresses, household water sources, estimated time since showerhead installation, usage frequency, cleaning frequency, and a description of the showerhead sampled.

Samples collected from the United States were mailed directly to the University of Colorado and stored in a -20°C freezer until processing; European samples were mailed directly to Copenhagen, Denmark, where they were stored at -20°C until European collection was completed, when they were sent overnight on dry ice to the University of Colorado, and stored at -20°C until processing.

Analyses of the samples showed that the genus Mycobacterium was consistently the most abundant genus of bacteria detected in residential showerheads; however, the researchers said that mycobacterial diversity and abundance was highly variable.

The data suggest that homes using municipal water had far higher levels of some mycobacteria, on average, compared with homes that used well water, a finding that is consistent with previous research concluding that water source plays a role in level of exposure to pathogenic mycobacteria. Mycobacteria were also more abundant in US households compared with European households, which the researchers attributed to the use of chlorine disinfectants in the US public water supply—which is less common in Europe.

Mycobacteria are significantly more resistant than other bacteria to chlorine and chlorine byproducts, the researchers note, and thus are expected to be more abundant in showerheads and water distribution systems where such disinfectants are used. “Strikingly, the abundance of taxa assigned to the genus Mycobacterium in showerhead biofilms in the United States was, on average, 2.3 times higher than that measured for showerheads in Europe (P <.01),” the authors wrote. Total chlorine concentrations in shower water in the US were 11 times higher, on average, than those measured in shower water in the European households tested.

“We identified geographic regions within the United States where showerheads have particularly high abundances of potentially pathogenic lineages of mycobacteria,” the researchers noted, “and these ‘hot spots’ generally overlapped those regions where NTM lung disease is most prevalent.” The researchers said their findings emphasize the public health relevance of mycobacteria in showerhead biofilms and demonstrate that mycobacterial distributions in showerhead biofilms are often predictable from household location and water chemistry. The knowledge “advances our understanding of NTM transmission dynamics and the development of strategies to reduce exposures to these emerging pathogens,” they concluded.

References

1. Gebert MJ, Delgado-Baquerizo M, Oliverio AM, et al. Ecological analyses of Mycobacteria in showerhead biofilms and their relevance to human health. mBio. 2018;9(5). pii: e01614-18. doi: 10.1128/mBio.01614-18.
2. Nishiuchi Y, Iwamoto T, Maruyama F. Infection sources of a common non-tuberculous mycobacterial pathogen, Mycobacterium avium complex. Front Med (Lausanne). 2017;4:27. doi: 10.3389/fmed.2017.00027. eCollection 2017.