Ecological analyses of mycobacteria in showerhead biofilms and their relevance to human health

Matthew Gebert, Manuel Delgado-Baquerizo, Angela Oliverio, Tara Webster, Lauren Nichols, Jennifer Honda, Edward Chan, Jennifer Adjemian, Robert Dunn, Noah Fierer

Preprint posted on 10 July 2018

Article now published in mBio at

Showerheads are a playground for mycobacteria. More chlorine disinfectant is not the answer.

Selected by Hannah Behrens

Categories: microbiology


Nontuberculous mycobacteria (NTM), relatives of the bacteria responsible for tuberculosis and leprosy, cause lung and wound infections. It has been suggested that biofilms on showerheads, which release infectious aerosols when the shower is turned on, might be a reservoir for NTM. This is a challenging reservoir, as it provides low levels of nutrients, high variations in temperature and water flow and even periods of drought. Nonetheless, NTMs seem to prevail in some showerheads, unrivalled by other bacteria which cannot compete in such adverse conditions.

Here the authors harness the power of citizen science to investigate how wide-spread NTM are on showerheads, and how their presence or absence correlates with different water qualities and with occurrence of NTM lung infections.

Key findings

The team sequenced relevant genes to identify different bacterial species in 656 showerhead samples which were sent to them from all over Europe and the US. The citizen participants had provided information on location and source of the water supply as well as analysed eight parameters of the water chemistry using test strips provided by the researchers. When Gebert and his team put together all these different pieces of information they found that

  • Mycobacterium is the dominant bacterial genus found on shower heads
  • Plastic showerheads harbour the least mycobacteria
  • Homes supplied with municipal water in the US have higher levels of mycobacteria than European homes or US homes supplied by wells
  • Levels of NTMs on showerheads correlate with the number of NTM infections in the region – a correlation mostly driven by Hawaii which has unusually high levels of both

The high levels of NTMs in municipal water in the US might be due to the high levels of chlorine disinfectants used there. These select for NTM, which are more resistant to chlorine than most other bacteria.

Interestingly, a previous suggestion that the presence of Methylobacterium in samples correlates with NTM prevalence was not confirmed by this study. Instead the presence of free living amoeba showed a weak correlation (Pearson’s r=0.25, p=0.02) with NTM levels. The authors suggest that this is because NTM can infect amoeba and multiply inside them.

Furthermore, the different parameters of water quality influenced the diversity of NTM and partly predicted which species of the NTM genus colonized the showerheads in different regions.

Figure 1A from the preprint: Municipal water contains more NTM than well water, US water more than European water (the reason might be chlorine levels, see Figure S1) and plastic shower heads harbour the least Mycobacteria.

What I like about this preprint

The authors combine two unconventional approaches: citizen science and culture-independent analysis. By choosing to involve citizens in the research efforts an impressive sample size and geographic variety was achieved.

Typically, analysis of such samples is done by culturing bacteria to identify them. Gebert and his colleagues tried this approach but found that many bacteria were lost compared to a culture-independent approach. Without growing the bacteria, amplification and sequencing of sentinel genes allowed them to detect the microorganism independent of their culture conditions.

These two unusual methods have allowed a fascinating aspect of NTM infections to be investigated: the role of the showerhead. These results pave the way for the destruction of NTM reservoirs and could directly impact thousands of people’s lives by preventing NTM infections.

Future directions: application and further research

This study is a big step forward in understanding the showerhead as a reservoir of NTM bacteria. The next step will be to show a causal relationship between this reservoir and NTM infections.

It is important that the findings are used to advise citizens and administrators on how to reduce NTM reservoirs. Such recommendations could be focused on the material of the showerhead or about cleaning protocols, as well as about the regulation of chlorine levels in municipal water supplies. This should occur in the context of intervention studies that investigate whether changes can reduce the number of NTM infections observed.

Questions for the authors

Were all showerheads colonized by NTM? In which geographical regions are showerheads likely to be a risk to their users? What can affected populations do to minimize their exposure to NTM?

Data on water chemistry was collected and evaluated with respect to the diversity of the NTM species found on showerheads. Could this data also be used to investigate predictors of total NTM abundance?

It is interesting that the previously observed correlated occurrence of Methylobacterium did not hold true in this study. Instead a correlation with amoeba was found. Could the evidence for the co-occurrence of Methylobacterium with NTM and for amoeba with NTM be shown side by side? It would provide an interesting perspective for the reader who has not seen the raw data.

Lastly, it would be great to hear from the authors themselves about where they see the future applications and what outstanding questions remain after this study.[1]

References/Further reading

  • Thomson R, et al. (2013) Isolation of nontuberculous mycobacteria (NTM) from household water and shower aerosols in patients with pulmonary disease caused by NTM. J Clin Microbiol 51(9):3006-3011.
    This study provided evidence that showerheads might be connected to showerhead bacteria. 35% of samples isolated from patients contained NTM bacteria which were also found in the water in their homes.


  • Nishiuchi Y, Iwamoto T, & Maruyama F (2017) Infection sources of a common nontuberculous mycobacterial pathogen, Mycobacterium avium complex. Front Med 4:27
    This review discusses the possible niches of a pathogenic NTM, including shower heads.


Tags: chlorine disinfectant, citizen science, crowd sourcing, culture-independent analysis, nontuberculous mycobacteria, ntm, shower head

Posted on: 24 July 2018

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