How many of us dare to look up and actually take notice of the cleanliness of the showerhead during our morning shower? How often do you ever take a little Clorox to the showerhead?

Well according to a study published in PNAS this month (PNAS, Sep 2009; 106: 16393 – 16399) by Leah M. Feazel, Laura K. Baumgartner, Kristen L. Peterson, Daniel N. Frank, J. Kirk Harris, and Norman R. Pace, titled: Opportunistic pathogens enriched in showerhead biofilms, the showerhead is a wonderfully warm, moist, and dark niche perfect for growth of microbial communities.

Why is this a problem?

Besides the fact that it’s raining bacteria on our heads, the authors explain that increasing incidence of pulmonary infections by nontuberculosis mycobacteria (NTM) and other opportunistic pathogens has been linked to the use of showers instead of baths. Identification of the microbial communities living in the showerhead as biofilms as well as the water moving through the shower (and causing aerosolization of the bacteria present) is an area that needs further review.

Methods:

For this study, they took swabs from interior surfaces of 45 showerheads from 9 US cities and collected water moving through the showerhead’s at 12 of the sites. They also compared showerheads and water from homes where the water was well fed vs. municipal water fed showers, which are more heavily chlorinated. The DNA was purified and rRNA genes were amplified with PCR and sequenced and then categorized phylogenetically to determine what genera were present in cloned libraries of the PCR products.

Results:

The results were very consistent. Sequencing analysis determined that a common resident of showerheads fed by municipal water is Mycobacterium species, and specifically, M. avium and M. gordonae. M. avium is an opportunistic pathogen and a major cause of pulmonary infections by NTM. Even samples that tested negative for Mycobacteria via the 16s gene sequencing experiments were found to be positive when examined using qPCR.

Mycobacterium spp. DNA predominated in biofilms collected from metropolitan water systems from New York City,  Denver , Illinois, and North Dakota.  In contrast, water coming though the contaminated showerheads resulted in much lower levels of the Mycobacterium spp. (about 100 fold less).

The authors speculate that the higher than average levels of Mycobacterium spp. collected from biofilms in showerheads with municipal water as a source is the result of chlorinated cleaning treatments. Mycobacterium has a competitive advantage over the other bacteria due to resistance to chlorine. Indeed, when samples from the Denver area were collected on repeat visits after which the showerhead had been cleaned with bleach, the presence of Mycobacterium spp. increased as compared to the sample collected on the first visit.

Surprise results?

Interestingly, well water fed showerheads had no Mycobacterium spp. DNA in the water or showerhead. However, some of the water wells examined in Denver that are located close to areas subjected to heavy drilling for oil and gas show high levels of the bacterial genera Methylocystis spp. and Methylobacteria spp, known methane and methanol metabolizing organisms. The authors note that the analysis of microbial communities in water can provide insight into the local geochemical environment.

The authors also tested whether the showerheads contaminated with Mycobacterium spp. would result in aerosolized bacteria (and thus be a cause of pulmonary infections). Aerosols were collected for 20 minutes from running shower water from three showerheads positive for Mycobacterium spp. and mycobacterium DNA was not isolated. It is likely that the greatest amount of aerosolization occurs during the initial flush of water through the showerhead and that after 20 minutes, the samples were extensively diluted.

Discussion:

This interesting study provides a thorough analysis of the difference between shower biofilms and the water moving through the biofilms and compares municipal water vs. well water on their impact on microbial communities. The presence of bacteria in our water and air is not a big surprise. There are at least 10 times more bacteria in and on the human body than human cells (approximately 10¹⁴ bacterial cells vs. 10¹³ human cells (ref 1-2)). Most bacteria in our environment are harmless to healthy individuals with functioning immune systems. However, Mycobacterium spp. can pose a health risk to even healthy individuals without additional risk factors such as smoking or immune deficiency. The authors conclude that more studies need to be performed on whether showerheads are linked to the rise in NTM caused illness.

Final note:

What should we do while we wait for further research on the impact of showerhead biofilm on public health? Most people probably let their shower run for a minute or two before getting into the water. This probably helps to flush out any heavy concentrations of biofilm that accumulated overnight and dilutes any aerosols. The other option? Take more baths.

References:

1. Savage, D. C. (1977), “Microbial Ecology of the Gastrointestinal Tract”, Annual Review of Microbiology 31: 107.
2. Berg, R. (1996), “The indigenous gastrointestinal microflora”, Trends in Microbiology 4: 430. 

Thanks for reading!

Suzanne (follow me on twitter: http://twitter.com/SuzyScientist)

This entry was posted on Saturday, September 26th, 2009 at 12:40 pm and is filed under Current Events, New papers in microbiology. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.