Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder

The World Inside Your Showerhead

Don't panic, but there is a largely unknown world of tiny creatures living inside your showerhead. Researchers at the University of Colorado Boulder are working to illuminate the secrets of this dark, damp microcosm.   

"Microbes are everywhere you look. There's an ecosystem inside your showerhead; it just happens to be man-made," said Noah Fierer, associate professor of Ecology and Evolutionary Biology and a fellow in the Cooperative Institute for Research in Environmental Sciences (CIRES) at CU Boulder. Fierer studies household microbial communities—from your cutting board to your television to your toilet. "Because humans spend so much time indoors, we are focusing on the microbes that live inside your homes," said Fierer, who, with research associate Matt Gebert, is putting the focus on this everyday bathroom fixture.

Matt Gebert uses a scanner to organize showerhead swab samples. Photo: Lauren Nichols.

Due to its unique environmental conditions, your bathroom showerhead is an ideal breeding ground for a dynamic community of specialized bacterial species. These organisms are nurtured by warm, moist conditions—yet must also endure short spurts of scalding temperatures, low-nutrient, chlorinated waters, and ample stretches of bone-dry surroundings.  

Microbial bacteria in this arena have the potential to influence multiple facets of human health. For example, those who are immunocompromised may be susceptible to illnesses, such as non-tuberculous mycobacterial disease, or NTM. The ecology of this disease is not well known, and this study (partly funded by the CIRES' Innovative Research Program) is starting to address where NTM-causing microbes may be found, and what types of bathroom conditions may encourage their growth. But these communities are complex: some people may even benefit from exposure to these microbes, gaining strengthened immune systems and increased resistance to disease.

The presence of specialized microbes in these extreme environments could also give microbiologists insight into what environmental factors drive the development of bacterial communities across the globe. Yet, despite their importance, to date, little is known about these isolated microbial civilizations.

For their research, Fierer’s group is tapping into a powerful technique known as citizen science, which calls upon members of the public to aid in scientific research—in this case, to collect samples from showers on two continents.   

The research team sent 1500 sampling kits to pre-registered households spanning 49 of 50 U.S. states, Puerto Rico, and parts of Europe. Once citizens received their kits, they completed a questionnaire revealing information such as showerhead type, shower metal type, and frequency of shower usage. Then they used a basic chemistry kit to take simple water quality measurements such as chlorine level, pH, and water hardness. Next came the swabbing. After unscrewing the face of the showerhead, each participant carefully ran a sterile swab along the interior, scooping up the delicate layer of microbial biofilm clinging to the metal's surface.

citizen science sample packets received from across the nation. photo: Matt Gebert.

These samples were anonymously mailed back to CU Boulder, where the team extracted DNA and genomically sequenced it using resources at the University of Colorado BioFrontiers Institute. Genetic sequencing technology is a powerful tool in the field of ecology and evolutionary biology: in this study, it allows the researchers to identify exactly what bacterial species were present in the shower slime. With this information, Fierer and Gebert will be able to determine the relative species abundance and microbial community structure of each showerhead ecosystem.  

Samples are now pouring in from households across the United States and Europe for this in-progress study, illustrating the strength of this research approach: helping to overcome the previous limits to what a single team of researchers could do by themselves. "Without citizen science," said Gebert, "there's no way we could get the sheer volume or range of samples we have now."

Not only does this research have the potential to influence the medical community, but it also gives microbiologists a better understanding of the wide variety of microbial niches in homes across the world. And this picture is always evolving. The more we humans change our surroundings, like changing the chemistry of our tap water, the more we are changing the types of microorganisms we interact with.

"We don't live in a sterile world," said Fierer. We are intimately connected with the microscopic population living among us, and with studies like this, scientists are developing a more complete picture of this dynamic, indoor microbial world. The answers may lie in what—or who—is living inside your showerhead.

CIRES is a partnership of NOAA and CU Boulder.