Yorgos Karahalis / Reuters

In 1654, Rembrandt painted a woman, in Amsterdam, bathing in a stream. As she lifts her nightdress above her knees and treads deeper, the woman is stepping from one world into another. Among art historians, the transition she is making is metaphorical. But to a biologist, it is also ecological.

This post is adapted from Dunn’s new book.

We imagine water to be clean, and we imagine clean to mean lifeless, and yet all the water you have ever bathed in, swum through, or drunk has been full of life, from bacteria to tiny crustaceans. So, too, the pipes in which it travels. As water passes through pipes in general and showerheads in particular, a thick biofilm builds up. Biofilm is a fancy word that scientists use to avoid saying “gunk.” It is made by individuals of one or more species of bacteria working together to protect themselves from hostile conditions—including the flow of water, which constantly threatens to wash them away—via their own excretions.

In essence, the bacteria poop a little indestructible condominium in your pipes, built of hard-to-break-down complex carbohydrates. But when the pressure is high enough, these species are let loose into the fine aerosol spray of water droplets pelting our hair and bodies and splashing up and into our noses and mouths. And in some regions, but not others, they increasingly seem to be making people sick.


The bacteria in biofilms making people sick are species of the genus Mycobacterium. Mycobacteria are different from most waterborne pathogens in that their normal habitat is not the human body. Instead, they live in the pipes themselves and become problematic only when they, quite accidentally from the perspective of their own well-being, make their way into human lungs.

The Mycobacterium species in showerheads are typically referred to as NTM, for nontuberculous mycobacteria. This means, as you may have inferred, other mycobacterial species are tuberculous, namely the species Mycobacterium tuberculosis and its close relatives.

Mycobacterium tuberculosis appears to have long associated with humans and our extinct relatives. The dangerous form of the pathogen evolved at about the time modern humans moved out of Africa, and spread with us as we moved across the globe. Once we domesticated animals, Mycobacterium tuberculosis evolved into Mycobacterium caprae in goats and Mycobacterium bovis in cows. We gave Mycobacterium tuberculosis to mice and seals, in which it evolved yet other forms. The seal version appears to have traveled to the Americas no later than 700 C.E., where it infected Native Americans (and then evolved into yet another specialized form).

In each case, the bacteria rapidly evolved special traits enabling them to better survive and spread among individuals of each new host. Mycobacterium tuberculosis is an emblematic example of evolution’s mechanisms every bit as elegant as that offered by the differences in beak shape among the species of Darwin’s finches.

Antibiotics, first developed in the 1940s, allowed us to gain a real victory against Mycobacterium tuberculosis, but today many strains of tuberculosis bacteria are resistant to most antibiotics. These resistant strains are (predictably) spreading. All of this is to suggest that the lineage of mycobacteria is one about which it would be good to have a robust awareness.

So far, the risk of infections due to nontuberculous mycobacteria is high only for immunocompromised people, people whose lungs have an unusual architecture, and people with cystic fibrosis. In these individuals, the pathogens can cause pneumonia-like symptoms, as well as skin and eye infections. Unfortunately, the risk of nontuberculous mycobacteria infections is increasing overall in the United States, but just how common infections are and how much more common they are becoming varies geographically.

In some regions, such as California and Florida, infections are common. In others, such as Michigan, they are rare. This difference could be due to differences in either the abundance or the presence of mycobacterial species in various regions. The species in Florida, for example, don’t seem to be the same as those in Ohio, and this might matter. Also, the mycobacterial species associated with infections tend to be the same species and strains found in showerheads, which are different from those associated with soil or other wild habitats.


The story of how water and the life in it get to our houses is both simple and extraordinarily complex. In many parts of the world, water comes from a well sunk into the aquifer beneath a house, or from a municipal water system that draws on an aquifer. Aquifer is a fancy word for the spaces in rocks that hold groundwater (i.e., water that is underground). The groundwater in aquifers ultimately comes from rain.

The infiltration of water into the earth gets progressively slower the deeper the water travels, so slow that the water in the deepest aquifers might be hundreds or even thousands of years old. When you dig a deep well, you tap into ancient, untreated water. This untreated water then flows up and directly into a home. Or it goes to a water-treatment plant. In many regions, such water-treatment plants remove big material from the water (sticks, mud, and the like) and then send it, with little more in the way of treatment, to your house via underground pipes.

Water is safe to drink if it has sufficiently low concentrations of pathogens and toxins. The deeper and older an aquifer, the more likely the water is to be free of pathogens and, biologically, safe to drink. Much of the world’s groundwater is safe to drink without any processing because of time, geology, and biodiversity. Geology influences the safety of the water inasmuch as some types of soils and rocks stop the spread of pathogens from surface waters. The biodiversity present in groundwater also helps to kill pathogens. Indeed, the more kinds of life present in groundwater, the less likely a pathogen is to survive.

The natural filtration of water by living organisms and time is of enormous benefit to humans. Unfortunately, in many regions, we haven’t set aside enough wild land for nature to do its work, or we have polluted groundwater, or, in some cases, there simply isn’t enough groundwater available to supply large human populations. Under such circumstances, we must rely on human ingenuity to make water from reservoirs, rivers, or other sources safe to drink.

Human ingenuity relies heavily on biocides. In the United States, all municipal (city) water is now treated with biocides at treatment plants. In addition, it is typically treated with extra biocide as it is leaving the treatment plant because the pipes in American water systems tend to be old, causing water to leak, stagnate, and become a breeding ground for pathogens. Even after disinfection with biocides, the water leaving treatment plants is not sterile. Instead, it is water in which the most susceptible species have been killed and the toughest species have survived, alongside the dead bodies of the susceptible species and the food those susceptible species were eating.

If ecologists have learned anything in the past hundred years, it is that when you kill species but leave the resources upon which they feed, the tough species not only survive but thrive in the vacuum created by the death of their competition. In the case of water systems, we would predict the species that thrive to be those that are resistant to or even just slightly more tolerant of chlorine or chloramine. Mycobacterial species tend to be very tolerant of chlorine and chloramine.


In 2014, I teamed up with Noah Fierer, a biologist at the University of Colorado at Boulder, and a large team of collaborators (including Matt Gebert, a technician in Noah’s laboratory who ultimately did most of the work) to begin what is probably the largest-ever study of the ecology of showers and showerheads. In the average American showerhead, the biofilm that grows contains many trillions of individual organisms, layered as much as half a millimeter thick. The mystery was why these showerheads sometimes abound in mycobacteria and in other cases lack them entirely.

Medical researchers have predicted that mycobacteria might be more common in well water inasmuch as it is less controlled, less treated, more susceptible to nature’s whimsy. But as ecologists, Fierer, Gebert, and I, along with the rest of our team, also had to contemplate the opposite—namely, that mycobacteria might actually be more common in the showerheads of people with municipal water, particularly that from treatment plants and countries that use chlorine or chloramine, particularly water from such plants in the United States.

When we examined our data, we found that the concentration of chlorine in the tap water from homes using municipal water in the United States was 15 times as high as that of homes with well water. Mycobacteria were twice as common in municipal water as in well water. In some showerheads from municipal water systems, 90 percent of the bacteria were one or another species of Mycobacterium. By contrast, many of the showerheads from houses with well water had no Mycobacterium. Instead, those biofilms tended to have a high biodiversity of other kinds of bacteria.

In Europe, the abundance of mycobacteria in showerheads from well-water systems was low, just as in the United States. But it was also low in European showerheads from houses with municipal water (half that of municipal systems in the United States), as might be expected given that many European municipal water systems do not use biocides at all. In our samples, the residual chlorine measures in European tap water were 11 times less than in tap water from the United States.

As we were pondering these results, Caitlin Proctor at the Swiss Federal Institute of Aquatic Science and Technology published a new study very much in line with what we were finding. Proctor and her colleagues compared the biofilms of the hoses that lead into showerheads from 76 homes around the world. They found that samples from cities that did not disinfect their water tended to be thicker (more gunk), but samples from those that did disinfect their water were more likely to be lower in diversity and more dominated by mycobacteria.

Our results match Proctor’s and seem to suggest that our fanciest treatment technology is creating water systems filled with microbes that are less healthful for humans than those found in untreated aquifers (or at least untreated aquifers that have been deemed safe). In our analysis, the mean abundance of the most pathogenic strains and species of Mycobacterium in showerheads in a particular state was highly predictive of the number of mycobacterial infections in that same state.


But there are twists in the story already. One of the twists is Christopher Lowry.

Lowry has spent 20 years studying Mycobacterium vaccae. He and his colleagues have found that exposure to this species boosts production of the neurotransmitter serotonin in the brains of mice and humans. Increased serotonin production tends to be linked to greater happiness and reductions in stress. Indeed, Lowry has shown that, at least in mice, inoculating individuals with Mycobacterium vaccae leads them to be more resilient to stress.

Lowry suspects that many Mycobacterium species may have effects similar to those he has observed. The only way to know for sure is to test them one by one, and so this is what Lowry is now doing. He is culturing the mycobacteria we have gathered in showerheads to see whether any other species behave like Mycobacterium vaccae. If they do, it may mean that some of the Mycobacterium falling on you from your showerhead may be beneficial in reducing your stress.

Lowry’s research reminds us that sorting out just which kinds of microbes are good and which are bad is gnarly, convoluted, and hard. Some mycobacterial strains may make you sick; others may make you happy. As for whether it is worth buying a new showerhead every so often, we don’t know yet. But I suspect that after reading this, you will go home and change yours anyway.


This post is adapted from Dunn’s new book, Never Home Alone: From Microbes to Millipedes, Camel Crickets, and Honeybees, the Natural History of Where We Live.

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