5
BATHING IN A STREAM OF LIFE
We must conclude that there are more animalcules or minute fishes in the sea, than has ever yet been thought of.
—ANTONY VAN LEEUWENHOEK
I bathe once a month, whether I need it or not.
—QUEEN ELIZABETH I
In wine there is wisdom, in beer there is freedom, in water there is bacteria.
—SIGN ON THE WALL IN A PUB IN DUMFRIES, SCOTLAND
IN 1654, REMBRANDT painted a woman, in Amsterdam, bathing in a stream. The woman has placed an elegant red robe on a rock. She wades in the water, lifting her nightdress above her knees so that it does not get wet as she treads deeper. It is night and dark and the woman’s skin glows as it disappears into the water. The painting evokes earlier works from ancient Rome and Greece. The woman who is stepping into the stream in Rembrandt’s painting is stepping from one world into another. Among art historians, the transition she is making is metaphorical.1 But to a biologist like me, it is also ecological. In entering the water, she is suddenly exposed to an entirely new group of species, of microbes, of fish, and much else. We imagine water to be clean and we imagine clean to mean lifeless, and yet all water you have ever bathed in, swam through, or drunk has been full of life.
The stream Rembrandt painted looks like the water in the small canals and streams near Amsterdam. The woman is probably Rembrandt’s lover, Hendrickje Stoffels. Even if Rembrandt did not intend to depict a particular body of water, his reference and inspiration would have been what he knew and had seen. Assuming this water was similar to the water of nearby Delft a decade or so later, it is likely to have been inhabited by microbes very much like those that Leeuwenhoek described living in the canal in front of his house. Of course, the water to which you expose yourself today is likely very different from that to which Rembrandt’s lover exposed herself. By different I don’t mean it is devoid of life; I mean only to say that as you slip into the bath or stand beneath the shower, you are covered in life-forms likely to have been rare in Delft, entirely different beasts. Recently, I found myself thinking a lot about these species.
It all started when Noah Fierer, my collaborator at the University of Colorado with whom I first studied the dust in houses, emailed me in the fall of 2014 to say he had an idea for a project. He had stumbled upon a showerhead mystery, and a real doozy at that. “Are you in?” he asked, before explaining what I might be in for. “I’ve been talking to people about showerheads and we need to work on them. It is going to be great.” What followed was the kind of shorthand conversation upon which discovery depends, a conversation in which Noah provided a brief sketch of an idea and then, having offered that sketch, assumed that, based on the sketch, I would fill in the blanks. “Cool either way,” he said, which is code for, “If you aren’t in, you are missing out and I won’t let you forget it, ever, but you don’t have to collaborate if you don’t want to. But if you are in, let’s get it moving.”2
The sketch was this. The tap water that flows into your house, and eventually into and through your showerhead, is alive. Leeuwenhoek found both bacteria and protists in rainwater as well as in the water from his well. So too have subsequent researchers. Tap water is no more or less full of life than is rainwater. In Denmark, where I work for part of the year, for instance, one can encounter small crustaceans in tap water.3 In Raleigh, where I can be found the rest of the year, tap water contains the bacterial species Delftia acidovorans with reasonable frequency.4 Delftia, which was first isolated in the soil of Leeuwenhoek’s own Delft, has the ability to concentrate minute quantities of gold found in water and precipitate them. It also has unique genes that allow it to thrive in mouthwash (or a mouth that has recently been mouth-washed). All of this has long been known and is interesting, but neither news nor Noah’s focus. What had intrigued Noah was the knowledge that 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.”
Biofilm is made by individuals of one or more species of bacteria working together to achieve the common goal of protecting themselves from hostile conditions (including the flow of water, which constantly threatens to wash them away). The bacteria make the infrastructure of the biofilm out of their own excretions.5 In essence, by working together, the bacteria poop a little indestructible condominium in your pipes, a condominium built of hard-to-break-down complex carbohydrates. Noah wanted to study the species living in the biofilm of showerheads, species fed by tap water and, when the pressure is high enough, let loose into the fine aerosol spray of water droplets pelting our hair and bodies and splashing up and into our noses and mouths.6 He wanted to study them because they were interesting but also because, 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, such as Vibrio cholerae. The normal habitat of the Mycobacterium species found in tap water is not the human body. Instead, they live in the pipes themselves. These pipe-loving mycobacteria (plumbingophiles) are not ordinarily pathogens. They become problematic only when they, quite accidentally (from the perspective of their own well-being), make their way into human lungs. In this, mycobacteria and several other pathogens associated with the new habitats we have made in houses (such as Legionella bacteria) represent a challenge very different from those we normally encounter when considering pathogens, a challenge associated with the ways we build our homes and cities.
The Mycobacterium species in showerheads are typically referred to as NTM, where NT stands for “nontuberculous” and M stands for “mycobacteria.” This means, as you may have inferred, other mycobacterial species are tuberculous, namely, the species Mycobacterium tuberculosis and its close relatives. We imagine the worst monsters of history to be beasts with bad breath and extra arms that one battles using a shield and a longsword, the beasts out of, say, Viking sagas. But the real demons of the past looked far more like Mycobacterium tuberculosis. Invisible to the naked eye, they “looked” like nothing other than their consequences, which were horrible deaths.
Mycobacterium tuberculosis is the cause of tuberculosis in humans. Tuberculosis killed one in five adults between 1600 and 1800 in Europe and North America.7 Mycobacterium tuberculosis appears to have long associated with humans and our extinct relatives and ancestors. The dangerous form of the pathogen evolved at about the time modern humans moved out of Africa (at about the same time that we have the first good evidence of houses and we started coughing on each other more). Mycobacterium tuberculosis spread with us. Once we domesticated goats and cows, we then gave them Mycobacterium tuberculosis, and, in their very different bodies, confronted with the realities of their unique immune systems, Mycobacterium tuberculosis evolved into Mycobacterium caprae in goats and Mycobacterium bovis in cows. We gave Mycobacterium tuberculosis to mice and it evolved into a form better able to take advantage of the immune systems of mice. We gave it to seals, in which it evolved yet another form. That form appears to have traveled, with seals, to the Americas no later than 700 CE, where it infected Native Americans (and then evolved into yet another specialized form).8
In each case, the bacteria rapidly evolved special traits enabling them to better survive and spread among individuals of each new host. The immune system and body of a seal are different from those of a human and so require special tricks. So too the immune systems and bodies of mice, or goats, or cows. Individual lineages of the microbe evolved these tricks. The human form of the pathogen even appears to be adapted to different populations of human hosts (which, because tuberculosis is deadly to even relatively young people, also led to adaptations of those human populations to the pathogen). 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. Our shining weapons, once the great silver bullets of medicine, now seem ever more like wooden swords. 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. Nothing prevents the nontuberculous mycobacterial species found in showerheads from adapting, as Mycobacterium tuberculosis has, to take advantage of us. They might adapt to better thrive in our water systems or, more worryingly, our bodies.
So far, the risk of infections due to nontuberculous mycobacteria is high only for immunocompromised people, people whose lungs have an unusual architecture, and individuals with cystic fibrosis. In these individuals, nontuberculous mycobacteria can cause symptoms like those of pneumonia, 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 vary geographically. Some regions seem to have many more infections than do others. 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.9 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.10
On the basis of the information I just gave you about mycobacteria, I could more or less guess what Noah had in mind for our impending showerhead investigation, the method for our plunge into the gunk. I could guess because in the years since we first studied forty houses in Raleigh, Noah and I have developed a way of working together that repeats itself. And, anyway, he had me at the words “showerhead mystery.” I responded to Noah’s email, committing, in a sentence or two, to coordinating the sampling of showerheads around the world.11 So began what is probably the largest ever study of the ecology of showers and showerheads. It is based on trust: I trust that nine times out of ten, if Noah is excited about something, then it is probably something pretty interesting.12 I’ve never heard anyone talk about trust in science, yet it influences what I do in the lab every day. A huge component of modern science is social, and inside a researcher’s most trusted social group, that quorum of very trusted colleagues, science moves faster. Everything happens faster. Conversely, most scientists have colleagues they don’t trust or colleagues with whom trust hasn’t yet been established. Such collaborations are slower, more deliberate, less able to respond to wild schemes in the middle of the night. I trust Noah, so, with him, I’m game for a wild scheme. We have now worked together on a half dozen major projects (beetle armpits, belly buttons, the microbes of forty houses, the microbes of a thousand houses, global forensics, and more). The science we work on together comes easily (albeit, as that list suggests, at times also idiosyncratically).
Figure 5.1 Map showing the prevalence of pulmonary nontuberculous mycobacteria cases in the United States between 1997 and 2007 among a sample of adults aged sixty-five and older. Hawaii, Florida, and Louisiana are among the states in which mycobacterial infections are most common per capita. Much as when Snow mapped cholera cases, for us, a key piece of solving the mycobacteria mystery is mapping where both NTM infections and Mycobacterium species occur. (Data from J. Adjemian, K. N. Olivier, A. E. Seitz, S. M. Holland, and D. R. Prevots, “Prevalence of Nontuberculous Mycobacterial Lung Disease in U.S. Medicare Beneficiaries,” American Journal of Respiratory and Critical Care Medicine 1 85 [2012]: 881–886.)
Earlier in 2014, I had just finished collecting data for a project in which my Danish colleagues and I engaged children in Danish schools to take samples of the life flowing from the water fountains and spigots of their schools. So I knew something about the life in water, but in considering the special case of showerheads, there was more to learn. In Denmark, we found thousands of kinds of bacteria in tap water, as have similar studies of tap water in the United States and elsewhere in the world. The species we or others have found in tap water include bacteria, amoebae, nematodes, and even small, leggy crustaceans. Although tap water is high in biodiversity, it is typically low in biomass, the living mass of life. Tap water does not contain much that might be construed (even by bacteria) as food. Nutritionally, it is a kind of liquid desert, such that many species persist, but none thrive. The biofilms in showerheads are different.
The water that flows through showerheads tends to be warm, which makes it easier for bacteria to grow. It also tends to pool for many hours between uses (which keeps the bacteria from drying out). Given these conditions, once bacteria and other microbes establish in biofilms along the pipes within showerheads, they have the environment they need. In that environment, like sea sponges, they are able to harvest whatever flows past them. The more the water flows, the more they can harvest. The resources available in any particular drop of water are modest, but the resources available in the gallons and gallons of tap water that flow collectively through a showerhead can actually be quite great. As a result, the biomass in showerheads is twice or more that in the tap water itself. What’s more, that biomass is composed of far fewer species than in the tap water, hundreds or even just tens rather than thousands.13 These species come to form relatively stable ecosystems in which each species plays a role. In biofilms one can even find predatory bacteria swimming, as Leeuwenhoek might have said, like “pike[s] through water.” Right now, in your showerhead, these tiny “pikes” are latching on to other bacteria, drilling holes in their sides, and releasing chemicals that digest them. Showerhead biofilms also sustain protists that eat the “pikes,” and even nematodes that eat the protists, as well as fungi doing their own fungal thing. This is the food web that falls upon you as you bathe. Each day, life falls on you, midmeal (theirs, not yours; though that too), flipping back and forth, stunned by the disruption.
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. When we began our project, no one could explain these differences. In considering an ecological system like a showerhead, about which little is known, my intuition about the first step is nearly always the same. My intuition, like that of any scientist, reflects some mix of my scientific training, what I am good at, and what I enjoy. What I always want to know first is how life with all of its attributes (its abundance, diversity, and even consequences) varies from one region to the next. In the context of showerheads, I wanted to know how many species the most diverse showerheads had, where showerheads were most diverse, and then, for Mycobacterium, how the identity and abundance of particular species vary from place to place. To me, until we know about the patterns in such variation, it doesn’t make much sense to take any next steps because we don’t really know what it is we need to explain (conversely, to some scientists, this step is not even viewed as part of science, which is to say we scientists differ as much, perhaps, as do our showerheads).
Our first step, then, was to engage people around the world to swab their showerheads and, having done so, send us back a sample of the scuz they found. Folks in my lab would then organize the data about the person who took the sample. The sample would then be sent to Noah’s lab, where his technicians, or postdoctoral researchers, would decode the sequence bits of the DNA to produce at least a coarse list of the bacteria and protists present in each sample, including mycobacteria and other potentially problematic species such as Legionella pneumophila, the cause of Legionnaire’s disease. Matt Gebert, one of Noah’s students, would then identify the specific species of Mycobacterium present in the sample by decoding a specific gene (hsp65) known to differ from one Mycobacterium species to the next. The samples would then be sent to other collaborators, each of whom would study some other aspect of the story, culturing the microbes from the showerheads, for example, in order to decode their entire genomes, base by base. It was to be an all-taxa biological inventory of the world’s showerheads. But first, we had to convince people to send us samples from their showerheads.
We used our social networks to search for people around the world willing to be part of our project. We tweeted. We wrote blog posts. We contacted friends and collaborators. We tweeted again. Many people were interested and signed up. We then got ready to send out kits, but before we could do that, people who had read the protocol started to send us questions about the project. Reaching out to thousands of potential project participants is a good way to quickly learn what you do and do not know about a particular topic and about the clarity of your protocol. Thousands of people simultaneously begin to pay attention to something they had not paid attention to quite so much before. This initial moment of project engagement can be revelatory, though not always in the ways one anticipates. In the context of showerheads, it quickly became clear that we did not know enough about the geography of showerheads themselves. In the American showerheads we worked with initially, we could unscrew the top of the showerhead, look the shower scuz right in the eyes (or where the eyes would be if shower scuz had eyes), and swab it. We had proposed this be done in Europe as well. We hadn’t accounted, however, for differences among countries in the type of showerheads people like to use. We began to receive emails from disgruntled Germans indicating that we knew nothing about how a German showers. German bathrooms (and, it would turn out, those of most other Europeans, though it was only the Germans who emailed) have showerheads attached (permanently) to flexible hoses. This precluded the kind of sampling we described in our protocol. The Germans were writing to tell us as much. The emails came to me. They went to various people in my lab. They even, when we didn’t answer fast enough, went to the department’s administrative assistant, Susan Marschalk. And then, when she didn’t respond fast enough (to say she was the wrong person to contact), emails were sent to other people with even less to do with the project. Department head. Deanlet.14 Frustrated emailers know no bounds. In response, we changed our protocol to accommodate European showerheads. We would soon realize that the hoses were not the only differences between showerheads in the United States and Europe, not hardly.
Showers are a very, very modern contrivance with complex consequences for our bodies, consequences no one anticipated when we first began to stand beneath them. For most of our mammalian history, our ancestors didn’t take showers or baths. They probably didn’t even really swim very often. They might have cleaned themselves, clumsily. Cats bathe themselves with their tongues. Dogs do the same, albeit less rigorously. But even a moment spent contemplating this possibility (try licking your own lower back) in our own history suggests that it has been a long time since this has been possible for us. Many nonhuman primates groom each other, but the grooming mostly has to do with picking off visible bits and pieces of things that might be lice (or might not be). Similarly, some mammals roll in soil or mud,15 but this, too, seems to be more about controlling animal parasites such as lice than it does with controlling microbes or odors. Some Japanese macaques bathe in hot springs, but they do so to warm up.16 Chimpanzees that live in savannahs get into the water occasionally, but only when it is really hot, presumably to cool off. Chimpanzees that live in rain forests don’t deign to dip.17 In short, if wild mammals are an indication, bathing is unlikely to have been a big part of our ancient past.
As for our more immediate past, our human past, real bathing, in water, is both recent in our history and more varied among cultures and eras than might be superficially apparent. Bathing is one of those features of human culture that proves that history is not necessarily a story of progress, or at least progress as we tend to imagine it, progress as the steady change of societies of the past toward ways of life ever more like our own.18 The Mesopotamians were not big bathers. Nor were the ancient Egyptians. The Indus River Valley people had a big central “great bath,” but we can’t be sure how they used it. It could have been for daily baths. It might have been for some sort of priestly, ritual ablutions.19 But it also could have been the place they killed cows before eating them. Archaeology is tricky that way. The first Western culture that embraced bathing was Greek. Greek bath culture was expanded upon by the Romans. Superficially, it is this Greco-Roman pro-bathing culture in which we still find ourselves today, a culture in which bathing is viewed as not simply hygienic but also, in some real way, goodly, or even godly. We look at Roman baths and we are reminded of our own baths. We and the Romans are the same (except we substituted football for gladiator fights and they staged events in which naked emperors fought ostriches).20 A clean life is a good life, a life to which the people of Western cultures have aspired since the classical period of Athens, a connection between our civilization and theirs. A well-bathed life is a good life. This is our subconscious mantra, one we wake up to each morning and embrace beneath the showerhead.
Yet, although the Greeks and Romans were both bathing cultures that valued hanging out naked in water, the water itself is likely to have been far from crystal clear. An excavation of the Roman baths at Caerleon, a site north of Newport in what is now Wales, discovered drains clogged with chicken bones, pig trotters, pork ribs, and mutton chops. These were the “light snacks” eaten “poolside.” And though Romans generally regarded bathing as healthful and even recommended baths as treatments for some ailments, individuals with wounds were warned not to bathe because the dirtiness of the water might lead to disease.21 The bathwater of Roman times would be more likely to cause disease than help prevent it.22
The Romans were far more likely to bathe, whatever the condition of the water, than those who followed them. The Visigoths, who came over the hill as the Western Roman Empire and Rome itself fell, with their shiny belt buckles and mustaches, were not much for baths. After the fall of Rome came a general shift toward less reading, less writing, and less infrastructure, including plumbing, and less bathing. This shift was persistent. It lasted, with local and largely ephemeral exceptions, from the end of the Western Roman Empire around 350 CE until well into the 1800s, which is to say nearly fifteen hundred years.23 Not only did Europeans bathe very little during this period, many even forgot how to do so. The Romans made their own soap for bathing, but the daily know-how necessary to produce soap was forgotten in many regions, so little was the stuff used. In 1791, a French chemist named Nicholas LeBlanc invented a way to make soda ash (sodium bicarbonate) cheaply, which could be mixed with fat to produce a hard bar of soap. But even this more effective soap remained a luxury item. Bathing with soap or without was, at best, a monthly sort of affair and often not even that common. Nor was the lack of bathing strictly a tradition of the common folk. The kings and queens of Europe talked about their annual baths.24
The fall of the Western Roman Empire thus had many consequences, some of which lasted until long after the Renaissance. With the Renaissance, art and science were reborn, but not bathing. Even Rembrandt’s lovely mistress, dipping her ankles in the water, is unlikely to have done so very often, and she may not have stepped much deeper in, the preference for bathing being to wash the feet and the hands, but not necessarily the rest of the body. And, given that the water into which she was dipping was likely to be the same water into which chamber pots were emptied, the parts of her that weren’t washed might have been more hygienic than those that were. Leave it to an ecologist to take the romance out of what seems to be an unambiguously romantic scene.
Overall, the question in the long history of bathing is why some people took it up again, rather than the reverse. Until very recently, most humans were unbathed. They would have smelled of the odors produced by the bacteria that grow on the human skin such as from armpit bacteria of the genus Corynebacterium. In the context of a city, the permanent funk rising up out of the pits of the people would have been matched only by the worse smells rising from other body parts. This would have been pungent in general, but especially when clothes weren’t washed very frequently. We like to imagine, from our modern perspective, that given the opportunity, people would take a bath or stand beneath a watering can. But they didn’t. Leeuwenhoek didn’t. Rembrandt didn’t. Then, in the 1800s, some people began to bathe regularly again. We can see the change as easily in the Netherlands, where it has been well studied, as we can see it anywhere. The answer has little to do with hygiene and much more to do with wealth and infrastructure.
In the early 1800s most water used in the cities of the Netherlands came from canals, rainwater collection, or, more rarely, wells. By that time, the surface water in urban and even many village canals was polluted with human and industrial waste. This pollution often affected shallow wells (much as it did later during the cholera outbreak in Soho in London) so much that the water smelled too bad to drink (as was also the case in London). Only the more affluent collected rainwater, and even then there was not typically enough water for daily use. Eventually, a few Dutch cities began the major shift to systems in which water was pumped from lakes and groundwater systems outside their boundaries into their centers. Two of the first cities to do so were Amsterdam, which had little of its own groundwater, and Rotterdam. In Amsterdam, it was necessary to pump water in to have enough for inhabitants and to stock ships traveling from the port. Rotterdam had enough of its own groundwater, but it had the problem that at low tide, the canals didn’t have enough pressure to wash the feces out of the city. And so the city needed to pump water in, not so much as a source of drinking or other daily water but instead to flush the feces out into the sea.
Once water began to be piped into cities, it became a commodity. The rich accessed this commodity by paying for pipes to be laid directly to their property; the middle class, by paying for buckets of pumped water. In this context, it didn’t take long for water and all that it could be used to do to become symbols of affluence. To be able to afford to flush away the odors of the toilet was prestigious. To be able to wash so often that the body did not smell was prestigious. The affluent installed water closets in their homes (to flush their waste) and then, more slowly, baths. Once this trend started, it never really stopped. It caught on in cities across Europe, cities in which to use a water closet was to be rich, to bathe was to be rich, and to be unable to bathe frequently was an honest indication of poverty or of the scarcity of clean water.25 In time, showers were invented as a new means of “getting clean.” In the coming years, this sense of cleanliness would be tied to the germ theory of disease and our desire to distance ourselves from all microbes in light of the knowledge that some cause disease. Since then, our desire to be clean and the amount of money we spend on getting clean have increased each year. Our desire to clean ourselves is fueled by a huge industry dedicated to convincing us we are dirty. We scrub, we buy sprays, and we stand, earnestly, beneath the shower’s spray. Then we rub ourselves with salves. Billions and billions of dollars are spent not only to clean ourselves in ever newer ways, with ever more products, but also to make our bodies smell like flowers, fruits, or musk after we have done so.
What is rarely discussed is what makes our bodies or even water itself “clean.” In the late 1800s in the Netherlands or London, clean meant that the water didn’t smell, and when you used it, along with soap, on your body, neither did you. Once it was discovered that pathogens such as Vibrio cholerae caused disease, clean meant that water lacked these pathogens (or at least that such pathogens were rare). Later, clean would also come to mean free of dangerous concentrations of particular toxins. What clean has never meant, and will never mean, is sterile. Every bit of water that has ever sprayed down on you from the shower, risen around you in the bath, or poured into you from a glass or a sealed bottle has been full of life.26 As is so often the case with the life in homes, what differs from one house or tap to the next is not the presence of life but instead its composition, which species are present and what those species do. This composition depends on where your water comes from in the first place.
THE STORY OF how water and the life in it get to our houses is both simple and extraordinarily complex. The simple part is the plumbing indoors. A water pipe comes into a house and branches into two. One branch travels to your water heater, where the water is warmed, before traveling again alongside the other pipe for water that is not warmed. The pair of branches then rebranch, in tandem, to get to each of your faucets and your showerheads.
The complexity has to do with what happens to the water before it reaches your house. The water’s path depends very much upon where you live. In many parts of the world, water comes from a well sunk into the aquifer beneath a house, or it comes from a municipal water system that draws on an aquifer. Aquifer is a fancy word for the spaces in rocks that hold groundwater (where groundwater really just means water that is underground).27 The groundwater in aquifers ultimately comes from rain. Rain falls on trees in forests, on grass in lawns, and on crops in fields. Over a period of hours, days, or even years (depending on local geology), rainwater seeps down into the soil, meter by meter. The infiltration of water into the Earth gets progressively slower the deeper the water travels. At great depths, the movement is very slow such 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 lacks pathogens (or has them in only very low concentrations) and if it has concentrations of toxins sufficiently low so as not to make us sick (with the concentration dependent on the toxin in question). 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. If a pathogen is a bacterium, it must compete for food, energy, and space. It must survive the antibiotics produced by other bacteria in the groundwater. It must avoid being eaten by predatory bacteria (such as species of the genus Bdellovibrio); it must also avoid being eaten by protists. Ciliates alone (such as which Leeuwenhoek encountered in his own water) can eat up to 8 percent of the bacteria around them a day. Choanoflagellates do even better and eat up to 50 percent of the bacteria around them a day. In addition, a pathogen must avoid being infected by bacteriophages, the specialized viruses that attack bacteria.28 The organisms at the top of the food chain in these ecosystems tend to be small arthropods such as amphipods or isopods that have, like cave animals, lost their pigmentation and vision and move through the world guided by their senses of touch and smell. They include so-called living fossils that are relatively unchanged after millions of years of isolation and endemic species found nowhere else. These animals tend to be present only when groundwater is biodiverse, each species carrying out its function. Their presence is thought of as an indicator of water health.29
The life in groundwater ecosystems seems remote and obscure and tends to be studied only at a great distance (by scientists with long poles, drills, and nets). Yet it is estimated that 40 percent of the living mass, the biomass, of all bacterial life on Earth may be in groundwater. Forty percent! In some places, groundwater ecosystems are connected into a vast network of pockets, streams, and underground reservoirs. In others, they are disconnected underground islands. Just which life is present in a particular bit of groundwater is heavily contingent on where the water is, how old it is, and whether it is connected to or disconnected from other groundwater systems. Much as each oceanic island has its own unusual species, so too each groundwater system appears to have unique species found nowhere else. The deep waters of Nebraska and those of Iceland are different in part because the organisms living in the two aquifers from which that water comes have been evolving along separate trajectories for millions of years.
It may seem strange to drink groundwater that has not been treated with some biocide. But many of us do. Most well water is not treated with any biocide, nor is much of the municipal water of Denmark, Belgium, Austria, or Germany. The water of Vienna, for example, flows untreated directly from a karst aquifer. That of Munich is pumped from the porous aquifers of a nearby river valley directly through pipes and out of taps. The natural filtration of water by living organisms and time is thus of enormous benefit to humans. The trick is that it requires large spaces to be set aside where nature can do its work; it needs natural watersheds to be preserved. It requires time too. And it requires us to avoid polluting the groundwater with pathogens and toxins. 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 turns out to be a useful, but slightly crappy replacement for nature.
Figure 5.3 An amphipod, Niphargus bajuvaricus, that lives in the groundwater in parts of Germany. This specimen was collected and photographed in Neuherberg, Germany. If this species tumbles out into your drinking glass, it is a sign that the aquifer out of which your tap water is flowing is probably healthy and biodiverse, a many-legged good omen. (Günter Teichmann, Institude of Groundwater Ecology, Helmholtz Center Munich, Germany).
Human ingenuity relies heavily on biocides. Beginning in the 1900s, water treatment plants in some regions began to use chlorine or chloramine to kill bacteria in water, with the aim of controlling pathogens. This was necessary in regions where the aquifers had become polluted. It was also necessary in the many regions where aquifers were insufficient to provide for growing human populations and water needed to be piped not from the ancient depths but instead from shallow rivers (such as the Thames in London), lakes, and reservoirs. In the United States, all municipal (city) water is now treated with biocides at treatment plants.30 In addition, because the pipes in water systems in the United States tend to be old relative to those in continental Europe and elsewhere, they leak and water stagnates.31 Whereas in natural aquifers older water is better water, the opposite is true in our pipes. In pipes, stagnation can favor the growth of pathogens. To counter such stagnation, water in the United States is typically treated with extra biocide as it is leaving the treatment plant beyond that which is used in similar European treatment systems. Sometimes the biocide is chlorine, sometimes chloramine. Sometimes it is a mix of these two. Water treatment plants can be technologically sophisticated and yet, to the extent to which nearly all rely primarily on getting rid of life through a series of sieving steps (through sand, through carbon, through a membrane), sometimes a dose of ozone, and then killing microbes with a biocide, they are really simple.32 Meanwhile, 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 last 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. They enjoy what ecologists call “competitive release.” They are released from competition, released too, often, from parasitism and predation. 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.
AS NOAH AND I, along with our other collaborators, began to consider the data from the showerhead study, we kept the differences between untreated groundwater, treated municipal water in the United States, and treated municipal water in Europe in mind. 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, Noah 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. Mycobacteria are relatively resistant to chlorine and chloramine. Perhaps tap water was treated with enough biocide to kill most species, most species except the mycobacteria. We found some precedent for this idea. One study of bacteria in showerheads had already noted that cleaning an individual showerhead from a Denver shower with bleach solution led to a threefold increase in the abundance of a species of Mycobacterium.33 It was an anecdote, but an interesting one.
When we looked at our data, we expected only about half a dozen different kinds of mycobacteria from showerhead samples. We expected to find those species cultured in medical study after medical study. Instead, we found dozens of species, quite a few of them apparently new to science. Just which species were present in a showerhead appeared to depend in part on region. Different species dominated in Europe than in North America (and not just because of the different types of showerheads). But even within the United States the species present in Michigan were different from those in Ohio, which were different again from those in Florida and Hawaii. These differences might be due to the different aquifers from which the water comes, or whether the water comes from an aquifer or surface water, or even some aspect of climate or ancient geology.
Yet, while the identity of the species of Mycobacterium present in a showerhead was hard to account for, the abundance of mycobacteria in the showerheads was more predictable. We measured the amount of chlorine in the tap water of the house of each of our participants. The concentration of chlorine in the tap water from homes using municipal water in the United States was fifteen times higher than that of homes with well water. This was enough of a difference, we thought, to have an effect. But we expected a modest effect. The effect was huge. In the United States, mycobacteria were twice as common in municipal water than in well water. In some showerheads from municipal water systems, 90 percent of the bacteria were one or another species of Mycobacterium. In contrast, many of the showerheads from houses with well water had no Mycobacterium. In place of Mycobacterium, the biofilms from houses with well water 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 the abundance of mycobacteria in Europe was also low in 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 eleven times less than in tap water in the United States. As we were pondering these results, Caitlin Proctor at the Swiss Federal Institute of Aquatic Science and Technology in Switzerland 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 shower heads from seventy-six homes around the world. They found that samples from cities that did not disinfect their water (including samples from Denmark, Germany, South Africa, Spain, and Switzerland) tended to be thicker (more gunk), but samples from those that did disinfect their water (including Latvia, Portugal, Serbia, the United Kingdom, and the United States) were more likely to be lower in diversity and more dominated by mycobacteria.
So far, our results match Caitlin Proctor’s results, which match what we expect if the subset of water treatment plants that use biocides kill many species and, in doing so, create conditions that allow mycobacteria to thrive. If true, this would mean that our fanciest water treatment technology is creating water systems filled with microbes that are less healthful for humans than those found in untreated aquifers (or at least that subset of untreated aquifers that has been deemed safe). We couldn’t explain all of the variation in the abundance of mycobacteria among houses. Nonetheless, we hypothesize that, in general, chlorine and chloramine use increases the abundance of mycobacteria in showerheads, which makes it more likely that people will develop a mycobacterial infection. 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, predictive of the pattern shown in Figure 5.1. But there are twists in the story already. One of the twists is Christopher Lowry.
Lowry has spent twenty years studying one particular Mycobacterium species, Mycobacterium vaccae. He and his colleagues have found that exposure to this Mycobacterium 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. Working with a colleague, Stefan Reber, in Germany, Lowry tested this by inoculating average-size male mice with Mycobacterium vaccae. He then introduced those mice, as well as average-size male mice without the bacteria (the control mice), into a cage with aggressive, sumo-size male mice. Afterward, he tested the stress-related compounds in the blood of the average-size male mice. The control mice pissed themselves, cried softly into their wood shavings, and registered high on every stress test. The males that had been treated with Mycobacterium vaccae weren’t stressed at all. Conversations are now ongoing about whether soldiers could be inoculated with Mycobacterium vaccae before they go to war to reduce the risk that they might suffer from posttraumatic stress disorder (inasmuch as they almost certainly will be exposed to traumatic stress). All of this sounds a bit crazy, but even in these early days, it has been recognized by Lowry’s peers as very important work. In 2016, the Brain and Behavior Research Foundation, for instance, ranked the work as one of the top ten (out of five hundred) contributions by researchers that it funded.34 Lowry suspects that many Mycobacterium species may have effects similar to those he has observed of Mycobacterium vaccae. 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.
The showerhead is one of the simplest ecosystems in your house. The average showerhead has dozens, and at most hundreds, of species in it rather than thousands. Even so, 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. Until we sort out which is which with some confidence, our results will prove totally dissatisfying to our participants (perhaps to you too). They are also dissatisfying to us. That is the thing about science. One imagines we do it out of joy and curiosity, and that is part of it, but sometimes we do it out of frustration. Sometimes, we do it because it is just so incredibly frustrating to not know an answer, even about something so immediate as our showerheads, that we have to go back into the lab and keep working, because the idea that no one yet knows what is going on, well, it keeps us up at night.
But what, then, should you do about your showerhead? We don’t know, but I’ll tell you what I think. Check back in a year or so and see whether I was right. I think that while some Mycobacterium species can be beneficial, the average species is at least a little bit of trouble, particularly for immunocompromised individuals. I think that these bad-news Mycobacterium species become more common the more we try to kill everything in our water and, in doing so, kill off Mycobacterium’s competition. We have shown that plastic showerheads tend to have less Mycobacterium than do metal showerheads, as might be expected if other bacteria are able to metabolize the plastic and, in doing so, outcompete the mycobacteria (Caitlin Proctor found a similar pattern in the hoses of showerheads). Finally, I think that the water that is healthiest for bathing is that which comes from aquifers rich with underground biodiversity including crustaceans. The crustaceans in these aquifers are an indication not of the dirtiness of the water but of its health. The trick is that these aquifers require time, space, and biodiversity to work. Also, they have to remain free of pollution. I suspect that this is a hard lesson for big cities to take to heart and so, over the coming years, I think we will try to kill everything in our water systems. Unfortunately, in doing so, we’ll also accidentally favor tough species (such as Mycobacterium and Legionella) that we don’t really want to have pouring over us quite so much. Meanwhile, we will begin to study natural aquifers in more detail and find that they differ in how effective they are at preventing toxins and pathogens from building up in our water systems. Having figured that out, we’ll try to replicate those natural aquifers. We won’t be great at it, but we’ll slowly figure out how to do a much better job than we are doing today, and the key will prove to be (as it often is) valuing biodiversity, valuing the work that nature does oh-so-much-more-effectively than we do. 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 your showerhead anyway.