The Body, page 4
Probably no mystery of the outer surface causes greater consternation, however, than our strange tendency to lose our hair as we age. We have about 100,000 to 150,000 hair follicles on our heads, though clearly not all follicles are equal among all people. You lose, on average, between fifty and a hundred head hairs every day, and sometimes they don’t grow back. About 60 percent of men are “substantially bald” by the age of fifty. One man in five achieves that condition by thirty. Little is understood about the process, but what is known is that a hormone called dihydrotestosterone tends to go slightly haywire as we age, directing hair follicles on the head to shut down and more reserved ones in the nostrils and ears to spring to dismaying life. The one known cure for baldness is castration. Ironically, considering how easily some of us lose it, hair is pretty impervious to decay and has been known to last in graves for thousands of years.
Perhaps the most positive way to look at it is that if some part of us must yield to middle age, the hair follicles are an obvious candidate for sacrifice. No one ever died of baldness, after all.
* “Corpuscle,” from the Latin, meaning “little body,” is a somewhat vague term anatomically speaking. It can signify either unattached, free-floating cells, as in blood corpuscles, or it can signify clumps of cells that function independently, as with Meissner’s corpuscles.
3 MICROBIAL YOU
And we are not at the end of the penicillin story.
Perhaps we are only just at the beginning.
—ALEXANDER FLEMING, NOBEL PRIZE ACCEPTANCE SPEECH, DECEMBER 1945
I
TAKE A DEEP breath. You probably suppose that you are filling your lungs with rich, life-giving oxygen. Actually, not really. Eighty percent of the air you breathe is nitrogen. It is the most abundant element in the atmosphere and it is vital to our existence, but it doesn’t interact with other elements. When you take a breath, the nitrogen in the air goes into your lungs and straight back out again, like an absentminded shopper who has wandered into the wrong store. For nitrogen to be useful to us, it must be converted into more sociable forms, like ammonia, and it is bacteria that do that job for us. Without their help, we would die. Indeed, we could never have existed. It is time to say thank you to your microbes.
You are home to trillions and trillions of tiny living things, and they do you a surprising amount of good. They provide you with about 10 percent of your calories by breaking down foods that you couldn’t otherwise make use of, and in the process extract beneficial nutriments like vitamins B2 and B12 and folic acid. Humans produce twenty digestive enzymes, which is a pretty respectable number in the animal world, but bacteria produce ten thousand, or five hundred times as many, according to Christopher Gardner of Stanford University. “Our lives would be vastly less well nourished without them,” he says.
Individually they are infinitesimally small and their lives are fleeting—the average bacterium weighs about one-trillionth of the weight of a dollar bill and lives for no more than twenty minutes—but collectively they are formidable indeed. The genes you are born with are all you are ever going to have. You can’t buy or trade for better ones. But bacteria can swap genes among themselves, as if they were Pokémon cards, and they can pick up DNA from dead neighbors. These horizontal gene transfers, as they are known, massively accelerate their capacity to adapt to whatever nature and science throw at them. The DNA of bacteria is less scrupulous in its proofreading, too, so they mutate more often, giving them even greater genetic nimbleness.
We can’t begin to compete with them for speed of change. E. coli can reproduce seventy-two times in a day, which means that in three days they can rack up as many new generations as we have managed in the whole of human history. A single parent bacterium could in theory produce a mass of offspring greater than the weight of Earth in less than two days. In three days, its progeny would exceed the mass of the observable universe. Clearly that could never happen, but they are with us already in numbers beyond imagining. If you put all Earth’s microbes in one heap and all the other animal life in another, the microbe heap would be twenty-five times greater than the animal one.
Make no mistake. This is a planet of microbes. We are here at their pleasure. They don’t need us at all. We’d be dead in a day without them.
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We know surprisingly little about the microbes in and on and around us because overwhelmingly they will not grow in a lab, which makes them exceedingly difficult to study. What can be said is that as you sit here now, you are likely to have something like 40,000 species of microbes calling you home—900 in your nostrils, 800 more on your inside cheeks, 1,300 next door on your gums, as many as 36,000 in your gastrointestinal tract, though such numbers must constantly be adjusted as new discoveries are made. In early 2019, a study of just twenty people by the Wellcome Sanger Institute in England found 105 new species of gut microbes whose existence had been quite unsuspected. Precise numbers will vary from person to person and within individuals over time depending on whether you are an infant or elderly, where and with whom you’ve been sleeping, whether you have been taking antibiotics, or whether you are fat or thin. (Thin people have more gut microbes than fat people; having hungry microbes may at least partly account for their thinness.) That is of course just the numbers of species. In terms of individual microbes, the number is beyond imagining, never mind counting: it’s in the trillions. Altogether your private load of microbes weighs roughly three pounds, about the same as your brain. People have even begun describing our microbiota as one of our organs.
For years, it was commonly stated that we each contain ten times as many bacterial cells as human ones. It turns out that that confident-sounding figure came from a paper written in 1972 that was little more than a guess. In 2016, researchers from Israel and Canada did a more careful assessment and concluded that each of us contains about thirty trillion human cells and between thirty and fifty trillion bacterial cells (depending on a lot of factors like health and diet), so the numbers are much closer to being equal—though it should also be noted that 85 percent of our own cells are red blood cells, which aren’t true cells at all, because they don’t have any of the usual machinery of cells (like nuclei and mitochondria), but are really just containers for hemoglobin. A separate consideration is that bacterial cells are tiny, whereas human cells are comparatively gigantic, so in terms of massiveness, not to mention the complexity of what they do, human cells are unquestionably more consequential. Then again, looked at genetically, you have about twenty thousand genes of your own within you, but perhaps as many as twenty million bacterial genes, so from that perspective you are roughly 99 percent bacterial and not quite 1 percent you.
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Microbial communities can be surprisingly specific. Although you and I will each have several thousand bacterial species within us, we may have only a fraction in common. Microbes are ferocious housekeepers, it seems. Have sex and you and your partner will perforce exchange a lot of microbes and other organic material. Passionate kissing alone, according to one study, results in the transfer of up to one billion bacteria from one mouth to another, along with about 0.7 milligrams of protein, 0.45 milligrams of salt, 0.7 micrograms of fat, and 0.2 micrograms of “miscellaneous organic compounds” (that is, bits of food). But as soon as the party is over, the host microorganisms in both participants will begin a kind of giant sweeping-out process, and within only a day or so the microbial profile for both parties will be more or less fully restored to what it was before they locked tongues. Occasionally, some pathogens sneak through, and that is when you get herpes or a head cold, but that is the exception.*1
Luckily, most microbes have nothing to do with us. Some live benignly inside us and are known as commensals. Only a tiny portion of them make us ill. Of the million or so microbes that have been identified, just 1,415 are known to cause disease in humans—very few, all things considered. On the other hand, that is still a lot of ways to be unwell, and together those 1,415 tiny, mindless entities cause one-third of all the deaths on the planet.
As well as bacteria, your personal repertoire of microbes consists of fungi, viruses, protists (amoebas, algae, protozoa, and so on), and archaea, which for a long time were thought to be just more bacteria but actually represent a whole other branch of life. Archaea are very like bacteria in that they are quite simple and have no nucleus, but they have the great benefit to us that they cause no known diseases in humans. All they give us is a little gas, in the form of methane.
It’s worth bearing in mind that all these microbes have almost nothing in common in terms of their history and genetics. All that unites them is tininess. To all of them, you are not a person but a world—a vast and jouncing wealth of marvelously rich ecosystems with the convenience of mobility thrown in, along with the very helpful habits of sneezing, petting animals, and not always washing quite as fastidiously as you really ought to.
II
A VIRUS, IN the immortal words of the British Nobel laureate Peter Medawar, is “a piece of bad news wrapped up in a protein.” Actually, a lot of viruses are not bad news at all, at least not to humans. Viruses are a little weird, not quite living but by no means dead. Outside living cells, they are just inert things. They don’t eat or breathe or do much of anything. They have no means of locomotion. We must go out and collect them—off door handles or handshakes or drawn in with the air we breathe. They do not propel themselves; they hitchhike. Most of the time, they are as lifeless as a mote of dust, but put them into a living cell, and they will burst into animate existence and reproduce as furiously as any living thing.
Like bacteria, they are incredibly successful. The herpes virus has endured for hundreds of millions of years and infects all kinds of animals—even oysters. They are also terribly small—much smaller than bacteria and too small to be seen under conventional microscopes. If you blew one up to the size of a tennis ball, a human would be five hundred miles high. A bacterium on the same scale would be about the size of a beach ball.
In the modern sense of a very small microorganism, the term “virus” dates only from 1900, when a Dutch botanist, Martinus Beijerinck, found that the tobacco plants he was studying were susceptible to a mysterious infectious agent even smaller than bacteria. At first he called the mysterious agent contagium vivum fluidum but then changed it to “virus,” from a Latin word for “toxin.” Although he was the father of virology, the importance of his discovery wasn’t appreciated in his lifetime, so he was never honored with a Nobel Prize, as he really should have been.
It used to be thought that all viruses cause disease—hence the Peter Medawar quotation—but we now know that most viruses infect only bacterial cells and have no effect on us at all. Of the hundreds of thousands of viruses reasonably supposed to exist, just 586 species are known to infect mammals, and of these only 263 affect humans.
We know very little about most other, nonpathogenic viruses because only the ones that cause disease tend to get studied. In 1986, a student at the State University of New York at Stony Brook named Lita Proctor decided to look for viruses in seawater—which was considered a highly eccentric thing to do because it was universally assumed that the oceans have no viruses except perhaps for a transient few introduced through sewage outfall pipes and the like. So it was a slight astonishment when Proctor found that the average quart of seawater contains up to 100 billion viruses. More recently, Dana Willner, a biologist at San Diego State University, looked into the number of viruses found in healthy human lungs—somewhere else that viruses were not thought to lurk much. Willner found that the average person harbored 174 species of virus, 90 percent of which had never been seen before. Earth, we now know, is aswarm with viruses to a degree that until recently we barely suspected. According to the virologist Dorothy H. Crawford, ocean viruses alone if laid end to end would stretch for ten million light-years, a distance essentially beyond imagining.
Something else viruses do is bide their time. A most extraordinary example of that came in 2014 when a French team found a previously unknown virus, Pithovirus sibericum, in Siberia. Although it had been locked in permafrost for thirty thousand years, when injected into an amoeba, it sprang into action with the lustiness of youth. Luckily, P. sibericum proved not to infect humans, but who knows what else may be out there waiting to be uncovered? A rather more common manifestation of viral patience is seen in the varicella-zoster virus. This is the virus that gives you chicken pox when you are small, but then may sit inert in nerve cells for half a century or more before erupting in that horrid and painful indignity of old age known as shingles. It is usually described as a painful rash on the torso, but in fact shingles can pop up almost anywhere on the body surface. A friend of mine had it in his left eye and described it as the worst experience of his life. (The word, incidentally, has nothing to do with the tiles of a roof. Shingles as a medical condition comes from the Latin cingulus, meaning a kind of belt; as a roofing material, it is from the Latin scindula, meaning a stepped tile. It is just by chance that they ended up in English with the same spellings.)
The most regular of unwelcome viral encounters is the common cold. Everyone knows that if you get chilled, you are more likely to catch a cold (that is why we call it a cold, after all), yet science has never been able to prove why—or even, come to that, if that is actually so. Colds unquestionably are more frequent in winter than in summer, but that may only be because we spend more time indoors then and are more exposed to others’ leakages and exhalations.
The common cold is not a single illness but rather a family of symptoms generated by a multiplicity of viruses, of which the most pernicious are the rhinoviruses. These alone come in a hundred varieties. There are, in short, lots of ways to catch a cold, which is why you never develop enough immunity to stop catching them all.
For years, Britain operated a research facility called the Common Cold Unit, but it closed in 1989 without ever finding a cure. It did, however, conduct some interesting experiments. In one, a volunteer was fitted with a device that leaked a thin fluid at his nostrils at the same rate that a runny nose would. The volunteer then socialized with other volunteers, as if at a cocktail party. Unknown to any of them, the fluid contained a dye visible only under ultraviolet light. When that was switched on after they had been mingling for a while, the participants were astounded to discover that the dye was everywhere—on the hands, head, and upper body of every participant and on glasses, doorknobs, sofa cushions, bowls of nuts, you name it. The average adult touches his face sixteen times an hour, and each of those touches transferred the pretend pathogen from nose to snack bowl to innocent third party to doorknob to innocent fourth party and so on until pretty much everyone and everything bore a festive glow of imaginary snot. In a similar study at the University of Arizona, researchers infected the metal door handle to an office building and found it took only about four hours for the “virus” to spread through the entire building, infecting over half of employees and turning up on virtually every shared device like photocopiers and coffee machines. In the real world, such infestations can stay active for up to three days. Surprisingly, the least effective way to spread germs (according to yet another study) is kissing. It proved almost wholly ineffective among volunteers at the University of Wisconsin who had been successfully infected with cold virus. Sneezes and coughs weren’t much better. The only really reliable way to transfer cold germs is physically by touch.
A survey of subway trains in Boston found that metal poles are a fairly hostile environment for microbes. Where microbes thrive is in the fabrics on seats and on plastic handgrips. The most efficient method of transfer for germs, it seems, is a combination of folding money and nasal mucus. A study in Switzerland in 2008 found that flu virus can survive on paper money for two and a half weeks if it is accompanied by a microdot of snot. Without snot, most cold viruses could survive on folding money for no more than a few hours.
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The two other forms of microbe that commonly lurk within us are fungi and protists. Fungi for a long time were a kind of scientific bewilderment, classified as just slightly strange plants. In fact, at a cellular level, they aren’t very like plants at all. They don’t photosynthesize, so they have no chlorophyll and thus are not green. They are actually more closely related to animals than to plants. It wasn’t until 1959 that they were recognized as quite separate and given their own kingdom. They essentially divide into two groups—molds and yeasts. By and large fungi leave us alone. Only about three hundred out of several million species affect us at all, and most of those mycoses, as they are known, don’t make you really ill, but rather cause only mild discomfort or irritation, as with athlete’s foot, say. A few, however, are much nastier than that, and the number of nasty ones is growing.
