* “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.