10% Human: How Your Body’s Microbes Hold the Key to Health and Happiness. Alanna Collen

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СКАЧАТЬ face and back are as different from those of the dry, exposed elbows as the tropical forests of Panama are from the rocks of the Grand Canyon. Where the face and back are dominated by species belonging to the genus Propionibacterium, which feed off the fats released by the densely packed pores in these areas, the elbows and forearms host a far more diverse community. Moist areas, including the navel, the underarms and the groin, are home to Corynebacterium and Staphylococcus species, which love the high humidity, and feed off the nitrogen in the sweat.

      This microbial second skin provides a double layer of protection to the body’s true interior, reinforcing the sanctity of the barrier formed by the skin cells. Invading bacteria with malicious intentions struggle to get a foothold in these closely guarded bodily border towns, and face an onslaught of chemical weapons when they try. Perhaps even more vulnerable to invasion are the soft tissues of the mouth, which must resist colonisation by a flood of intruders smuggled on food and floating in the air.

      From the mouths of their volunteers, researchers working on the Human Microbiome Project took not just one sample, but nine, each from a slightly different location. These nine sites turned out to have discernibly different communities, within mere centimetres of one another, made up of around 800 species of bacteria dominated by Streptococcus species and a handful of other groups. Streptococcus gets bad press, on account of the many species that cause diseases, from ‘strep throat’ to the flesh-eating infection necrotising fasciitis. But many other species in this genus behave themselves impeccably, crowding out nasty challengers in this vulnerable entrance to the body. Of course these tiny distances between sampling locations within the mouth may seem insignificant to us, but to microbes they are like vast plains and mountain ranges with climates as different as northern Scotland and the south of France.

      Imagine, then, the climatic leap from the mouth to the nostrils. The viscous pool of saliva on a rugged bedrock replaced by a hairy forest of mucus and dust. The nostrils, as you might expect from their gatekeeper status at the entrance to the lungs, harbour a great range of bacterial groups, numbering around 900 species, including large colonies of Propionibacterium, Corynebacterium, Staphylococcus and Moraxella.

      Heading down the throat towards the stomach sees the enormous diversity of species found in the mouth drop dramatically. The highly acidic stomach kills many of the microbes that enter with food, and just one species is known for certain to reside there permanently in some people – Helicobacter pylori, whose presence may be both a blessing and a curse. From this point on, the journey through the digestive tract reveals an ever-greater density – and diversity – of microbes. The stomach opens into the small intestine, where food is rapidly digested by our very own enzymes and absorbed into the bloodstream. There are still microbes here though; around ten thousand individuals in every millilitre of gut contents at the start of this 7-m-long tube, rising to an incredible ten million per millilitre at the end, where the small intestine meets the large intestine’s starting point.

      Just outside the safe-house created by the appendix is a teeming metropolis of microbes, in the heart of the microbial landscape of the human body – the tennis-ball-like caecum, to which the appendix is attached. This is the epicentre of microbial life, where trillions of individual microbes of at least 4,000 species make the most of the partially digested food that has passed through round one of the nutrient-extraction process in the small intestine. The tough bits – plant fibres – are left over for the microbes to tackle in round two.

      The colon, which forms most of the length of the large intestine, running up the right-hand side of your torso, across your body under your rib cage, and back down the left-hand side, provides homes for microbes, numbering one trillion (1,000,000,000,000) individuals per millilitre by now, in the folds and pits of its walls. Here, they pick up the scraps of our food and convert them into energy, leaving their waste products to be absorbed into the cells of the colon’s walls. Without the gut’s microbes, these colonic cells would wither and die – whilst most of the body’s cells are fed by sugar transported in the blood, the colonic cells’ main energy source is the waste products of the microbiota. The colon’s moist, warm, swamp-like environment, in parts completely devoid of oxygen, provides not only a source of incoming food for its inhabitants, but a nutrient-rich mucus layer, which can sustain the microbes in times of famine.

       The human gut.

      Because HMP researchers would have to cut open their volunteers to sample the different habitats of the gut, a far more practical way of collecting information about the gut’s inhabitants was to sequence the DNA of microbes found in the stool. On its passage through the gut, the food we eat is mostly digested and absorbed, both by us and our microbes, leaving only a small amount to come out the other end. Stool, far from being the remains of our food, is mostly bacteria, some dead, some alive. Around 75 per cent of the wet weight of faeces is bacteria; plant fibre makes up about 17 per cent.

      At any one time, your gut contains about 1.5 kg of bacteria – that’s about the same weight as the liver – and the lifespans of individuals are a matter of just days or weeks. The 4,000 species of bacteria found in the stool tell us more about the human body than all the other sites put together. These bacteria become a signature of our health and dietary status, not only as a species, but as a society, and personally. By far the most common group of bacteria in the stool are the Bacteroides, but because our gut bacteria eat what we eat, bacterial communities in the gut vary from person to person.

      The gut microbes aren’t just scavengers, though, taking advantage of our leftovers. We have exploited them too, especially when it comes to outsourcing functions that would take us time to evolve for ourselves. After all, why bother having a gene for a protein that makes Vitamin B12, which is essential for our brain function, when Klebsiella will do it for you? And who needs genes to shape the intestine’s walls, when Bacteroides have them? It’s much cheaper and easier than evolving them afresh. But, as we will discover, the role of the microbes living in the gut goes far beyond synthesising a few vitamins.

      The Human Microbiome Project began by looking only at the microbiotas of healthy people. With this benchmark set down, the HMP went on to ask how they differ in poor health, whether our modern illnesses could be a consequence of those differences, and if so, what was causing the damage? Could skin conditions like acne, psoriasis and dermatitis signal disruption to the skin’s normal balance of microbes? Might inflammatory bowel disease, cancers of the digestive tract and even obesity be due to shifts in the communities of microbes living in the gut? And, most extraordinarily, could conditions that were apparently far removed from microbial epicentres, such as allergies, autoimmune diseases and even mental health conditions, be brought on by a damaged microbiota?

      Lee Rowen’s educated guess in the sweepstake at Cold Spring Harbor hinted at a much deeper discovery. We are not alone, and our microbial passengers have played a far greater role in our humanity than we ever expected. As Professor Jeffrey Gordon puts it:

      This perception of the microbial side of ourselves is giving us a new view of our individuality. A new sense of our connection to the microbial world. A sense of the legacy of our personal interactions with our family and environment early in life. It’s causing us to pause and consider that there might be another dimension to our human evolution.

      We have come to depend on our microbes, and without them, we would be a mere fraction of our true selves. So what does it mean to be just 10 per cent human?

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