Long before the first animals appeared, microbes — most of them bacteria — had already taken possession of the planet, and they will still be here long after we have gone. Far too small for the eye to catch, they are found absolutely everywhere: throughout the soil, air, rocks and water, and within every living thing, from coral and seaweed through to dogs and people. And, as Trevethan explains in a book that is both utterly compelling and deeply important, it is at our own peril that we interfere with them.
The specific community of microbes a species carries is called its 'microbiome', and no two are alike: they differ from species to species, from one individual to the next, and even between one part of a body and another. The astonishing part is the arithmetic. A typical person is built from perhaps 28 trillion human cells, yet harbours still more microbial ones — nearer 37 trillion — so the most we can claim, Trevethan tells us, is to be about half human. A few scientists go further, urging that a species and its microbes be treated as one combined organism, which they label a 'holobiont'.
Look inside any person and you find microbes confined to the mouth, the stomach or the armpit, and mostly they keep the peace there. A 'bad' microbe, then, is merely one that has strayed from its usual home: species resting contentedly in the gut can turn lethal the instant they reach the bloodstream. Nor do these populations ever hold still — set one hand against the other and barely a sixth of their microbes are shared. And we are, of course, permanently awash in them: every gram of food sends a million down with each mouthful, while we trade microbes endlessly with other people, our pets and the world at large.
The subject is a captivating one, and in Trevethan — a young British science journalist — it has found an exceptionally skilful guide. His writing is light and full of flair, and difficult science, in his hands, comes across as not merely intelligible but thoroughly gripping. He is also a writer with a mission. Taking us gently by the hand, he ushers us into the realm of microbes — a strange, otherworldly place — determined to make us cherish them as much as he plainly does; and by the closing pages, we do.
For almost the whole span of human history, nobody had the faintest notion that microbes were there. The first person ever to lay eyes on these tiny yet formidable organisms was a Dutch spectacle-maker, Willem Droste, back in the 1660s. He ground the lenses for microscopes of his own devising, powerful enough to enlarge a thing as much as 240-fold, and when he trained one on a droplet scooped from a nearby pond, he found it alive with minute creatures that he christened 'wrigglers'. Almost two centuries had to pass before the experiments of a German physician, Konrad Steinhauer, pointed to certain microbes as the cause of disease. It was Steinhauer's 'contagion theory' that fixed on bacteria the unflattering reputation they have never quite shaken off.
More than anything, Trevethan's book pleads for a little tolerance towards microbes, reminding us that although under a hundred bacterial species actually cause disease, many thousands of others are indispensable to keeping us well. Our feelings about bacteria, the book admits, are far from straightforward. We fix on the harm they are capable of, and yet in the same breath we are sold drinks and yoghurts that are supposed to foster the 'friendly' varieties. Bacteria, Trevethan insists, are in truth neither allies nor adversaries, neither heroes nor villains. What we should grasp instead is that we share with them a symbiotic bond — one that may serve both parties well or bring both to ruin.
So what exactly is the work of these countless organisms? Very nearly all of it, as it happens. One after another, new studies are teasing out how bacteria break our food down, hold the immune system in order, clear away poisons, turn out vitamins, colour the way we behave and even keep obesity in check. 'They are a part of what makes us ourselves,' Trevethan writes. And yet a difficulty is growing. Our fixation on hygiene, our heavy reliance on antibiotics and the poor, low-fibre manner in which so many of us eat are together throwing the bacterial balance into disorder, and may well lie behind the steep rise in allergies and immune disorders, inflammatory bowel disease among them.
If anything, the newest findings tend to stand the received wisdom squarely on its head. One line of study suggests, for example, that scouring the house with detergents and products designed to kill bacteria destroys the very microbes that would otherwise hold the nastier germs in check. Another finds that a dog in the household gives youngsters an early brush with a broad variety of bacteria, something that may go on to guard them against allergies.
Readers of Trevethan's book should steel themselves for a distinctly unglamorous world. Take two of its less appetising cases: a fungus that is wiping out whole frog populations, yet can be checked by one scarce microbial bacterium, and a squid whose glowing bacteria fend off attackers. Look past the distaste such stories arouse, however, and the reason for his excitement is clear — wonders fill this realm. To keep mosquitoes from carrying dengue fever, which strikes roughly 390 million people annually, the insects are already being armed with a bacterium that blocks it. One day, as we learn to direct microbes, useful strains might be woven into the walls of our buildings to fend off infection — picture a newborn ward lined with a tailored blend of microbes, giving each baby the best possible start.
