I Contain Multitudes by Ed Yong Bodley Head 4½/5 stars We are not alone. We have never been alone. We are possessed. Our inner demons cannot be cast out, because they did not move in and take possession: they were here before us, and will live on after us. They are invisible, insidious and exist in overwhelming numbers. They manage us in myriad ways: deliver our minerals and vitamins, help digest our lunch, and provide in different ways all our cheese, yoghurt, beer, wine, bread, bacon and beef. Microbes can affect our mood, take charge of our immune system, protect us from disease, make us ill, kill us and then decompose us. As complex, multicellular life forms, we are their puppets. We spread them, introduce variety into their brief lives and provide them with all they need to replicate and colonise new habitats. We are perambulating tower blocks, each occupied by maybe 40 trillion tiny tenants. Our skins are smeared with a thin film of microbial life, with ever greater numbers occupying every orifice and employed in colossal numbers in our guts. Giving babies antibiotics may increase risk of them having food allergies Yet, until late in human history, we didn’t know they were there at all. We still do not know who they all are, or what they do. We discover new things almost every day. In July, German microbiologists announced a new antibiotic that kills the hospital superbug MRSA. It was produced by a seemingly inconsequential microbe fighting for space in the impoverished habitat that is the human nostril. Staphylococcus lugdunensis can produce a toxin that can see off MRSA, even if it is outnumbered 10 to one. You won’t find this particular microbe in Ed Yong’s marvellous, thrilling and richly annotated book, but don’t worry. Unless you are a microbiologist, almost all of it will be new to you. I call it marvellous: everything about the microbial world is to be marvelled at. And it is a page-turner in a very old-fashioned sense. All life is here, and death too, and sex and violence, including deviations you had never dreamed of. Wolbachia is a genus first discovered in 1924, inside a mosquito. Actually, it is in almost everything. Most of the planet’s animal species are arthropods – spiders, flies, scorpions, mites, woodlice and so on – and Wolbachia infects at least two in every five. It manipulates their sex lives: it kills male wasps, and the females replicate by cloning themselves. It selectively kills the male embryos of blue moon butterflies of Fiji and Samoa, so that females outnumber males by 100 to one. It changes insect sperm so that males can only effectively fertilise eggs that are also infected with the same strain of Wolbachia , and all of this happens because the microbe makes the journey through the generations via the ovaries and in the eggs. Infection experts warn of more US superbug cases in coming months All this helps put the rest of us in our place. These things are tiny. They exist as just one cell, each vastly smaller than almost any single human cell. They have no brain or nervous system, but they exist in monstrous numbers – every hour you sneeze, cough, splutter or just breathe an estimated 37 billion into the air around you – and untold variety. Collectively, they both compete and cooperate. The rest of us spread our variant genes by replicating: change happens with a new generation. Microbes can swap those handy mutations that deliver advantage with their neighbours: change happens on the spot, and this ability, called horizontal gene transfer, swiftly delivers microbial strains that can learn to resist whatever antibiotics we throw at them. Microbes have been around, and have learned a thing or two. Complex life has 500 million years of evolutionary history: microbial life is at least 3.5 billion years old. We are their offspring. We share our genes with them, we incorporate their DNA. To a microbiologist, all plants, birds, mammals, insects, molluscs, reptiles, fish and amphibians are just the last few twigs of one tip of the eukaryote branch of the microbial imperium. And although microbial life extends kilometres below the ocean basalt, rides on the dust and water vapour high in the transatlantic clouds, and makes a living in thermal springs, mine dumps, alkaline lakes and even radioactive waste, Yong’s book addresses only the microbiome, our name for those microbes that live parasitically on, or in symbiosis with, or just manipulate, animals. ‘Plagued’ 3,000 years ago: Humans carried deadly bacteria earlier than thought We meet Vibrio fischeri , which occupies the Hawaiian bobtail squid, turning on the luminescence machinery that cancels its shadow and makes it invisible to submarine predators. A bacterium called Tremblaya colonised the citrus mealybug, and then discarded the genetic material it no longer needed as a mealybug freeloader; another bacterium called Moranella colonised Tremblaya . There are now three parties in the symbiosis, between them contributing the nine enzymes necessary to create one amino acid. None of the three could survive without the other two. Unlike ruminating cattle or the great grazing herds of the Serengeti, Yong says, humans could just about scrape by without their bacterial companions, but it is clear that our lives would be the shorter without them. We have an inner life, in every sense, and are the richer for it: richer still for this witty and compelling book.