I Contain Multitudes: The Microbes Within Us and a Grander View of Life

by Ed Yong

Plants are by far the most abundant source of food on land, but it takes more enzymatic power to digest them. Compared to animal flesh, plant tissues contain more complex carbohydrates, such as cellulose, hemicellulose, lignin, and resistant starches. Vertebrates don’t have the molecular chops for breaking these apart. Bacteria do. The common gut bacterium B-theta has over 250 carb-busting enzymes on its own; we have fewer than 100, despite owning a genome that’s 500 times bigger. By sundering plant carbohydrates with their broad toolkits, B-theta and other microbes release substances that directly nourish our own cells. Collectively, they provide 10 per cent of our energy intake, and a whopping 70 per cent of a cow’s or sheep’s. To eat a diet of plants, animals need microbes in both great diversity and great abundance.