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

by Ed Yong

the lean communities were inherently superior. Instead, Ridaura had tipped the battles in their favour by feeding her mice with plant-heavy chow. The complex fibres in these meals created many opportunities for microbes with the right digestive enzymes – ‘job openings for them to fill’, in Gordon’s words. The obese communities had few species that could fill those positions but the lean communities were brimming with qualified candidates, including fibre-busting specialists like B-theta. So, when obese communities colonised lean guts, they found that every morsel of food was already being devoured and every niche had been filled. By contrast, when the lean communities entered obese guts, they found a glut of uneaten fibre – and flourished. Their success only evaporated when Ridaura fed the mice with fatty, low-fibre chow, designed to represent the worst extremes of the Western diet. Without fibre, the lean communities couldn’t establish themselves or stop the mice from putting on weight. They could only infiltrate the guts of mice that ate healthily.