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

by Ed Yong

Gulping down a yoghurt is like ingesting scarcity. Rarity, too: the bacteria in these products are not important members of the adult gut. They largely belong to the same category that Metchnikoff canonised – makers of lactic acid, like Lactobacillus and Bifidobacterium, which were chosen more for practical reasons than scientific ones. They are easy to grow, they are already found in fermented foods, and they can survive the trip through both a commercial packaging plant and a consumer’s stomach. ‘But most of them never arose in the human gut, and they don’t have the factors that allow them to dwell for a long time there,’ says Jeff Gordon. His team confirmed this by monitoring the gut microbiomes of volunteers who ate twice-daily servings of Activia yoghurt for seven weeks. The bacteria in the yoghurt neither colonised the volunteers’ guts, nor changed the composition of their microbiomes. It’s the same problem that Herter and Kendall identified in the 1920s, and that Matthew Becker and others saw when working on probiotics for frogs. They’re like a breeze that blows through two open windows.

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