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

by Ed Yong

Here is a strange but critical sentiment to introduce in a book about the benefits of living with microbes: there is no such thing as a “good microbe” or a “bad microbe”. These terms belong in children’s stories. They are ill-suited for describing the messy, fractious, contextual relationships of the natural world.6 In reality, bacteria exist along a continuum of lifestyles, between “bad” parasites and “good” mutualists. Some microbes, like Wolbachia, slide from one end of the parasite-mutualist spectrum to the other, depending on the strain, and on the host they find themselves in. But many exist at both ends of the continuum at once: the stomach bacterium Helicobacter pylori causes ulcers and stomach cancer, but also protects against oesophageal cancer – and it’s the same strains that account for both these pros and cons.7 Others can change roles in the same hosts, depending on the context. All of this means that labels like mutualist, commensal, pathogen, or parasite don’t quite work as badges of fixed identity. These terms are more like states of being, like hungry or awake or alive, or behaviours like cooperating or fighting. They’re adjectives and verbs rather than nouns: they describe how two partners relate to one another at a given time and place.