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

by Ed Yong

A cut or a bruise can split some of your cells apart and spill fragments of mitochondria into your blood – fragments that still keep some of their ancient bacterial character. When your immune system spots them, it mistakenly assumes that an infection is under way and mounts a strong defence. If the injury is severe, and enough mitochondria are released, the resulting body-wide inflammation can build into a lethal condition called systemic inflammatory response syndrome (SIRS).10 SIRS can be worse than the original injury. Absurdly, it’s simply the result of a human body mistakenly overreacting to microbes that have been domesticated for over two billion years. Just as a weed is a flower in the wrong place, our microbes might be invaluable in one organ but dangerous in another, or essential inside our cells but lethal outside them. “If you go immunosuppressed for a little bit, they’ll kill you. When you die, they’ll eat you,” says coral biologist Forest Rohwer. “They don’t care. It’s not a nice relationship. It’s just biology.” So, the world of symbiosis is one in which our allies can disappoint us and our enemies can rally to our side. It’s a world where mutualisms shatter for the matter of a few millimetres.