Why do microbes so easily slide between pathogen and mutualist? For a start, these roles are not as contradictory as you might imagine. Think about what a ‘friendly’ gut microbe needs to set up a stable relationship with its host. It must survive in the gut, anchor itself so it doesn’t get swept away, and interact with its host’s cells. These are all things that pathogens must do, too. So both characters – mutualists and pathogens, heroes and villains – often use the same molecules for the same purposes.
