All life — plants, animals, people — depends on peaceful coexistence with a swarm of microbial life that performs vital services from helping to convert food to energy to protection from disease. Now, with the help of a squid that uses a luminescent bacterium to create a predator-fooling light organ and a fish that uses a different strain of the same species of bacteria like a flashlight to illuminate the dark nooks of the reefs where it lives, scientists have found that gaining a single gene is enough for the microbe to switch host animals.