To understand how microbes impact humans, it is critical to examine the ways other species have interacted, evolved with, and created symbiosis with microbes. This chapter gives numerous examples of interactions with microbes and explains their importance to the way of life of certain creatures.

For example, the luminous bacteria Vibrio fischeri is an important symbiont to the Hawaiian bobtail squid. These bacteria live in the light organs on the underside of the squid and act to cancel out any shadow that the squid may cast, thus keeping it hidden from predators (50). Margaret McFall-Ngai and Ned Ruby study the relationship between the bacteria and the squid by breeding the squid in a lab, a process that provides them with sterile hatchlings to work with and allows them to control the introduction of the microbes. They have found that the microbes are attracted to the mucus of the light organs, and at least five bacterial cells must make contact with the squid to switch on a set of genes in the squid that further attract V. fischeri and deter other microbes (51). Some of the genes produce antimicrobials that kill other bacteria but leave V. fischeri unharmed.