The winnowing:: Establishing the squid-Vibrio symbiosis

Most symbiotic associations between animals and microorganisms are horizontally transmitted - the microorganisms are acquired from the environment by each generation of the host. How are exclusive partnerships established in the context of the thousands of other microbial species that are present in the environment? Similar to winnowing during a harvest, the symbiosis between the squid Euprymna scolopes and its luminous bacterial symbiont Vibrio fischeri involves a step-wise elimination of potential interlopers that ensures separation of the 'grain' from the 'chaff'.