text: Ecologically, V. fischeri is known to have symbiotic associations with a number of eukaryotic hosts, including the Hawaiian Bobtail Squid (Euprymna scolopes).  In this relationship, the squid host maintains the bacteria in specialized light organs.  The host provides a safe, nutrient rich environment for the bacteria and in turn, the bacteria provide light.  Although bioluminescence can be used for mating and other purposes, in E. scolopes it is used for counter illumination to avoid predation.
  The autoinducer molecule used by V. fischeri is N-(3-oxohexanoyl)-homoserine lactone.  This molecule is produced in the cytoplasm by the LuxI synthase enzyme and is secreted through the cell membrane into the extracellular environment.   As is true of most autoinducers, the environmental concentration of N-(3-oxohexanoyl)-homoserine lactone is the same as the intracellular concentration within each cell.  N-(3-oxohexanoyl)-homoserine lactone eventually diffuses back into cells where it is recognized by LuxR once a threshold concentration (~10 μg/ml) has been reached.  LuxR binds the autoinducer and directly activates transcription of the luxICDABE operon.  This results in an exponential increase in both the production of autoinducer and in bioluminescence.  LuxR bound by autoinducer also inhibits the expression of luxR, which is thought to provide a negative feedback compensatory mechanism to tightly control levels of the bioluminescence genes.