The Vibrio fischeri belongs to the Gram-negative rods that are natural inhabitants of seawater. It is a species of bioluminescent bacterium frequently found in symbiotic relationship with marine animals like bobtail squid. Their bioluminescence stems from their expression of a series of proteins contained in the lux operon.
In the presence of toxic substances or due to other detrimental effects its bioluminescence is reduced, which can be measured by luminometer.