Many species of bacteria use quorum sensing to coordinate gene expression according to the density of their local population.Vibrio fischeri has bioluminescent properties and is found predominantly in symbiosis with various marine animals, such as the bobtail squid.Bobtail squid have a symbiotic relationship with bioluminescent bacteria, which inhabit a special light organ in the squid's mantle.Luminous bacteria are the most widely distributed light-emitting organisms with the majority existing in seawater and the remainder living in the terrestrial or freshwater environment.Virulence is the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host.Molecular biology chiefly concerns itself with understanding the interactions between the various systems of a cell.Carly Denly, the producer of Nova scienceNOW segment, conducted an interview with Bonnie Bassler on October 19, 2006.