Photobacterium  Photobacterium is a genus of gram-negative bacteria in the family "Vibrionaceae". Members of the genus are bioluminescent, that is they have the ability to emit light. Many species, including "Photobacterium leiognathi" and "Photobacterium phosphoreum", live in symbiosis with marine organisms. Species such as "Photobacterium profundum" are adapted for optimal growth in the deep cold seas making it both a psychrophile (an organism capable of growth and reproduction in cold temperatures) and a piezophile (an organism which thrives at high pressures). There are currently 16 species with numerous subspecies known within the genus "Photobacterium". The development of 16S RNA sequencing has led to many species being shifted into and out of this genus. "Photobacterium" can be distinguished from other genera based on identifiable characteristics. "Photobacterium" are primarily marine organisms (hence the use of sodium for growth). They may be free-living or found as colonies associated with certain species of fish. These organisms do not contain any pigmentation and therefore will appear white or colorless. When there is a high density of cells forming a colony, they will exhibit fluorescence. However, the fluorescence is based on the accumulation of autoinducers which is proportional to cell density and therefore free-living photobacterium will not fluoresce. Their association with fish may be: symbiotic growth within fish for the formation of light organs, as a neutral entity on the surface or within the intestines of fish, as decomposers of dead fish, or as an agent of disease. Some of the 15 known species of "Photobacterium" have evolved into pathogens of marine life. Some of these diseases affect commercially important fish and can therefore indirectly impact human health through their consumption. This genus has been shown to degrade the chitin of the Deep Sea Tanner Crab ("Chionoecetes tanneri"). The species "Photobacterium damselae" are among the most virulent and are divided into two subspecies: "piscicida" and "damsela". "P. damselae" subspecies "piscicida" is the causative agent of fish pasteurellosis. Bacterial colonies grow on the infected fish's spleen and kidney, eventually leading to mortality. This disease accounts for severe losses in some fish farming enterprises with some of the most susceptible fish including Yellowfin tuna ("Thunnus albacares"), certain Seabreams ("Sparus" spp.), Striped bass ("Morone saxatilis"), and White perch ("Morone americana").