Photobacterium is a genus ofgram-negative, oxidase-positive and catalase-positivebacteria in the familyVibrionaceae.[1] Members of the genus arebioluminescent, that is they have the ability to emitlight.
Many species, includingPhotobacterium leiognathi andPhotobacterium phosphoreum,Photobacterium ganghwense,Photobacterium marinum live insymbiosis with marine organisms.[1] S.I. Paul et al. (2021)[1] isolated and identified multiple strains ofPhotobacterium frommarine sponges of theSaint Martin's Island Area of theBay of Bengal,Bangladesh.
Species such asPhotobacterium profundum are adapted for optimal growth in the deep cold seas making it both apsychrophile (an organism capable of growth and reproduction in cold temperatures) and apiezophile (an organism which thrives at high pressures).
Colony, morphological, physiological, and biochemical characteristics ofPhotobacterium species are shown in the Table below.[1]
| Test type | Test | Characteristics |
|---|---|---|
| Colony characters | Size | Medium |
| Type | Round | |
| Color | Creamy | |
| Shape | Convex | |
| Morphological characters | Shape | Rod |
| Physiological characters | Motility | + |
| Growth at 6.5% NaCl | + | |
| Biochemical characters | Gram's staining | – |
| Oxidase | + | |
| Catalase | + | |
| Oxidative-Fermentative | Oxidative | |
| Motility | + | |
| Methyl Red | + | |
| Voges-Proskauer | – | |
| Indole | + | |
| H2S Production | – | |
| Urease | V | |
| Nitrate reductase | + | |
| β-Galactosidase | – | |
| Hydrolysis of | Gelatin | + |
| Aesculin | V | |
| Casein | V | |
| Tween 40 | + | |
| Tween 60 | + | |
| Tween 80 | + | |
| Acid production from | Glycerol | + |
| Galactose | + | |
| D-Glucose | + | |
| D-Fructose | V | |
| D-Mannose | – | |
| Mannitol | V | |
| N-Acetylglucosamine | V | |
| Amygdalin | + | |
| Maltose | V | |
| D-Melibiose | + | |
| D-Trehalose | + | |
| Glycogen | V | |
| D-Turanose | V |
Note: + = Positive; – =Negative; V =Variable (+/–)
There are currently 16 species with numerous subspecies known within the genusPhotobacterium.[2] The development of 16S RNA sequencing has led to many species being shifted into and out of this genus.[3]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 ofautoinducers 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 oflight 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.[3]
Some of the 15 known species ofPhotobacterium have evolved intopathogens 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 thechitin of the Deep Sea Tanner Crab (Chionoecetestanneri). The speciesPhotobacterium damselae are among the most virulent and are divided into two subspecies:piscicida anddamsela.[4]P. damselae subspeciespiscicida is the causative agent of fishpasteurellosis. 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 includingYellowfin tuna (Thunnus albacares), certainSeabreams (Sparus spp.),Striped bass (Morone saxatilis), andWhite perch (Morone americana). This subspecies is not pathogenic for humans.[5]
Fish-virulent strains of this subspecies ofP. damselae causesepticemia in species of fish such as;damselfish (Family Pomacentridae),eels (Anguilla anguilla),sandbar/brown sharks (Carcharhinus plumbeus), Yellowtails (Seriola quinqueradiata), seabreams (Sparus spp.) andturbots (Scophthalmus spp.). This subspecies has been shown to be pathogenic for humans as it has been isolated from human wounds and has been shown to cause primary septicimia in healthy humans.[6]
Fish infected by the subspeciesdamselae initially experience a reduced appetite accompanied by lethargicness and ulcerative lesions along their flank and head regions. Their stomachs will distend and they will experience extensive hemorrhaging especially in their eyes, mouth, and musculature. There will also bepetechiation of the gills and liver along with the characteristic accumulation of mucus around the gills. It has been noted that the infected fish will swim violently a few minutes before death occurs.
Evidence ofepizootic outbreaks gained from an increase in ulcers noted among the fish populations seem to correlate to warmer seasons, suggesting a seasonal distribution in the incidence of the disease, dependent upon the waters temperature and salinity along with a decreased resistance caused by physiological changes experienced by the host during sexual maturity.
Seawater is the most likely mode of transmission of the virulent cells of the pathogen. Once it comes into contact with the outer surface of the fish, it is able to adhere to skin and resist the bactericidal action of the skinmucus layer, thus suggesting that the skin is the site of entry into the host. This bacterium could therefore represent a significant threat to aquacultured fish species, especially those living in crowded and stressed conditions, where the spread of the disease could be accelerated through direct contact and thus pose a threat to humans.[7]
Hilgarth, Maik et al. “Photobacterium Carnosum Sp. Nov., Isolated from Spoiled Modified Atmosphere Packaged Poultry Meat.” Systematic and Applied Microbiology 41.1 (2018): 44–50.
Vezzi, A.; Campanaro, S.; D'Angelo, M.; Simonato, F.; Vitulo, N.; Lauro, F. M.; Cestaro, A.; Malacrida, G.; Simionati, B. (March 4, 2005). "Life at depth: Photobacterium profundum genome sequence and expression analysis".Science.307 (5714):1459–61.Bibcode:2005Sci...307.1459V.doi:10.1126/science.1103341.ISSN 0036-8075.JSTOR 3841724.OCLC 1644869.PMID 15746425.S2CID 21290953.(subscription required)Authority control databases: National |
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