Aliivibrio fischeri

Aliivibrio fischeri (also called Vibrio fischeri) is a Gram-negative, rod-shaped bacterium found globally in marine environments. A. fischeri has bioluminescent properties, and is found predominantly in symbiosis with various marine animals, such as the Hawaiian bobtail squid. It is heterotrophic, oxidase-positive, and motile by means of a single polar flagella. Free-living A. fischeri cells survive on decaying organic matter. The bacterium is a key research organism for examination of microbial bioluminescence, quorum sensing, and bacterial-animal symbiosis. It is named after Bernhard Fischer, a German microbiologist. rRNA comparison led to the reclassification of this species from genus Vibrio to the newly created Aliivibrio in 2007. However, the name change is not generally accepted by most researchers, who still publish Vibrio fischeri (see Google Scholar for 2018-2019). The genome for A. fischeri was completely sequenced in 2004 and consists of two chromosomes, one smaller and one larger. Chromosome 1 has 2.9 million base pairs (Mbp) and chromosome 2 has 1.3 Mbp, bringing the total genome to 4.2 Mbp. A. fischeri has the lowest G+C content of 27 Vibrio species, but is still most closely related to the higher-pathogenicity species such as V. cholerae. The genome for A. fischeri also carries mobile genetic elements. A. fischeri are globally distributed in temperate and subtropical marine environments. They can be found free-floating in oceans, as well as associated with marine animals, sediment, and decaying matter. A. fischeri have been most studied as symbionts of marine animals, including squids in the genus Euprymna and Sepiola, where A. fischeri can be found in the squids' light organs. This relationship has been best characterized in the Hawaiian Bobtail Squid (Euprymna scolopes), where A. fischeri is the only species of bacteria inhabiting the squid's light organ. A. fischeri cells in the ocean inoculate the light organs of juvenile squid and fish. Ciliated cells within the animals' photophores (light-producing organs) selectively draw in the symbiotic bacteria. These cells promote the growth of the symbionts and actively reject any competitors. The bacteria cause these cells to die off once the light organ is sufficiently colonised. The light organs of certain squid contain reflective plates that intensify and direct the light produced, due to proteins known as reflectins. They regulate the light for counter-illumination camouflage, requiring the intensity to match that of the sea surface above. Sepiolid squid expel 90% of the symbiotic bacteria in their light organ each morning in a process known as 'venting'. Venting is thought to provide the source from which newly hatched squid are colonized by A. fischeri. The bioluminescence of A. fischeri is caused by transcription of the lux operon, which is induced through population-dependent quorum sensing. The population of A. fischeri needs to reach an optimal level to activate the lux operon and stimulate light production. The circadian rhythm controls light expression, where luminescence is much brighter during the day and dimmer at night, as required for camouflage.