Vibrio fischeri

Vibrio fischeri is a rod-shaped bacterium found globally in the marine environments. It has bioluminescent properties, and is found predominantly in symbiosis with various marine animals, such as the bobtail squid. It is heterotrophic and moves by means of flagella. Free living V. fischeri survive on decaying organic matter (see saprotroph). The bacterium is a key research organism for examination of microbial fluorescence, quorum sensing, and bacterial-animal symbiosis.

Ecology
Planktonic V. fischeri are found in very low quantities (almost undetectable) in almost all oceans of the world, preferentially found in temperate and sub-tropical waters. These free-living V. fischeri subsist on organics within the water. They are found in higher concentrations in symbiosis with certain deep sea marine life within special light-organs; or as part of the normal enteral (gut) microbiota of marine animals. Additionally the bacteria can be pathogenic to certain species of marine invertebrates, some of which are commercially farmed in aquaculture. This disease is known as luminous vibriosis.

Symbiotic relationship
Symbiotic relationships in monocentrid fishes and sepolid squid appear to have evolved separately. The most prolific of these relationships is with the Hawaiian bobtail squid (Euprymna scolopes).

Free-living V. fischeri in the ocean waters inoculate the light organs of juvenile squid and fish. Ciliated cells within the light organs selectively draw in the symbiotic bacteria. These cells promote the growth of the symbionts and actively reject any competitors. Through quorum sensing the bacteria cause these cells to die off once the light organ is sufficiently colonised.

The light organ of certain squid contain reflective plates that intensify and direct the light produced, due to proteins known as reflectins. They regulate the light to keep the squid from casting a shadow on moonlit nights, for example.

Sepolid squids expel 90% of the symbiotic bacteria in its light organ each morning in process known as "venting". Venting is hypothesised to provide the free-living inoculum source for newly hatched squids.

Bioluminescence
The bioluminescence of V. fischeri is caused by transcription induced by population-dependent quorum sensing. The luminescence is only seen when population density reaches a certain level.

The luminescence appears to follow a circadian rhythm, that is, it is brighter during the nighttime than daytime.

Bioluminescence levels have also been shown to be proportionally related to both protection against ultraviolet radiation damage to genes and the pathogenicity of bioluminescent V. fischeri.

Genetics of bioluminescence
The bacterial luciferin-luciferase system is encoded by a set of genes labelled the Lux operon. In V. fischeri five such genes (LuxCDABE) have been identified as active in the emission of visible light, and two genes (LuxR and LuxI) are involved in regulating the operon. Several external and intrinsic factors appear to induce and inhibit the transcription of this gene set and produce or suppress light emission. More research is being done to improve our understanding of these processes.

List of synonyms

 * Achromobacter fischeri (Beijerinck 1889) Bergey et al. 1930
 * Bacillus fischeri (Beijerinck 1889) Trevisan 1889
 * Bacterium phosphorescens indigenus (Eisenberg 1891) Chester 1897
 * Einheimischer Leuchtbacillus Fischer 1888
 * Microspira fischeri'' (Beijerinck 1889) Chester 1901
 * Microspira marina (Russell 1892) Migula 1900
 * Photobacterium fischeri Beijerinck 1889
 * Vibrio noctiluca Weisglass and Skreb 1963
 * From NCBI Taxbrowser