Evolution and Systematics
Members of Vibrio fisheri bacterial colonies synchronize bioluminescent light production via a cell-to-cell signalling mechanism known as quorum sensing.
"Although this species can be found in open seawater, it also occupies an unusual ecological niche; V. fischeri is a symbiont, which colonizes the light-producing organ of certain marine fish and squid…Although the potential benefits to the host of having a mobile biological 'light bulb' are superficially obvious (attracting prey, repelling predators etc.), the question of precisely how V. fischeri produces light, and why this only occurs at high cell densities, occupied researchers for several years. The problem was all-the-more intriguing since the bacteria within the light organ apparently coordinate their efforts to produce light; the transition to light production is sharp, and involves a concerted effort on behalf of the whole population. A key breakthrough came when Hastings and colleagues discovered that cell–cell signalling lies at the heart of this remarkable biological switch.12,13" (Welch et al. 2005:197)
Learn more about this functional adaptation.
- Welch, M.; Mikkelsen, H.; Swatton, J. E.; Smith, D.; Thomas, G. L.; Glansdorp, F. G.; Spring, D. R. 2005. Cell-cell communication in Gram-negative bacteria. Mol Biosyst. 1(3): 196-202.
- Atkinson S; Williams P. 2009. Quorum sensing and social networking in the microbial world. J R Soc Interface. 6(40): 959-78.
Aliivibrio 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 bobtail squid. It is heterotrophic and moves by means of flagella. Free-living A. fischeri survive on decaying organic matter (see saprotroph). The bacterium is a key research organism for examination of microbial bioluminescence, quorum sensing, and bacterial-animal symbiosis. It is named in honor of Bernhard Fischer, a German microbiologist. 
Planktonic A. fischeri are found in very low quantities (almost undetectable) in almost all oceans of the world, preferentially found in temperate and subtropical waters. These free-living A. fischeri subsist on organics within the water. They are found in higher concentrations in symbiosis with certain deep sea life within special light organs; or as part of the normal enteral (gut) microbiota of marine animals.
Free-living A. fischeri in the ocean 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. 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 a process known as "venting". Venting is hypothesised to provide the free-living inoculum source for newly hatched squids.
The bioluminescence of A. fischeri is caused by transcription of the Lux operon, induced by population-dependent quorum sensing. The luminescence is seen only when population density reaches a certain level. The luminescence appears to follow a circadian rhythm. That is, it is brighter during the nighttime than during the 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 A. fischeri.
Genetics of bioluminescence
The bacterial luciferin-luciferase system is encoded by a set of genes labelled the Lux operon. In A. 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.
The bioluminescent, Gram-negative bacterium Aliivibrio fischeri is one of many species of bacteria that commonly form symbiotic relationships with marine organisms (Distal, 1993). Marine organisms contain bacteria that use bioluminescence so they can find mates, ward off predators, attract prey, or communicate with other organisms (Widder, 2010). In return, the organism the bacteria are living within provides the bacteria with a nutrient-rich environment (Winfrey et al., 1997). The lux operon is a 9-kilobase fragment of the A. fischeri genome that controls bioluminescence through the catalyzation of the enzyme luciferase (Meighen, 1991). The lux operon has a known gene sequence of luxCDAB(F)E, where lux A and lux B code for the components of luciferase, and the lux CDE codes for a fatty acid reductase complex that makes the fatty acids necessary for the luciferase mechanism (Meighen, 1991). Lux C codes for the enzyme acyl-reductase, lux D codes for acyl-transferase, and lux E makes the proteins needed for the enzyme acyl-protein synthetase. Luciferase produces blue/green light through the oxidation of reduced flavin mononucleotide and a long-chain aldehyde by diatomic oxygen. The reaction is summarized below (Silverman et al., 1984): FMNH2+O2+R-CHO → FMN + R-COOH + H2O + Light
The reduced flavinmononucleotide (FMNH) is provided by the fre gene, also referred to as LuxG. In A.fischeri, it is directly next to LuxE (giving LuxCDABE-fre) from 1042306 to 1048745 
To generate the aldehyde needed in the reaction above, three additional enzymes are needed. The fatty acids needed for the reaction are pulled out from the fatty acid biosynthesis pathway by the enzyme acyl-transferase. Acyl-transferase reacts with acyl-ACP to release R-COOH, a free fatty acid. R-COOH is reduced by a two-enzyme system to an adehyde. The reaction is shown below: R-COOH+ATP+NADPH→ R-CHO+AMP+PP+NADP+ (Winfrey et al., 1997). Although the lux operon encodes the enzymes necessary for the bacteria to glow, bioluminescence is regulated by autoinduction. An autoinducer is a transcriptional promoter of the enzymes necessary for bioluminescence. Before the glow can be luminized, a certain concentration of an autoinducer must be present. So, for bioluminescence to occur, high colony concentrations of A. fischeri should be present in the organism (Winfrey et al., 1997).
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
- Madigan M, Martinko J (editors) (2005). Brock Biology of Microorganisms (11th ed. ed.). Prentice Hall. ISBN 0-13-144329-1.
- Holt JG (editor) (1994). Bergey's Manual of Determinative Bacteriology (9th ed. ed.). Williams & Wilkins. ISBN 0-683-00603-7.
- George M. Garrity: Bergey's Manual of Systematic Bacteriology. 2. Auflage. Springer, New York, 2005, Volume 2: The Proteobacteria, Part B: The Gammaproteobacteria ISBN 0-387-24144-2
- Urbanczyk, H.; Ast, J. C.; Higgins, M. J.; Carson, J.; Dunlap, P. V. (2007). "Reclassification of Vibrio fischeri, Vibrio logei, Vibrio salmonicida and Vibrio wodanis as Aliivibrio fischeri gen. nov., comb. nov., Aliivibrio logei comb. nov., Aliivibrio salmonicida comb. nov. and Aliivibrio wodanis comb. nov.". International Journal of Systematic and Evolutionary Microbiology 57 (12): 2823–2829. doi:10.1099/ijs.0.65081-0. PMID 18048732.
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