Vibrio fischeri

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Vibrio fischeri
Scientific classification
Kingdom: Bacteria Phylum: Proteobacteria Class: Gammaproteobacteria Order: Vibrionales Family: Vibrionaceae Genus: Aliivibrio Species: V. fischeri
Binomial name
Vibrio fischeri

Description and significance

Vibrio fischeri is a gram-negative marine bioluminescent bacterium that forms symbiosis with various species of fish and squid (Ruby, Oxygen). V. fischeri is a member of the Vibrionacea family of marine γ-proteobacteria which includes many species having symbiotic and pathogenic relationships with animals (Ruby, Complete genome). The bioluminescent bacteria chemically produces light in a reaction where a substrate molecule, luciferin is oxidized by an enzyme, luciferase. This process emits light energy rather than heat energy (Herring, Bioluminescence). The proteins necessary for the production of bioluminescence are encoded in a set of genes called the lux operon. The expression of the lux operon and other genes depend upon the presence of a signal molecule known as N-acyl homoserine lactone (AHL). The accumulation of the AHL is a function of population density and hence, bioluminescence can only occur after the bacterium reaches a critical population threshold. The expression of genes through the interaction of bacteria through a signal molecule is known as quorum sensing (NCBI). Intercellular communication via signal molecules has been shown to regulate genes whose products are needed for establishment of virulence, symbiosis, biofilm formation, plasmid transfer and morphogenesis in a variety of microorganisms (Wiley, Microbiology). Prior to its discovery in V. fischeri, quorum sensing and other mechanisms for bacterial cell communication was unknown. The particular symbiosis between a strain of V. fischeri and its host, the Hawian bobtail squid Euprmma scolopes, has been studied extensively. Comparative studies between symbiotic species, like V. fischeri, and human pathogenic species, like Vibrio cholerae, are being examined to elucidate the mechanisms for pathogenesis and pathogenic/host relationships (Ruby, complete).

Genome structure

To date, the complete genomes of two strains of V. fischeri, ES114 and MJII have been sequenced. Strain ES114 isolated from E.scolopes contains two circular chromosomes and a circular plasmid pES100. The genetic material is composed of DNA and the total genome is 4.284 Mbp in length. Chromosome 1, the larger of the two chromosomes contains 2586 genes, while Chromosome 2 and the plasmid contains 1175 and 57 genes respectively. The lux operon system, located on chromosome 2, contains luxI, luxR and luxCDABEG. LuxI encodes for AHL synthase, the enzyme that produces the autoinducer AHL. The Lux R gene produces the AHL dependent transcriptional activator protein that binds to the AHL and promotes the transcription of the lux operon. Luciferase, the enzyme responsible for catalyzing the light reaction, is encoded by luxA and lux B. LuxA produces the alpha chain subunit and luxB, the beta subunit of luciferase. LuxCDE are required for aldehyde synthesis (lux system,UK). LuxG encodes flavin mononucleotide reductase.

Ecology

Vibrio fischeri can be found in the upper 1000m of the ocean, living freely, or amongst the mixed biota in the guts of marine animals but is more commonly found as symbionts

Pathology

Vibrio fischeri is non-pathogenic to humans but three other members of the Vibrios family are. These are; V. cholerae, V.parahaemolytus, V. vulnificus.

References

http://www.isbc.unibo.it/Files/BC_PlanktonNekton.htm

http://web.uconn.edu/mcbstaff/graf/VfEs/VfEssym.htm