Vibrio fischeri
A Microbial Biorealm page on the genus Vibrio fischeri
Classification
Higher order taxa
Bacteria; Proteobacteria; Gammaproteobacteria; Vibrionales; Vibrionaceae; Vibrio; Vibrio fischeri
Species
Vibrio fischeri
Description and significance
The family Vibrionaceae are motile, gram-negative rods that are natural inhabitants of seawater but can be found in fresh water. Vibrio fischeri, a species of bioluminescent bacterium, is frequently found in symbiotic relationships with marine animals like the bobtail squid. Their bioluminescence stems from their expression of a series of proteins contained in the lux operon.
Genome structure
The genome of Vibrio fischeri strain ES114 consists of 4,284,050 bp. Approximately 61.6% of the genome is AT rich and its coding density is 86.2% (1129 bp/gene). It contains two chromosomes both of which are circular. Chromosome 1 is 2,906,179 bp and chromosome 2 is 1,332,022 bp. This strain of bacteria is characterized by a 45.8 kbp plasmid (cir) called pES100. The pES100 plasmid encodes a putative type IV secretion system. The carriage of a plasmid that is similar to the pES100 plasmid is common among symbiotic strains of Vibrio fischeri, but is not required for host association.
Cell structure and metabolism
Vibrio fischeri is a gram negative bacteria composed of a cell wall that consists of an outer membrane containing lipopolysaccharides, a periplasmic space with a peptidoglycan layer, and an inner, cytoplasmic membrane.
One of its most distinguishing characteristics, bioluminescence, is controlled by a small set of genes known as the lux operon.
Luminescence is a chemical reaction in which the enzyme, luciferase, oxidizes organic compounds, such as long chain aldehyde and reduced flavin mononucleotide, so as to release free energy in the form of blue-green light at 490nm. The reaction that occurs in Vibrio fischeri is as follows: FMNH2 + RCHO + O2 --> FMN + RCOOH + H2O + hv(490nm)
Ecology
Luminescent bacteria are found in free-living, symbiotic, saprophytic or parasitic relationships. The symbiotic relationship between the squid and Vibrio fischeri provides an example of specific cooperativity during the development and growth of both organisms. For instance, once Vibrio fischeri cells utilize type IV pili to interact with the squid host, maturation of the light organs begin. Vibrio fischeri are helpful to the squid, a nocturnal forager, by erasing the shadow that would normally be seen as the moon's rays strike the squid, protecting the squid from its predators. The squid, in turn, provide the bacteria with shelter and a stable source of nutrients.
Pathology
Among the Vibrio species that are capable of causing human disease, there are two main groups: Vibrio cholerae infection and noncholera Vibrio infections. Vibrio infections are often characterized as a foodborne disease as a result of consumption of contaminated seafood, exposure of wounds to contaminated seawater, or injury caused by shark and alligator bites.
Infections associated with noncholera Vibrio species are gastroenteritis, wound infection and septicemia. V vulnificus is responsible for causing septicemia. Several mechanisms contribute to the virulence of Vibrio vulnificus. For instance, iron is an important growth factor. However, because free iron is absent in humans, Vibrio vulnificus produces siderophores that acquire iron from transferrin or lactoferrin and deliver it to the bacteria. The inability to produce siderophores leads to the reduction of virulence. Among all foodborne diseases, Vibrio vulnificus infection is linked with the highest case fatality rate.
Patient symptoms associated with gastroenteritis include diarrhea, abdominal cramps, nausea, vomiting, headaches, bloody stools, etc. Symptoms associated with noncholera wound infection include swelling, pain, erythema, bullae, and necrosis. And symptoms associated with septicemia include fever, hypothermia, hypotension, acute respiratory distress syndrome, and multiple organ dysfunction.
Application to Biotechnology
The ability of Vibrio fischeri cells to produce bioluminescence results from the expression of the lux operon.
The lux operon is a 9 kb fragment that consists of genes that code for the subunits of luciferase (luxAB) and for enzymes (luxCDE) that convert compounds to oxidizable substrates. This 9 kb fragment includes all that is necessary to carry out the functions of luminescence in other organisms such as E. Coli. It has been found that expression of luminescence is dependent on cell density and thus light is not seen unless the bacteria are in high concentration, like in light emitting organs of fish and squid. This phenomenon is known as quorum sensing.
The rationale behind quorum sensing has to do with the proteins encoded on the lux operon. The lux operon is actually set up like two differently transcribed operons. As mentioned before, luxA and luxB code for the subunits of the enzyme luciferase. LuxCDE codes for enzymes that convert fatty acids into aldehydes which are needed for the reaction to proceed. These genes, in addition to luxI and luxG, make up one operon. LuxI is at the head of this first operon and is responsible for the production of the auto inducer protein, homoserine lactone. This is the molecule that is involved in sensing the concentration of bacterium in a space. This protein can easily diffuse out of the cell, and if Vibrio fischeri were found floating freely in the ocean, as is sometimes seen, this auto inducer would diffuse out of the cell and float away. However, when in a restricted space, it is forced to react with the protein produced from the second operon, the regulator, luxR. This results in increasing the affinity of RNA polymerase to the promoter region of the first operon and eventually producing luminescence. Luminescence is then used to measure the level of gene expression in an organism.
Current Research
In order for researchers to understand the signalling communication between Vibrio fischeri and its host, the squid, they started to identify bacterial genes that played a role in bacterial colonization of the symbiotic light organ. They predicted that Vibrio fischeri mutants that were defective in their ability to reach high cell densities in the light organs would also display defects in their symbiotic luminescence levels. They were indeed correct. They identified two mutants that had significant colonization defects, KV712 and KV733, by screening a library of mutant Vibrio fischeri cells. The sequence similarity between the gene defective in KV712, now called rscS (regulator of symbiotic colonization) and sensory kinases allowed them to predict the role of RscS in the symbiosis. Researchers believed that the periplasmic loop of RscS recognized a signal from the squid, transmitted the signal to a response regulator protein(RscR) thatin turn functioned to up-regulate the transcription of genes required for the symbiotic phase of the Vibrio fischeri life cycle.
Currently, researchers are trying to find the critical time points during which bacterial signaling occurs. In order to aid in the process, they have generated cDNA libraries of aposymbiotic and symbiont juveniles at these time points. They are now subtracting these libraries to determine the gene expression induced by interaction with Vibrio fischeri. Once genes have been identified, they will then conduct research concerning the timing and location of expression in colonized host tissues.
Discoveries have been made that the bacterial symbiont, Vibrio fischeri, deregulates the expression of the peroxidase gene in tissues where it serves as a beneficial symbiont, but up-regulates the expression of the gene in tissues where Vibrio fischeri is viewed as a pathogen. This illustrates the fact that some of the same genes are involved in the control of both beneficial and pathogenic associations, and that it is their modulation that defines outcome of the relationship.
References
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Geszvain, K., and K. L. Visick. (2006). Roles of bacterial regulators in the symbiosis between Vibrio fischeri and Euprymna scolopes, p. 277-290. In J. Overmann (ed.), Molecular basis of symbiosis. Springer-Verlag, Germany.
Hallin PF, Ussery DW (2004). "CBS Genome Atlas Database: A dynamic storage for bioinformatic results and sequence data." Bioinformatics. 2004 Dec 12;20(18):3682-6. Epub 2004 Jul 15.
Hoi Ho, Thong Huy Do, Tony Tran Ho, Derek Lee. "Vibrio infections." 2007 January. Emedicine Specialties.
Miyamoto, M.C., Lin,H.Y., Meighen,A.E.(2000, May). "Control of bioluminescence in Vibrio fischeri by the luxO signal response regulator." Molecular Microbiology,36(3), 594-607.
Ruby, E.G., Urbanowski, M., Campbell, J., Dunn, A., Faini, M., Gunsalus, R., Lostroh, P., Lupp, C., McCann, J., Millikan, D., chaefer, A., Stabb, E., Stevens, A., Visick, K., Whistler, C., and Greenberg, E.P. "Complete genome sequence of Vibrio fischeri: A symbiotic bacterium with pathogenic congeners." Proc. Natl. Acad. Sci. USA. In press (2005), published online 9 February 2005.
Small, AL and MJ McFall-Ngai (1999). " A halide peroxidase in tissues that interact with bacteria in the host squid Euprymna scolopes." J Cellul Biochem 72:445-457.
Stevens AM, Greenberg EP. “Quorum sensing in Vibrio fischeri: essential elements for activation of the luminescence genes.” J Bacteriol. 1997 Jan;179(2):557-62.
Visick, KL and MJ McFall-Ngai (2000) Minireview. An exclusive contract: Specificity in the iVibrio fischeri-Euprymna scolopes partnership. J Bacteriol 182:1779-1787.
Edited by Maxine Mathew student of Rachel Larsen
