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 (Hallin PF, Ussery DW). 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 similar to the pES100 plasmid is common among symbiotic strains of Vibrio fischeri, but is not required (Ruby, E.G., Urbanowski, M, et al.).
Cell structure and metabolism
Vibrio fischeri is an oxidase-positive, 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 (Hoi Ho, et al.).
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) (Davis, Aubrey).
Ecology
Luminescent bacteria are found in free-living, symbiotic, saprophytic or parasitic relationships. The symbiotic relationship between Euprymna scolopes, a small Hawaiian squid, and Vibrio fischeri provides an example of specific cooperativity during the development and growth of both organisms (Geszvain, K., and K. L. Visick). 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.
Quorum sensing bacterias help the regulation of the gene expression of Vibrio Fisheri in response to changes in cell density. Quorum sensing bacteria produce and release autoinducers. Autoinducers modify gene expression after a sudden change in the cell population density. Quorum sensing bacteria relies upon the synthesis of a signal molecule (N-acylhomoserine lactone or AHL). The protein LuxI synthesizes AHL. Then, the AHL diffuses out the bacterial cells. Since, more bacteria are present, the cell density increases. After stimulation, The LuxR AHL complex bind to the lux box causing the activation of luminescence genes. V.fisheri exists at low cell densities when they are free-living. On the contrary, V.fisheri has high cell densities when they are colonising the light organ.
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 due to consumption of contaminated seafood, exposure of wounds to contaminated seawater, or injury caused by shark and/or alligator bites (Finkelstein RA).
Infections associated with noncholera Vibrio species are gastroenteritis, wound infection and septicemia, which is blood poisoning. Vibrio vulnificus is responsible for causing septicemia. Several mechanisms contribute to the virulence of Vibrio vulnificus. For instance, iron is an key growth factor. However, because it is not present in humans, Vibrio vulnificus constructs siderophores that get their iron from transferrin or lactoferrin and transport it to the bacteria. The inability to make siderophores decreases the virulence of Vibrio vulnificus. Among all foodborne diseases, Vibrio vulnificus infection is linked with the highest 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 (Hoi Ho, et al.).
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 autoinducer 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 (Stevens AM, Greenberg EP). Luminescence is then used to measure the level of gene expression in an organism.
Current Research
In order for researchers to understand how Vibrio fischeri and its host, Euprymna scolopes, communicate, they began to look for bacterial genes that were involved in the colonization of the symbiotic light organ. They expected that Vibrio fischeri mutants that were unable to reach high cell densities in the light organs would also reveal deficiencies in their symbiotic luminescence levels. They were indeed correct. They identified two mutants, KV712 and KV733, that had significant colonization defects by screening a library of mutant Vibrio fischeri cells (Miyamoto, M.C., Lin,H.Y., Meighen,A.E.). The similarity of the sequences between the gene defective in KV712, also known as 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 the signal sent by the squid. The signal was then transmitted to a response regulator protein(RscR), which in turn functioned to increase the transcription of genes required for the symbiotic phase of the Vibrio fischeri life cycle (Yip, E.S., et al.).
Currently, researchers are trying to find the critical time points during which bacterial signaling occurs. In order to facilitate the process, they have constructed cDNA libraries at these time points of both aposymbiotic and symbiont juveniles (Visick, KL and MJ McFall-Ngai). They are now subtracting these libraries to determine the gene expression brought about by interaction with Vibrio fischeri. Once potential genes have been identified, they will then conduct further research concerning the timing and location of gene expression in colonized host tissues.
Discoveries have been made that Vibrio fischeri de-regulates the expression of the peroxidase gene in tissues where it acts as a beneficial symbiont and conversely up-regulates the expression of the peroxidase gene in tissues where it is viewed as a pathogen (Small, AL and MJ McFall-Ngai). This illustrates the fact that some of the same genes are involved in the control of beneficial and pathogenic associations. Therefore, it is the modulation of the genes that describes the outcome of the relationship.
References
Davis, Aubrey. Biology (BIMM) 101 Lab Manual. p. ii. AS Soft Reserves, Winter 2007.
Finkelstein RA. "Cholera, the cholera enterotoxins, and the cholera enterotoxin-related enterotoxin family." p. 85. In Owen P, Foster TS (eds): Immuno-chemical and Molecular Genetic Analysis of Bacterial Pathogens. Elsevier, Amsterdam, 1988. http://www.gsbs.utmb.edu/microbook/ch024.htm
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. http://www.meddean.luc.edu/lumen/DeptWebs/microbio/kv.htm
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. http://www.cbs.dtu.dk/services/GenomeAtlas/show-genus.php?KLSO=ASC&KLSK=ORGANISMSORT&kingdom=Bacteria&GLgenus=Vibrio&GLSHWPLA=on&GLSHWMERG=on&GLspecies=fischeri&GLsupStrain=ES114
Hoi Ho, Thong Huy Do, Tony Tran Ho, Derek Lee. "Vibrio infections." 2007 January. Emedicine Specialties. http://www.emedicine.com/med/topic2375.htm
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. http://aem.asm.org/cgi/content/full/70/4/2520
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. http://www.pnas.org/cgi/content/abstract/102/8/3004
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. http://www.kewalo.hawaii.edu/labs/mcfall-ngai/
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. http://jb.asm.org/cgi/content/abstract/179/2/557
Visick, KL and MJ McFall-Ngai (2000) Minireview. "An exclusive contract: Specificity in the iVibrio fischeri-Euprymna scolopes partnership." J Bacteriol. 182:1779-1787. http://www.hawaii.edu/zoology/faculty/mcfall-ngai.htm
Yip, E. S., K. Geszvain, C. R. DeLoney-Marino, and K. L. Visick. (2006). "The symbiosis regulator RscS controls the syp gene locus, biofilm formation and symbiotic aggregation by Vibrio fischeri." Mol. Microbiol. 62:1586-1600. http://www.meddean.luc.edu/lumen/DeptWebs/microbio/KV/research.htm
"QS in Vibrio Fischeri." The University of Nottingham. Web. 08 Apr. 2011. <http://www.nottingham.ac.uk/quorum/fischeri2.htm>.
Edited by Maxine Mathew student of Rachel Larsen
