Research Paper: Luminous Bacterium Vibrio Fischeri

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Luminous Bacterium Vibrio Fischeri

Vibrio species are gram negative rods that are facultative anaerobes and are mainly found in aquatic environments. Vibrio are distinct from the Enterobacteriaceae in that they react positively for oxidase and have polar flagella. They are a known cause of gastrointestinal diseases in humans, such as Vibrio cholerae, the causative agent of cholera (Murray, 1998). However, the luminous bacterium, Vibrio fischeri, is a water borne organism that can be found both free living or in a symbiotic, mutualistic relationships with squid. Bioluminescence is the chemical production of light within the body of primarily marine fish and invertebrates (Sea and Sky, 2010). The chemical reaction that produces light occurs when the chemical luciferin and oxygen are mixed together in the presence of the enzyme luciferase. The purpose of bioluminescence can be for navigation, communication or camouflage. At the molecular level this reaction is governed by a well described genetic system known as the Lux Operon.

The V. fischeri and squid symbiosis is of great historical significance for many reasons. It is a valuable model for examining bioluminescence, pheromone signaling and symbiotic bacteria-animal interactions (Lyell, 2010). Pheromone signaling, such as quorum sensing, is an important form of bacterial communication. In quorum sensing, small molecules known as autoinducers are exchanged intercellularly (Breitbach, 2010). These molecular messages give cell density dependent cues to bacteria as to when to turn on or off certain genes such as the genes responsible for bioluminescence. The formation of a symbiotic state involves a complex "communication" between the bacteria and host (Nyholm, 2008). Study of this relationship offers insights into each organism's well-being as well as the maintenance of a mutual relationship as opposed to a pathogenic one. The system has offered numerous opportunities to look at the problems presented by bacteria-host interactions and insight into how each organism overcomes these obstacles (Visick, 2009). The formation of a symbiotic state can help us elucidate the healthy state of organisms and what factors are disrupted by pathogenic invasion (Chun, 2008). Also, the study of symbiotic relationships can lead to insight into the host-pathogen relationships and suggest potential medical targets for research (Breitbach, 2010). This particular symbiotic system also has the advantage of being replicable in the laboratory setting. This enhances the ability of researchers to manipulate and study the entire bacteria-host relationship rather than with each organism separately in artificial environments. Additionally, since the symbiotic relationship of V. fischeri and squid is affected by environmental factors such as nutrient availability, temperature, and salinity, this system offers a unique perspective into determining how quickly an organism can adapt to changing environments. This will be especially important in the context of global warming because the ability to adapt will determine whether an organism will survive as its environment changes potentially very rapidly (Nyholm, 2008).

V. fischeri are found free living in aquatic environments. They can be found in both fresh and salt water habitats including lakes, rivers and marine habitats (Nyholm, 2008). Squid take in seawater through their mantle cavity, which exposes bacteria, including V. fischeri, to the light organ. Squid light organs are known to contain only one to three species of Vibrio or Photobacterium. This illustrates the specificity of the squid-bacteria relationship, because other bacteria are constantly ventilated through the squid mantle and yet are not successful at colonizing the light organ. One amazing statistic is that during each ventilation by a juvenile squid only one V. fischeri will be present in the mantle cavity and has less than one second before being expelled from the cavity (Nyholm, 2008). Against these great odds V. fischeri are able to successfully outcompete other bacteria and colonize the light organ.

Although this is a mutually beneficial relationship for both the squid and the bacteria, the squid has many bacteriocidal defenses in its light organ. First, V. fischeri aggregate on the surface of the light organ in a biofilm. Then, they must swim against a cilia induced current through a narrow tube. Motility is extremely important in V. fischeri's ability to colonize the light organ. The environment of this narrow tube contains antibacterial chemicals and toxins, which V. fischeri has adapted to overcome. Once in the light organ crypt, squid immune molecules similar to macrophages known as hemocytes protect the crypt space. Somehow, V. fischeri are recognized by the hemocytes as "friendly" and are not encapsulated by hemocytes in adult squid (Nyholm, 2008). Also, on a daily basis, the squid empty out the crypt spaces of the majority of V. fischeri and the whole process starts over again. It is thought that this serves as the inoculum for juvenile squid that are born without bacteria in the light organ. All this occurs so that the squid can take advantage of V. fischeri's bioluminescent quality. The light organ of squid functions as a form of camouflage. In this way predators of the squid that are below cannot see the shadow of the squid against moonlight from above (Nyholm, 2008). V. fischeri gain a nutrient rich environment to live in from the squid.

The symbiotic relationship of V. fischeri with squid offers such a wealth of knowledge that much research has focused on the different aspects of this relationship as well as environmental factors affecting the relationship. Much about the effect of external, environmental factors is summarized in an article by Nyholm and Nishiguchi (2008). Since V. fischeri are transmitted to squid from the environment, environmental factors play a role in the conferring of symbiotic competence to the microorganisms. Some research suggests that temperature controls which Vibrio species, fischeri or logei, will be more successful at colonizing the light organ of squid. Both species are found more abundantly in the wintertime when more nutrients are available due to the disappearance of the thermocline, which is in contrast to the original hypothesis that V. logei species would be more abundant due to their cold loving nature. This indicates that nutrient availability plays a stronger role in selecting the abundance of free living, aquatic Vibrio species than temperature. Also, V. fischeri species that live in an environment that is more varied are more sensitive to changes in both salinity and temperature than those living in less variable environments. Whether this could lead to a competitive advantage for V. fischeri that can more quickly adapt to changing environments will be interesting to find out. It is likely that a lot competition between bacterial species occurs in the aquatic environment where nutrients are relatively scarce as compared to the nutrient rich environment of the squid light organ.

Other research has focused on the transcriptional regulation of genes within V. fischeri that are important in the bacterial-host relationship. One key aspect of V. fischeri ability to form a successful symbiosis is the formation of a biofilm that allows colonization and aggregation of V. fischeri on the light organ epithelium (Visick, 2009). The syp gene locus was found to be critical in the formation of this biofilm and is regulated by the sensor kinase RscS, which phosphorylates SypG. SypG then can activate syp transcription. Mutants lacking RscS are unable to successfully initiate a symbiotic relationship with squid. It is especially important that biofilm formation by V. fischeri in culture has been positively correlated with an important function in the bacterial-animal association. Another study involving the molecular regulation of the Lux Operon in V. fischeri further points out the strength of a model system the can be observed in the laboratory intact. In this study pheromone signaling in an ecologically relevant context could be studied. The researchers have elucidated a very complex regulatory network that governs bioluminescence in V. fischeri and numerous environmental factors that play a role in the transcriptional control of the Lux Operon. It has turned out to be far more complicated than previously thought and rivals that of mammalian transcriptional control. This study has shown the importance of magnesium, redox state and inorganic phosphate in regulation of the Lux Operon and in future research these factors should be looked at in the host light organ. Another study found that several factors have a hierarchical effect on the host (Chun, 2008). The researchers used microarray technology to analyze the differential gene regulation of squid when exposed to non-symbiotic bacteria, wild type symbiotic bacteria and bacterial mutants deficient in either bioluminescence or autoinducer production. The researchers found that the presence of bacteria produced the greatest response in squid, then the bacteria's ability to luminesce and finally the bacteria's ability to produce autoinducers. Some conserved gene homologs were found in the V. fischeri and squid symbiosis that were also important in other host-pathogen associations. This only further underscores the importance of V. fischer and squid as a model, but also indicates that there are some conserved genes that are important for epithelial colonization regardless of the type of symbiosis.

On another interesting front, one research group has found a novel way to control quorum sensing in V. fischeri using a chemical embedded in a biofilm that can be… [END OF PREVIEW]

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Luminous Bacterium Vibrio Fischeri.  (2010, November 15).  Retrieved May 20, 2019, from https://www.essaytown.com/subjects/paper/luminous-bacterium-vibrio-fischeri/8235

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"Luminous Bacterium Vibrio Fischeri."  Essaytown.com.  November 15, 2010.  Accessed May 20, 2019.
https://www.essaytown.com/subjects/paper/luminous-bacterium-vibrio-fischeri/8235.