Understanding More of Vibrio’s Fischeri Life Cycle

Now, that we have a better understanding of this organism itself, it is time to further understand it’s life cycle.

Vibrio fischeri are usually found in marine subtropical waters as well as freshwater. Even though my previous posts were more focused in the Vibrio fischeri living in a symbiotic relationship with the bobtail squid, this organism is also capable of free-living; they just happen to be more commonly found living in symbiosis. However, the emittance of light through quorum sensing only occurs when this bacterium is living in colonies.

 

 

But You might ask:

“How do the bacteria find the squid or vice versa?”

Bobtail squid offspring are not hatched with the bacteria, they have to “get” them from their surroundings. In fact, within minutes after being hatched the squid will acquire millions of Vibrio fischeri. The fascinating part of all of this to me is that newborn squids will produce reactive oxygen species (ROS) in order to prevent the attachment of bacterium other than Vibriofisheri.

 In the same manner, the Vibrio fischeri bacterium present in the surrounding will trigger the new hatch squid into secreting mucus from the appendage of its light organ, and then Vibrio fischeri which is attracted to the mucus components will display chemotaxis moving towards the light organ for attachment. Upon attachment, will occur the colonization phase, where the light organ will by the beginning of the night be completely formed and functional.

With the night, comes the moon, with the moon, comes the light and shadow. So, the light organ allows the bobtail squid to disguise and not cast a shadow, making them a less visible prey for its predators and making them a greater predator for its prey. More food for the bobtail squid also means more nutrients for the Vibrio fischeri, making them a perfect partner for a happy mutualistic symbiotic relationship. 

But, as we all know, happiness does not last forever, in this case, it only lasts until the morning arrives. Furthermore, the bobtail squid will expel (yes, kick them out) 95 % percent of the Vibrio fischeri. The reason, why it leaves approximately 5% is to facilitate the replication of more Vibrio fischeri which by the night time will have covered the light organ all over again, instead of having to get them again from its surrounding.

This indicates, that Vibrio fischeri are capable of replicating fast but it still not known the exact growth rate constant for its natural habitat but when mimicking its natural environment in the laboratory the results are the following. In favorable conditions, with the perfect salt and iron concentration, the bacterium takes advantage of its short generation time, in the lab when media mimics its natural habitat the growth rate constant is K= 1.2738. 

In addition, Vibrio fischeri is a relatively easy bacterium to culture in the laboratory setting since they do not need high ending apparatus to do so. They are facultative anaerobes, capable of respiratory and fermentative metabolism making them chemoorganotrophic. So, in the lab, the exposure to oxygen will positively affect them and their optimal growth temperature occurs at room temperature which is not hard to provide. Moreover, it is possible to maintain their bioluminescent glow by subculturing them 2 to 3 times per week in photobacterium agar.

Vibrio fischeri, is considered a heterotrophic bacterium which means that they are not capable of producing its own food by carbon fixation and needs to consume sources of organic carbon for nutrition. Since, most of them live in symbiotic relationship with marine organism their nutrient source depends in what kind of host they are, for example, the ones that reside in the bobtail squids receives nutrients that can vary from brine shrimp, mosquitofish, and prawns.

One interesting fact the nutrition of Vibrio fischeri is that they require an intake of iron because iron is a key growth factor and seems to play a role in the regulation of bioluminescence. Moreover, how the iron helps in the regulation is currently being further studied, but there is a hypothesis that the iron helps in the regulation through inhibiting the synthesis of luciferase which is the enzyme responsible for bioluminescence ability of the organism.

Vibrio fischeri reproduces by binary fission. In the process, a single cell will further divide into two identical daughter cells, followed by the same process over and over by the daughter cells (personally, I find fascinating how bacteria can reproduce through basically making clones of themselves). 

Vibrio fischeri does not have a unique enzyme in the context of replication. However, like briefly mentioned before they have a unique gene calledLuxthat synthesizes autoinducers. Upon release of these autoinducers, it will help the bacteria to “determine” cell density. The higher the concentration for autoinducers the higher the cell density. The same autoinducers will bind to a regulatory gene in which will turn it on the Lux lac operon genes to synthesize the luciferase, which is their unique enzyme that catalyzes a redox reaction in which the byproduct is light.

 

 

FIGURE 6. Lux pathway controlling bioluminescence in V. fischeri. At low cell density, the sensor kinases AinR and LuxP/Q are predicted to exhibit net kinase activity leading to the phosphorylation of LuxU and subsequent phosphotransfer to LuxO. Phospho-LuxO induces the expression of the inhibitory sRNA, qrr1, which leads to the degradation of litR mRNA. LitR is the transcriptional activator of luxR, which encodes a protein required for expression of the luxCDEBAG operon. Thus, at low cell density, litR translation is inhibited and the cells do not produce high levels of light. At high cell density, two distinct autoinducer molecules made by AinS (C8-HSL, diamonds) and LuxS (AI-2, circles) are predicted to be at sufficient concentrations to switch the activity of the SKs from net kinase to net phosphatase activity. This leads to dephosphorylation of the downstream regulators, litR translation, and transcription of luxR. LuxR, in conjunction with the autoinducer produced by LuxI (3-oxo-C6-HSL, triangles), leads to the transcription of the lux operon and bioluminescence (reviewed in Stabb et al., 2008). LuxR is also predicted to weakly bind to C8-HSL, which allows for the initiation of luxCDABEG expression. See text for caveats to this model.

 

Now, last observation:

“How cool it would be if we were able to auto-induce ourselves into expressing cool genes like that?”

 

 

I hope you enjoyed learning with me

 

See you guys soon.

 

References/Sources:

 

  1. Doino, J.A., and M. J. McFall-Ngai. 1995. A transient exposure to symbiosis-competent bacteria induces light-organ morphogenesis in the host squid. Biol. Bull. 189:347-355.
  2. Fidopiastis PM, Miyamoto CM, Jobling MG, Meighen EA, Ruby EG. 2002. LitR, a new transcriptional activator in Vibrio fischeri, regulates luminescence and symbiotic light organ colonization. Mol. Microbiol. 45:131-43.
  3. Ruby, E. G. and M. Urbanowski. 2004. Complete genome sequence of Vibrio fischeri: A symbiotic bacterium with pathogenic congeners. PNAS 102: 3004-3009.
  4. “V. Fischeri.” Strain Info. Genomic Standards Consortium, n.d. Web. 25 Oct. 2011. <http://www.straininfo.net/genomes/12986>.
  5. https://microbewiki.kenyon.edu/index.php?title=Vibrio_fischeri_BI246_General_Microbiology_Skidmore_College_Fall_2011&action=edit
  6. https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=3553747
  7. http://www.phys.ufl.edu/REU/2007/reports/johnson.pdf
  8. http://en.citizendium.org/wiki/Vibrio_fischeri#cite_note-12

 

 

 

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