Aliivibrio fischeri
The bacterium is a key research organism for examination of microbial bioluminescence, quorum sensing, and bacterial-animal symbiosis.
[7] Ribosomal RNA comparison led to the reclassification of this species from genus Vibrio to the newly created Aliivibrio in 2007.
Aliivibrio fischeri utilizes chitin as a primary carbon and nitrogen source in its symbiosis with the Hawaiian bobtail squid.
In the squid’s light organ, A. fischeri breaks down chitin into N-acetylglucosamine (GlcNAc), which acts as both a nutrient and a chemoattractant, guiding colonization.
The bacteria metabolize GlcNAc through fermentation or respiration, supporting energy needs and bioluminescence, which are crucial for the mutualistic relationship with the squid.
[7] This mutualistic symbiosis provides A. fischeri with nutrients and a protected environment and helps the squid avoid predation using bioluminescence.
A. fischeri provides luminescence by colonizing the light organ of the Hawaiian bobtail squid,[17] which is on its ventral side.
[17] This strategy prevents the squid from casting a shadow on the ocean floor, helping it avoid predation during feeding.
[7][17] Morphological changes made by A. fischeri do not occur when the microbe cannot luminesce, such as a decrease in the number of pores in the light organ.
Additionally, if colonization by A. fischeri is abruptly removed by antibiotics, the ciliated epithelium of the light organ will regress.
In the process of colonization, ciliated cells within the animals' photophores (light-producing organs) selectively draw in the symbiotic bacteria.
These cells create microcurrents that, when combined with mucus,[16] promote the growth of the symbionts and actively reject any competitors.
[2] The population of A. fischeri needs to reach an optimal level to activate the lux operon and stimulate light production.
The circadian rhythm controls light expression, where luminescence is much brighter during the day and dimmer at night, as required for camouflage.
Several external and intrinsic factors appear to either induce or inhibit the transcription of this gene set and produce or suppress light emission.
[citation needed] A. fischeri is one of many species of bacteria that commonly form symbiotic relationships with marine organisms.
[21] The lux operon is a 9-kilobase fragment of the A. fischeri genome that controls bioluminescence through the catalytic activity of the enzyme luciferase.
Luciferase produces blue/green light through the oxidation of reduced flavin mononucleotide and a long-chain aldehyde by diatomic oxygen.
In addition, C8-HSL binds to another transcriptional regulator, LitR, giving the ain and lux systems of quorum sensing slightly different genetic targets within the cell.
[34] It plays a key role in studying the effects of chemical mixtures, helping identify synergistic or antagonistic toxic interactions.
Natural transformation of A. fischeri facilitates rapid transfer of mutant genes across strains and provides a valuable tool for experimental genetic manipulation in this species.
(A) At low cell density, the autoinducers (3OC6-HSL – red dots), produced by LuxI, diffuse through the cell membrane into the growth medium
(B) As the cell growth continues, the autoinducers in the medium start to accumulate in a confined environment. A very low intensity of light can be detected.
(C) When enough autoinducers have accumulated in the medium, they can re-enter the cell where they directly bind the LuxR protein to activate luxICDABEG expression.
(D) High levels of autoinducers activate the luminescent system of A. fischeri . A high intensity of light can be detected.