EXTENSION This extension explores the cellular mechanisms of quorum sensing by examining the case study of bioluminescent Vibrio bacteria. To learn more about these bacteria, please see this introductory video by Dr. Bonnie Bassler: - YouTube.
Bonnie Bassler’s TED Talk on the subject: How bacteria "talk" - Bonnie Bassler - YouTube.
All articles listed below are open access.
Talà A, Delle Side D, Buccolieri G, et al. Exposure to Static Magnetic Field Stimulates Quorum Sensing Circuit in Luminescent Vibrio Strains of the Harveyi Clade. PLoS One. 2014; 9(6): e100825. Published online 2014 Jun 24. doi: 10.1371/journal.pone.0100825 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4069165/
In their Introduction, this paper provides a thorough summary of the current research on Vibrio bioluminescence.
“In Vibrio spp. bioluminescence is regulated by quorum sensing (QS) , the communication circuit that many bacteria use to sense population density and regulate, in a coordinate fashion, a diverse array of physiological activities that are presumably productive only when groups of cells act in concert . QS regulation of Harveyi clade Vibrio spp. is rather complex. These bacteria produce and respond to three autoinducers (AIs): HAI-1 ( N -[β-hydroxybutyryl] homoserine lactone), a species-specific AI –, CAI-1 ([ S ]-3-hydroxytridecan-4-one), a genus-specific signal –, and AI-2 ([2 S ,4 S ]-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran borate), an interspecies signal –.
HAI-1, CAI-1, and AI-2 are detected by the membrane-bound, two-component sensors LuxN, CqsS, and LuxQ, respectively –. At low cell density, when the AIs concentrations are low, the sensors act as kinases and transfer phosphate to the histidine phospho-transfer protein LuxU, which in turn, passes the phosphate to the response regulator LuxO –. Phospho-LuxO, together with sigma 54, activates the expression of the genes encoding five regulatory small RNAs (sRNAs) called Quorum Regulatory RNA 1–5 (Qrr1–5) –. In conjunction with the RNA chaperone Hfq, the sRNAs destabilize the mRNA encoding the master quorum-sensing regulator LuxR . Because LuxR is required for transcription of the luxCDABE operon, coding for the luciferase and the fatty acid reductase complex, under low-cell-density conditions luminous bacteria produce reduced levels of luminescence that may be below detection. At high cell density, the presence of the AIs converts the sensors from kinases to phosphatases, resulting in dephosphorylation and inactivation of LuxO, no Qrr expression, and stabilization of the luxR mRNA, leading to LuxR production. Thus, under high-cell-density conditions, light is produced.”
Verma SC and Miyashiro T. Quorum Sensing in the Squid - Vibrio Symbiosis. Int J Mol Sci. 2013 Aug; 14(8): 16386–16401. Published online 2013 Aug 7. doi: 10.3390/ijms140816386 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3759917/
Verma and Miyashiro expand upon the specifics of the symbiotic relationship between Vibrio bacteria and the Hawaiian bobtail squid, Euprymna scolopes . “The symbiosis is established when V. fischeri cells migrate via flagella-based motility from the surrounding seawater into a specialized structure injuvenile squid called the light organ. The cells grow to high cell densities within the light organ where the infection persists over the lifetime of the animal. A hallmark of a successful symbiosis is the luminescence produced by V. fischeri that camouflages the squid at night by eliminating its shadow within the water column.”
Figure 1. Please see Figure 1 from Verma and Miyashiro. “The QS network of V. fischeri. V. fischeri has three QS systems: LuxI-LuxR, AinS-AinR, and LuxS-LuxP/Q. In the absence of C8-HSL and AI-2 autoinducers, LuxO is phosphorylated by the kinase activities of the histidine kinases AinR and LuxQ. Phosphorylated LuxO activates expression of the sRNA Qrr1, which degrades via Hfq the mRNA of litR, thereby reducing the level of the transcription factor LitR. Accumulation of C8-HSL and AI-2 at high cell density results in decreased phosphorylation of LuxO, which enhances the level of LitR. LitR activates transcription of luxR, which encodes the transcription factor that, when bound by the autoinducer 3-oxo-C6-HSL, directly regulates expression of the luminescence (lux) genes. C8-HSL can also affect luminescence by directly binding to LuxR. The LuxR/C8-HSL complex can activate transcription of the lux genes, although less effectively than the LuxR/3-oxo-C6-HSL complex. In addition to encoding the light-producing enzyme luciferase, the lux operon contains luxI, which encodes the synthase LuxI that synthesizes 3-oxo-C6-HSL. As described in the main text, synthesis of both C8-HSL and 3-oxo-C6-HSL is autoregulated by separate positive feedback loops. OM = outer membrane, IM = inner membrane.”
There are also images and micrographs of the light organ in E. scolopes :
Figure 2. “The light organ of a juvenile E. scolopes harboring V. fischeri. (A) Bright field image showing the ventral side of a juvenile E. scolopes. The dark structure highlighted in the box is the light organ. Scale bar = 1 mm. E = eye; (B) Differential interference contrast (DIC) image of a 48-h p.i. light organ colonized with GFP-labeled V. fischeri cells (green). Scale bar = 100 μm. Ap = appendages; (C) Confocal image of a light organ crypt colonized with GFP-labeled V. fischeri cells (green). Host actin is stained with phalloidin (blue). Scale bar = 10 μm.”
Henke JM and Bassler BL. Three Parallel Quorum-Sensing Systems Regulate Gene Expression in Vibrio harveyi . J Bacteriol. 2004 Oct; 186(20): 6902–6914. doi: 10.1128/JB.186.20.6902-6914.2004 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC522208/
For the interested student, Bassler (who delivered the aforementioned TED talk) and Henke discuss further the quorum sensing systems in Vibrio harveyi , as well as quorum sensing in Vibrio cholerae.
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