Quorum Sensing


Many bacterial processes are dependent on the presence of a population of cells. Quorum sensing is a term that has been coined to describe the ability of bacteria to sense the population of bacteria around them. Bacteria achieve this by detecting the concentration of signalling molecules that they secrete into the surrounding environment.

Keywords: communication; signalling; bacteria; gene regulation

Figure 1.

Quorum sensing control of bioluminescence in Vibrio fischeri. At low cell density (top panel) there is a basal level of transcription of the lux operons. This results in low levels of the lux transcriptional activator, LuxR and the AHL signal generator, LuxI. 3‐Oxo‐C6‐HSL is synthesized by LuxI. However, due to the low cell density the concentration of the signal molecule remains low. At a high cell density (bottom panel), the concentration of 3‐oxo‐C6‐ HSL surpasses a threshold concentration and activates LuxR, resulting in the induction of the left (OL) and right (OR) lux operons. This results in the production of more LuxR and more AHL, creating a positive feedback loop. At the same time there is a rapid induction of the bioluminescence genes, which results in the production of light. SAM, S‐adenosylmethionine; ACP, acyl carrier protein.

Figure 2.

Schematic representation of AHL‐dependent quorum sensing in Gram‐negative bacteria.

Figure 3.

Structures of the N‐acylhomoserine lactone family of molecules. The figure illustrates the structural features of AHL molecules purified from bacterial culture supernatants. AHL molecules differ in substitutions of groups at the C‐3 position of the acyl chain (R1) and also in the composition of the acyl chain itself (R2).

Figure 4.

The use of the Chromobacterium violaceum biosensor (CVO26) to detect AHL signal molecules. (a) A ‘T‐streak’ test for the production of AHLs. CVO26 biosensor (vertical) and two strains of Aeromonas (A. salmonicida, top; A. hydrophila, bottom) have been streaked on to an agar plate and incubated at 30°C overnight. The diffusion of AHL molecules from the Aeromonas strains has induced pigment production in the C. violaceum biosensor closest to the test bacteria. (b) An extract of Erwinia carotovora culture supernatant run on a TLC plate and overlaid with an agar lawn containing the C. violaceum biosensor (CVO26). In addition to the extract, AHL standards have also been included. Lane 1, E. carotovora extract; lane 2, AHL standards C6‐HSL (bottom) and 3‐oxo‐C6‐HSL (top); lane 3, AHL standards C8‐HSL (bottom) and C4‐HSL (top) and lane 4, AHL standard 3‐oxo‐C8‐HSL. Direction of flow of the solvent is bottom upwards.

Figure 5.

AHL degradation pathways. (a) Lactonase or alkaline pH‐driven opening of the HSL ring to form the corresponding homoserine compound. (b) Alkaline pH‐driven intramolecular rearrangement to form the corresponding tetramic acid. This reaction only occurs with 3‐oxo‐substituted AHLs. (c) Cleavage of the acyl chain from the HSL moeity by an acylase to release the corresponding fatty acid and HSL.

Figure 6.

Structures of some non‐AHL quorum sensing signal molecules. PQS, 2‐heptyl‐3‐hydroxy‐4‐quinolone; HHQ, 2‐heptyl‐4‐quinolone; 3OH‐PAME, 3‐hydroxy‐palmitic acid methyl ester; DSF, cis‐11‐methyl‐2‐dodecenoic acid; AI‐2, furanosyl borate diester; AIP‐1, peptide thiolactone; A‐factor, γ‐butyrolactone.

Figure 7.

Quorum sensing control of the agr locus in Staphylococcus aureus. The agr locus consists of two divergent operons (P2 and P3) encoding two proteins (AgrB and AgrD) involved in the production of the agr peptide signal, two proteins that detect the signal (AgrA and AgrC) and an RNA molecule (RNAIII) that acts as the effector, modulating virulence gene expression. Only when the concentration of the signalling peptide (AIP‐1 peptide shown) surpasses a threshold concentration does activation of the sensory cascade take place. Once activated, AgrC is thought to trans‐activate AgrA, which in turn activates the bidirectional transcription of the agr P2 and P3 operons.



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Further Reading

Demuth DR and Lamont R (eds) (2006) Bacterial cell‐to‐cell communication. Role in virulence and pathogenicity. In: Advances in Molecular and Cellular Microbiology (No.11). Cambridge: Cambridge University Press.

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Shapiro JA and Dworkin M (eds) (1997) Bacteria as Multicellular Organisms. New York: Oxford University Press.

Williams P (ed.) (2006) Quorum sensing in human pathogens. International Journal of Medical Microbiology 296: 57–170.

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Holden, Matthew TG, Diggle, Stephen P, and Williams, Paul(Dec 2007) Quorum Sensing. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001426.pub2]