QUORUM SENSING IN GRAM-NEGATIVE BACTERIA

Four common characteristics found in almost all known gram-negative QS systems have been discussed by Ng and Bassler (2009).

AIs are AHLs or other molecules synthesised from S-adenosyl methionine (SAM) and diffuse easily through the bacterial membrane thereafter.
Specific receptors which reside either in the inner membrane or in the cytoplasm are bound by autoinducers.
Typically, quorum sensing modifies dozens to hundreds of genes that underpin different biological processes.
Through an AI-driven activation of QS, known as autoinduction, there is an increased synthesis of the AI. It creates a feed-forward loop that potentially encourages synchronous gene expression in the population.

Acyl-homoserine lactones are the most common class of AIs in gram-negative bacteria, having a core N-acylated homoserine-lactone ring carrying the acyl chain from C4 to C18 in length, with changes in the C3 position or unsaturated double bonds.
Only in rare cases does the expression of target genes in another species influence the AI of one bacterial species. Two main factors control this signalling specificity: the substratum specificity of the LuxI-like proteins and the specificity of the binding of the LuxR-like proteins to their cognate AHLs.

QUORUM SENSING IN GRAM-NEGATIVE BACTERIA — **Quorum-sensing synthases, autoinducers, and receptors.**

In most cases, by deriving the lactone moiety from SAM, with the specific acyl chain obtained from fatty acid anabolism, the LuxI enzymes produce AHLs. However, some exceptions are also found.
In the photosynthetic bacterium Rhodopseudomonas palustris, 4-coumaroyl-homoserine lactone synthase (RpaI) produces p-coumaroyl-homoserine lactone (HSL), for which the acyl group is a host metabolite, p-coumarate.
By utilising bacterial substrates, some plant-associated bacteria synthesise unusual HSL AIs. Aeromonas spp., for instance, Isovaleryl-HSL and Bradyrhizobium japonicum are produced, and cinnamoyl-HSL is produced by Bradyrhizobium BTAi.
Similarly, certain microbes such as Ralstonia solanacearum and Xanthomonas campestris have also been reported with atypical AIs.
The protein PhcB in R. Solanacearum synthesises one of the two related AIs that regulate virulence and biofilm formation, i.e. (3-OH PAME) and (R)-methyl-3-hydroxymyristate ((R)-3-OH MAME).
3-hydroxypalmitic-acidmethyl-ester. To facilitate transitions between its planktonic and biofilm-associated lifestyles, Xanthomonas campestris uses the diffusible signal factor cis-11-methyl-2- dodecenoic acid.
However, for accurate execution of QS, several issues such as the presence of several AIs, mixed species consortia, internal modifications, and external fluctuations need to be sorted out.
By uncovering common network design principles that occur in QS systems, systems biology has shed light on these aspects.
Four well-known QS pathways with cytoplasmic DNA-binding transcription factors in P. aeruginosa are currently present.
The QS systems in P. aeruginosa, with LasR at the top of the cascade and receptors, are organised in a hierarchy.
The second examples of a canonical QS circuit relying on membrane-bound receptors are provided by Vibrio harveyi and Vibrio cholerae.

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Quorum sensing in Bacteria and other Organisms