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.
- 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.