Team:Aix-Marseille/QS

Quorum sensing

Quorum sensing is a mechanism that allows bacteria to coordinate their behaviour depending on the bacterial population density [1]. It allows bacteria to adopt collective patterns of gene regulation to have, at a population level, an advantageous phenotype. Quorum sensing allows Xylella fastidiosa to produce a biofilm, a sticky extracellular matrix composed of DNA, proteins and polysaccharides, which is one of the key problems in the plant disease because it blocks xylem vessels.

Utilization of the DSF 2-cis-decenoic acid to stop the biofilm formation of bacteria.

In order to produce this biofilm, to communicate and coordinate the gene expression of the whole colony, X. fastidiosa mainly uses a Diffusible Signal Factor (DSF) which is a fatty acid with a 2-cis unsaturation[2]. The same system of communication regulates virulence. In addition to quorum sensing, bacteria can also use a similar mechanism called quorum quenching to modify the quorum sensing of other bacterial populations and outcompete them. Therefore, to stop X. fastidiosa forming the biofilm that kills plants we choose to quench it quorum sensing activity.

Pseudomonas aeruginosa under stress produces a similar fatty acid named 2-cis-decenoic acid, that is able to stop biofilm formation by many organisms[3]. Thus, we thought of including this specific DSF in KILL XYL to stop X. fastidiosa biofilm formation.

Design

T--Aix-Marseille--pQS2.png

As we wanted to limit the number of GMOs in our product we wanted to produce and purify the 2-cis-decenoic acid in E. coli. We choose to not use P. aeruginosa because of it pathogenicity.

Several enzymes are necessary for the production of 2-cis-decenoic acid by Pseudomonas aeruginosa, thay are not all present in E. coli. Thus, to produce the fatty acid, we designed biobricks to produce the three enzymes lacking in E. coli for the production of 2-cis-decenenoic and we optimized their sequences for production in E. coli. Part BBa_K2255000 is an enoyl-CoA hydratase, BBa_K2255001 an acyl-CoA isomerase and BBa_K2255002 a thioesterase.

We want to optimize the production of the different proteins so we designed these parts so they can be assembled to form an operon. Thus, BBa_K2255001 and BBa_K2255002 have a Ribosome Binding Site (RBS) integrated into the biobrick. To produce large amounts of these enzymes, we decided to add a strong and constitutive promoter in E.coli (BBa_K608002).

Results

Biofilm production without fatty acid after 24, 48, 72, and 96 hours.
Biofilm production with fatty acid 24, 48, 72, and 96 hours.

As a proof of concept, we studied the effect of the fatty acid on X. campestris. We used several different experimental conditions, such as the concentration of product and type of tube. After growing bacteria in media containing the product, we measured the effect of it on biofilm production using crystal violet visually (Protocol). Biofilm production can also be quantified with TECAN and is proportional with absorbance. We measured fatty acid effect by adding it at the start of growth. Then we observed biofilm production after 24, 48, 72, and 96 hours.

These results are very encouraging. We can see a significant decrease of biofilm production after 48, 72 and 96 hours. However, they need to be reproduced and the biofilm quantified and studied in more detail.


References

  1. Rutherford, S. T. & Bassler, B. L. Bacterial Quorum Sensing: Its Role in Virulence and Possibilities for Its Control. Cold Spring Harb Perspect Med 2, a012427 (2012).
  2. Ionescu, M. et al. Promiscuous Diffusible Signal Factor Production and Responsiveness of the Xylella fastidiosa Rpf System. mBio 7, e01054–16 (2016).
  3. Amari, D. T., Marques, C. N. H. & Davies, D. G. The Putative Enoyl-Coenzyme A Hydratase DspI Is Required for Production of the Pseudomonas aeruginosa Biofilm Dispersion Autoinducer cis-2-Decenoic Acid. J. Bacteriol. 195, 4600–4610 (2013).