A Novel Molecular Approach
For our biosensor, our team genetically modified E. coli cells to be able to elecit a fluorescent response to autoinducers produced by the pathogen Pseudomonas aeruginosa during quorum sensing. In the case of P. aeruginosa, these autoinducers are N-3-oxo-dodecanoyl-L-homoserine lactone, or 3-oxo-C-12-HSL for short. The quorum sensing system of this pathogen contains the LasR activator which binds 3-oxo-C-12-HSL, and the LasI promoter, which is activated by the LasR-HSL complex. Both LasR activator and LasI promoter are available as BioBricks C0179 and J64010.
As a reporter gene, we use GFP. However, expression of GFP is not simply controlled through the LasI promoter activity in our approach. Instead, our sensor cells contain genes for a constitutively expressed fusion protein consisting of GFP and a dark quencher, and an HSL-inducible protease. We use the REACh protein as dark quencher for GFP and the TEV protease to cleave the complex; here you can read more about the REACh construct and the TEV protease.
When P. aeruginosa cells are stuck on our agar chip and come close to our sensor cells, the latter take up the HSL molecules secreted by the pathogens. Inside the sensor cells, the autoinducer binds to the LasR gene product and activate the expression of the TEV protease. The protease then cleaves the GFP-REACh construct. When illuminated with light of 480 nm, the excitation wavelenght of GFP, our sensor cells in the vicinity of P. aeruginosa give a fluorescence signal. On the other hand, sensor cells that were not anywhere close to the pathogens do not express the protease. Therefore, the GFP will still be attached to the dark quencher in these cells, and no fluorescence is produced.
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