2D Biosensor

Principle of Operation

Cellock Holmes is devised based upon a SynBio approach comprised of a two-dimensional biosensor and a measurement device. The two-dimensional biosensor is designed to recognize quorum sensing molecules secreted by the pathogen cells and to generate a distinct fluorescence signal, while the measurement device is designed to recognize and analyse the produced signal.

Our sensor cells are immobilized in agar chips. To make the chips, we mix the Cellock Holmes cells with liquid LB agar. In the course of our project, we designed a casting mold specifically for the production of our agar chips. When the agar has cooled down, the chips are out of the mold and ready to use.

Add: [Photo of our casting mold]

To detect P. aeruginosa cells, the chip is placed on a hard surface that is potentially contaminated with the pathogen. Subsequently the chip is introduced into our measurement device where it is incubated at 37 °C. After a short time, the chip is illuminated with blue light. The device takes a picture of the chip and the software Measurarty analyzes any fluorescent signal. Depending on the intensity of the signal and the size of the spot, Cellock Holmes can calculate concentration and distribution of P. aeruginosa on the sampled surface.

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 [http://parts.igem.org/Part:BBa_C0179 C0179] and [http://parts.igem.org/Part:BBa_J64010 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.

Add: [Fig. 1: Scheme of our molecular approach]

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.