Quorum sensing Revisited

One of the triggering mechanisms of our Pico plumber, in order to start glue production, is quorum sensing. Quorum sensing is a response of bacteria to their population density. In the natural system of the marine bacterium Vibro fischeri a small molecule, termed autoinducer, is constantly produced at a low level. This autoinducer molecule (HSL) can freely diffuse in an out the E.Coli membrane. The HSL molecule then diffuses into the surroundings. Other bacteria in the surrounding environment can sense each other via diffusion of HSL through their cell membrane. In the absence of a high density of cells HSL rapidly diffuses into the environment and the HSL concentration is too low to trigger quorum sensing. Quorum sensing is based on a lux-type regulated transcriptional system controlling the production of the autoinducer. In Vibro fischeri, LuxR is encoded on the left side of the operon whereas LuxI is produced on the right side. LuxI and an enzyme always present in the cell, called SAM, together generate in an enzymatic reaction the autoinducer, HSL. HSL together with LuxR form a transcriptional activator for the lux operon, also known as lux box. This complex enables a stronger production of LuxI and LuxR and can been seen as an amplifying loop for LuxI production [1][2][3].

The issue

In the planning stage of the project, we designed the quorum sensing circuit with a constant medium production of LuxI in order to activate a lux box on the plasmid where the glue production takes place in the presence of sufficient cell density:

We are using the lux operon and LacO to give us AND–gate logic to start glue production (Here denoted as gene X and gene Y). It is to be expected that if IPTG is present without any HSL within the cell, the promoter starts to produce the glue at a low level. In the presence of HSL and IPTG the promoter starts fabrication of the glue at its maximum level. A possible source of error in our design is whether or not our system might trigger glue production and cell lysis on its own. Having in mind that we have a constituative medium strength promoter, we needed a lower concentration of LuxI. To achieve this LuxI was tagged and a Scheint Orgaon sequence was used to limit ribosome affinity to 60%.

The Model

We ran our simulation using the IPTG and HSL concentration outside the cell as inputs. We changed both IPTG and HSL concentration at different times by a switch from low to high to see how the inputs affected the simulation. In our later simulations we will simulate both the IPTG and HSL inputs more realistically. We did this for stochastic model.