Network details

Our system is divided in two modules:

The signaling module

The signaling module allows our bacteria strain to integrate the input signal = the presence of a pathogene.

You can find here the mathematical model and numerical simulation this module.

The idea behind this module comes from the iGEM London Imperial College 2010 Team's work on Parasight [1].

Staphylococcus aureus secretes the exfoliative toxin B [2] which cleaves a specific amino-acids sequence (Desmoglein 1). This specific sequence can be used as a linker between a membrane protein and a dipeptide.
Once S. aureus is present, the linker is cut by the toxin and the dipeptide is released.

The dipeptide binds an engineered receptor [3] [4] that consists of:

the extracellular part of Tap [5], a dipeptide receptor involved in the chemotaxism
the intracellular part of EnvZ [6], a histidine kinase involved in the osmoregulation

Once the dipeptide binds the Tap part [7], the intracellular EnvZ part allows the phosphorylation of OmpR [8] [9], which is a constitutively produced transcriptional activator.

OmpR phosphorylation allows the activation of the ompC promoter [10]. We introduced cyaA (that code for adenyl cyclase) downstream of this promoter.

Amplification module

The amplification module allows our bacteria to amplify the input signal and to produce an output signal = fluorescence.

As for the previous module you can read here our mathematical model and numerical simulation.

The activation of the ompC promoter allows the production of Adenyl cyclase [11]. Adenyl cyclase catalyses the conversion of ATP (Adenosine Tri-Phosphate) into cAMP (cyclic Adenosine Mono-Phosphate).

The binding of cAMP to CRP (cAMP Receptor Protein) leads to the production of AraC by activating the pmalT promoter [12].
In the presence of arabinose, AraC and cAMP-CRP, cooperatively activate the paraBAD promoter [13], thus forming an "AND" gate. This allows the production of:

Adenyl cyclase which reproduces cAMP, forming thus a positive amplification loop
GFP (Green Fluorescent Protein) = the output signal

Cell to cell communication module

When a bacterium detects S. aureus, it produces several molecules of GFP and even more cAMP. cAMP diffuses through the membrane and activates the amplification loop in neighbor bacteria [14], which triggers in turn the production of GFP and cAMP.
This leads to GFP production by the entire population, triggered by a single bacterium that has detected the pathogen in the first place: