<img src="https://static.igem.org/mediawiki/2014/c/c8/Populationdesig1_Uppsala14.PNG"></img><h2>The <i>Y.enterocolitica</i></h2><p>The <i>Y.enterocolitica</i>, noted p, has been shown to be immobile at 37 degrees Celsius (8), and does therefore not need a term describing its random walk. Drift will be neglected since most bacteria attach to the intestinal wall and might therefore not be influenced by drift nor intestinal movement. Neither will growth of the <i>Y.enterocolitica</i> be included since the generation time is about 30min in optimal conditions for most bacteria. Considering the rough competitive climate in the intestine, it would be hard for the <i>Y.enterocolitica</i> to grow. It is therefore not worth to add a growth term due to its complexity.<br><br>The death of <i>Y.enterocolitica</i> due to sufficient concentrations of the colicin will be included in the model via a heaviside-step function. We can control when the <i>Y.enterocolitica</i> should be eliminated by setting up a threshold, K_c. When the colicin is above K_c the prey will be eliminated via the -p term, as seen in equation below.</p><img src="https://static.igem.org/mediawiki/2014/8/88/Populationdesign2_Uppsala14.PNG"></img><br><h2>The sensing module</h2><p>To incorporate the sensing of <i>Y.enterocolitica</i> we added a PDE to explain how the concentration of OHHL change over time. OHHL is produced by <i>Y.enterocolitica</i> at a constant production rate,( beta_p), in our formulas. The diffusion of the molecule is modeled via Fick’s law with the diffusion coefficient D_o. Since the OHHL is not stable in the intestine we also added a degradation constant, eta_o. Lastly we added consumption of the molecule by our Bactissile with a consumption rate, alpha_b.</p><img src="https://static.igem.org/mediawiki/2014/0/09/Populationdesign3_Uppsala14.PNG"></img><h2>The killing module</h2><p>Finally, we needed the killing module. Since the killing is determined by the concentration of colicin we added a PDE for the colicin. The colicin is produced by attacking Bactissiles with a production rate of, beta_c. As with the OHHL we also introduce both diffusion with diffusion constant, D_c, and degradation with constant, eta_c. However we have no consumption because the <i>Y.enterocolitica</i> will probably lyse upon death and the molecules can be reused.</p><img src="https://static.igem.org/mediawiki/2014/e/e5/Populationdesign4_Uppsala14.PNG"></img><h2>Domain and boundary</h2><p>Further, we need to define a domain for our system to be active in. The domain will consist of a rectangle representing the intestinal wall cut open and stretched out. This creates the advantageous 2D domain that could easily be studied. The intestine is folded over and over again, which creates opportunities for bacteria and cells to interact between the walls folded close together. Interactions between areas of the fold will however be neglected, due to high complexibility. Since most bacteria is attached to the intestinal wall they cannot diffuse over to the other wall and neglecting that diffusion will not be unrealistic.  The main loss is the diffusion of molecules over the surface. If the diffusion is fast enough the molecules would be able to reach the wall on the other side and trigger both detection and death of cells. If this would happen in real life it would not make our system less useful since the real life result will be better then the result proposed by the model.<br><br>The boundary of the domain was defined by setting the concentration of all molecules and bacteria to zero at the edges leading out of the defined part of the intestine. This is an approximation of the greater area outside the small intestine which will lead to concentrations approaching zero as the distance from the diffusion source increases . If the diffusion source would be set close to the domain boundaries this approximation would not be realistic, instead the domain should then be expanded to allow diffusion calculations to take place.</p><h2>Initial values</h2><p>To start our model we need to set up initial values, a time = 0 scenario, that is the moment before the medicine reached the intestine. We want to place a colony of <i>Y.enterocolitica</i> that has already built up a steady-state of OHHL around itself, in our model. Therefore we need to define the location x_p, y_p of the <i>Y.enterocolitica</i> and the radius of the colony, r_p. This can then be inserted via an if-statement into MATLAB initial code. The concentration of OHHL in the location can be calculated via running a simulation on a large time frame, eg. 3 hours, and then observe  at what value OHHL concentrations converges over time.<br><br>The position of the <i>Y.enterocolitica</i> colony was set to the middle of the system to make sure that the boundary conditions assumption would not effect our model too much. The radius of the colony was set to 3.16 (sqrt(10)) mm since no good data was found. The estimation originates from a test run to get nice results and internal guess.<br><br>The initial density of <i>Y.enterocolitica</i> was set to 1*10^8 cells/mL since it is known that they are constantly in their exponential phase during host infection(8). The initial concentration of our Bactissile was set to 1*10^7 cells/mL since it is likely that it is lower than the <i>Y. enterocolitica </i> concentration. One might suggest that the density should be set even lower, but due to restricted capacity to run long simulations we decided to go with 1*10^7 cells/mL.</p><h2>Parameters</h2><p>Some of the parameter values were found in articles while others were estimated, see table 1. A few parameters needed to be reworked, or did not precisely fit our system and will be discussed further.</p>';