The fur-lac-cecropin AD system

In order to measure the ability of engineered bacteria to kill iron bacteria, we modeled the production of the cecropin AD and the sterilizing ability of the cecropin. We want to model the fur-box fell the concentrate of Fe2+ to drive the LacI to reverse the signal to produce the cecropin AD which play a bactericidal effect. So our model used in the project includes four parts: the iron sensor, the inverter system, the sterilizing system and the chelator system.

The first part focuses on the sensor system to find the Fe2+ needed by our system. And the strength of the best promoter with the fur-box of the three kinds of fur-box was determined. Second, the inverter system our team implemented in the bacteria focused on the cecropin AD produced with time. The third part modeled the sterilization rate of the cecropin AD. The last part integrate those previous parts leading to the cecropin production and sterilization. The model show the time it takes for our bacteria system to work.

1.Sensor model

1.1 Introduction

We first modeled the sensing system using ODEs with the help of experimental results to determine one of our parameters ki1.We had three kinds of fur-box designs. We model our three kinds of fur-box (shown in the figure 2) to find the optimal fur-box and the strength of the promoter with the best kind of fur-box. We finally corrected our model through the experiments. We make this framework like figure 1.

1.2 Methods and materials:

1.2.1 The dynamic simulation of sense iron to FBS:

(1) the iron-FUR complex formation:

$$2\cdot FUR+2\cdot Fe\leftrightarrow Fe_{2}FUR_{2}$$

We think this equation to:

$$FUR+Fe\leftrightarrow FeFUR:K_{FeFUR}$$

We just want to use differential equations more easily. And we can easily divide our [FeFur] by two to get the real complex concentration.

(2) We can easily make the formation (v) and the dissociation (v') speeds:

$$V=K_{FeFUR}\cdot \left[ FUR\right] \cdot \left[ Fe\right]$$

$$V'=d_{ff}\cdot \left[ FeFUR\right] $$

• K_FeFUR : Formation constant of FeFur complex (m^-1∙s^-1)

• d_ff : FeFUR degradation rate (min^-1)

We model the iron input in the bacteria using a linear function of the external iron concentration Ferext with the factor p which is the cell-wall permeability for iron.

$$\dfrac {d\left[ Fe\right] }{dt}=p\cdot Ferext-K_{FeFUR}\cdot \left[ FUR\right] \cdot \left[ Fe\right] +d_{ff}\cdot \left[ FeFUR\right]$$

$$\dfrac {d\left[ FUR\right] }{dt}=FurO-K_{FeFUR}\cdot \left[ FUR\right] \cdot \left[ Fe\right] +d_{ff}\cdot \left[ FeFUR\right]$$

• p : Permeability of cell wall (min^-1)

• d_ff : FeFUR degradation rate (min^-1)

We track the free Fe-FUR complex but not those attached to a Fur Binding Sites in our model.

$$\dfrac {d\left[ FeFUR\right] }{dt}=K_{FeFUR}\cdot \left[ FUR\right] \cdot \left[ Fe\right] -d_{ff}\cdot \left[ FeFUR\right] -\dfrac {1}{N_{A}V}.\dfrac {dFBS}{dt}$$

• NA : Avogadro’s constant (mol^-1)

• V : Volume of a bacterium (m³)

• FBS : the number of inhibited Fur Binding Sites

We use our Logistic function under its differential form to simulate the inhibition phenomenon. Since it is the Fe-FUR that represses it, the LacI can be expressed as a logistic fuction of the Fe-FUR:

$$\dfrac {dFBS}{d\left[ FeFUR\right] }=\dfrac {K_{i}1}{K_{f}}\cdot FBS\left( \left[ FeFUR\right] \right) \cdot \left( 1-\dfrac {FBS\left( \left[ FeFUR\right] \right) }{N_{pla1}}\right) $$

• K_f : fixation rate of FeFUR (min^-1)

• K_i1 : constant repesents the inhibition power (min^-1)

• N_pla1 : pET28-a plasmid number (nb/cell)

K_i1 is a non-dimensional parameter which repesents the inhibition power, and K_f is the fixation rate of the Fe-FUR on the FBS. N_pla1 is the number of pasmids containing the sensor system.

1.3 Result

We want to know the fittest ki1 for the model to sense the iron and the concentrate of iron. We make three kinds of fur-box for our sensor system. We want to know which is our best choice. Our experiment result show in the figure 3.

Then we try to change the value of K_i1 to model different strength of promoter with fur-box in our experiment which show in the figure 4. We want our system to make sense in the high level of Fe²⁺, so we choose the fur-2 system. And we finally set the KI1:6.3*10^-5.

2 Inverter model

2.1 Introduction

Build an inverter model downstream, which allowed us to answer the following question: “How much concentration of cecropin AD dose our bacteria produce?” and “How long does our system delay working?”

We make the system framework shown in the figure 5. We built the inverter system using lacI-lacO on the basis of sensor model to determine the concentrate of cecropin AD and the need of time.

2.2 Methods and materials:

2.2.1 The dynamic simulation of inverter model:

LacI production：

The [mRNA] and [LacI] equations are alike. The prodction rates are K_r for the mRNA and K_L for the LacI. Since FBS represents the number of inhibited Fur Binding Sites, we have to substract it from N_pla1.

$$\dfrac {d\left[ mRNA\right] }{dt}=\left( N_{pla1}-FBS\right) \cdot K_{r}-D_{mRNA}\cdot \left[ mRNA\right] $$

$$\dfrac {d\left[ LacI\right] }{dt}=K_{L}\cdot \left[ mRNA\right] -D_{LacI}\cdot \left[ LacI\right] $$

• Kr : production rate of mRNA (min^-1)

• DmRNA : mRNA degradation rate (min^-1)

• KL : production rate of LacI (min^-1)

• DLacI : LacI degradation rate (min^-1)

In the sensor and inverter system the cecropin AD is regulated by the lacI-PlacO. We try to measure the concentrate and rate of the production by the measure of mcherry. The mcherry expression is repressed by FBS the same :

Cecropin AD production：

$$\dfrac {dMcherry_{expressed}}{dFBS}=K_{i2}Mcherry_{expressed}\left( FBS\right) \left( 1-\dfrac {Mchervy\left( FBS\right) }{N_{pla2}}\right) $$

• K_i2 : the constant of inhibition power (min^-1)

• N_pla2 : pET28-a plasmid number (nb/cell)

K_i2 is the inhibition power and N_pla2 is the number of plasmimds containing the mcherry. FBS and Mcherryexpressed are both ruled by a normal logistic function. If we were to track the number of expressed LacI or Mcherry, we would be using two inverted logistic fuctions to model a double inverter. Since FBS represents the number of repressed genes and Mcherryexpressed the number of expressed genes, the double inverter is still there. Finally we equate the concentration of Mcherry to the concentration of cecropin AD.