【Why Do We Design This Experiment】

Bacillus subtilis has been widely applied as engineered bacteria, especially in food industry and pharmaceutical industry, for its safety and excellent secretion capacity. Therefore, after comparing characters of distinct mutants we selected Bacillus subtilis WB800N mutant as our engineered bacteria and looked up plenty of papers to select the optimal conditions for our experiment. To our disappointment, very few experiments have been done on WB800N mutant, and most optimization experiments regarding Bacillus subtilis focus solely on the optimization of production of specific proteins produced by Bacillus subtilis. Consider the final goal of our project, it is imperative to design this experiment on our own to find out the best condition for Bacillus subtilis WB800N.

【Methodology】

Any optimization designs will inevitably involve the ideology of Design of Experiment (DOE), which includes several dependent plots. Among them Orthogonal Design and Response Surface Design, RSM for short, are the most common two in biological experiments. Generally, Orthogonal Design consumes less time and has been used more widely, yet it is not logically rigorous in mathematics, and sometimes it overlooks interactions and alias between or among factors. In contrast, RSM is constructed on rigorous mathematical theories and excels in data analysis. Having weighing the features of the two methods carefully, we finally chose RSM.

【Sweeping Factors】

The first step of any methods of DOE is to investigate all variables that affect the results and select controllable factors for the experiment. In terms of this experiment, all factors can be categorized into two kinds: environment factors, like temperature, the rotation speed of the shaker, and the components of the medium. We have looked up several papers about the optimization experiments on Bacillus subtilis, finding the rotation speed of shakers ranging from 100 r/min to 250 r/min, and generally rotation speed only plays a tiny role. Additionally, our lab has only two shakers. While we can place twenty different mediums into one shaker at a time, we must run the shakers every time we alert the speed, which surely consumes longer time. Thus, we fixed the rotation speed of shakers at 200r/min.However, temperature and inoculation time are both vital environment factors whose effects cannot be ignored.
Inoculation amount and pack amount are also two factors that affect results slightly. We fixed them at 5 percent and 30mL/500mL respectively according to earlier authentic experiments.
A typical medium consists of carbon source, nitrogen source and inorganic salt, all of which are essential to ensure the regular metabolism of engineered bacteria. Finally in light of convenience, we infered the components of typical LB medium and determined three independent medium factors: peptone, yeast extract and sodium chloride (NaCl). Peptone provides nitrogen and carbon for the colonies, while yeast extract contains most required inorganic salt, therefore we did not list any inorganic salt except NaCl. We had no idea why NaCl is listed alone, and we suspected the influence of NaCl as yeast extract had already contains sodium.
Thus, we had five independent factors: temperature, inoculation time, peptone, yeast extract and NaCl. We further investigated some papers and defined their ranges. The following table displays their levels, and the unit of peptone, yeast extract and NaCl is g/L:

Factor	Low	High
Temperature	25℃	35℃
25℃	12h	24h
Peptone	5	15
Yeast Extract	2.5	7.5
NaCl	5	15

【Designs&Results】

The methodology of RSM can be divided into two subplots: Central Composite Designs (CCD) and Box-Behnken Designs. Generally the overall runs of Box-Behnken Designs is fewer when the factors are fixed, but Central composite designs are often recommended when the design plan calls for sequential experimentation because these designs can incorporate information from a properly planned factorial experiment. In our experiment, time is more precious than reagents, and as time itself is also an independent factor, Box-Behnken Designs would not have saved any time if adopted. Thus we selected CCD.
CCD itself can also be classified into three subplots: Central Composite Circumscribed Design (CCC), Central Composite Inscribed design(CCI) and Central Composite Face-centered Design(CCF). The alpha value of CCC is related to the number of factors, whereas in CCF α is fixed at 1, and only CCC is rotatable. The rotational invariance empowers CCC to be mathematically preferred, yet the value of alpha in a five-factor-CCC is over 2. In other words, if we adopted CCC, we would get some absurd treatments where the concentration of some specific actual material were negative. If we narrowed down the range to ensure any concentration is positive, the ranges of all three medium factors would be too narrow to yield cogent results. Therefore, we finally selected CCF.
We conducted our experiments according to the following table, which was calculated by Minitab, and the results, which were measure by OD value, were also included:
The result of No.42 medium is destroyed due to some unfortunate reason. Additionally, multiple center points, which means conducting multiple experiments at the center points with identical treatments, is a very common phenomenon in DOE, yet we decided to do only experiment at the center point and reuse its result due to our limited time and reagents.
Estimated Regression Coefficients for OD
S = 0.295758 PRESS = 7.78904
R-Sq = 92.25% R-Sq(pred) = 78.45% R-Sq(adj) = 87.41%
Suppose we redefine the factors accoding to the following table:
According to the ANOVA calculated by minitab, we got the expression of OD:
f(T,t,p,y,c)=2.87625+0.60225T+0.28447t+0.18665p+0.16776y-0.01626c-0.549T^2-0.18725t^2-0.08325p^2-0.10625y^2-0.05075c^2-0.35558Tt+0.17881Tp+0.04544Ty+0.0155Tc+0.02575tp+0.06112ty-0.00144tc-0.07469py+0.09150pc-0.04450yc
P represents confidence coefficient, which is a key judgment to check the reliability of the fitting function. In other words, if P=0.05, the probability that this term is wrong is 5%. The coefficient of determination (R) was calculated to be 0.9225, indicating that the model could explain 92% of the variability .From the above table we can identify eight statistically significant and reliable terms:
Constant;
Temperature;
Time;
Yeast Extract;
Peptone;
Temperature*Temperature;
Temperature*Time;
Temperature*Yeast Extract;
The influences of linear terms predominated, except NaCl, which substantiated our suspicion whereas most square terms and interaction terms were ignorable and statistically unreliable. Temperature and time and two most influential factor.
As our world is three-dimensional but the intact response surface is six-dimensional, it is impossible to draw the intact surface. Yet we could fix some factors to lower the dimensional, which empowers us to imagine the full surface. Here are some surfaces and contours of our fitting surface, we can extrapolate this super surface by combining these pictures:

【Designs&Results】

In our experiment , we discovered that the promoter P43 is too weak to express amilCP for a blue-color display. So we redesigned the circuits , aiming to find a more feasible plan, and test the part amilCP simultaneously. The new disign contains two circuits:

1 Pgrac-amilCP-Terminator

We use the Promoter grac , a promotor with lac promotor ,on the PHT vector to express amilCP, instead of Promoter 43. The Promoter grac is strong enough ,so we can easily see the results theoretically.

pgrac-amilCP overlap PCR（Right three tracks） PSBC3 Double Digestion（Left four tracks）

2 P43-amilcp-SigB-Terminator

We plan to use a positive feedback to magnify the expressing of amilCP. Because the P43 is a sigma B factor binding promotor, we designed a circuit, that the P43 is fused to the sigma B factor. We hope this could increase the response of P43.
Course of time limitation, we haven’t put this design to experiment.

【designing of the suicide system】

We design a circuit of killing switch based on its endogenous genetic system.
In B.subtilis, when it comes to the stationary phase, the environmental pressure increases and nutrition becomes limited, so B begin to produce spores. Now the community will be divided into two different parts. One of them are trying to kill others to get enough nutrient , delaying the production of spores and achieving a competitive advantage. Killing is mediated by the exported toxic protein SdpC. SdpI will appear on the membrane surface to avoid itself from being damaged. SdpI could bind free SdpC and autopressor SdpR, to remove SdpR’s inhibition against I and R, to produce more SdpI to offset SdpC, finally guaranteeing the subgroup alive, thereby delaying the spores production.
https://static.igem.org/mediawiki/2013/2/2b/Reporter_3.png We transfer SdpC which is fused by promoter SdpI/R into high copy plasmids in order to damage the balance of the system, thereby killing whole colony. When SdpC appears, SdpI on the membrane will bind free SdpC and adsorb SdpR to cease its inhibition against SdpI P/R, trying to produce more SdpI. At the same time, it will activate the promoter SdpR/I in our circuits and generate more SdpC.The system would fall into an infinite loop, and according to our modeling ,the amount of SdpC increases beyond the ability of SdpI.Thus,the cells with protection mechanism will crack and die because of too much SdpC. All above forms the killing device. We Also designed a test circuit,which contains promotor grac and sdpABC only,aiming to determine the ability of SdpC.

Results

https://static.igem.org/mediawiki/2013/e/ec/Reporter_4.tif
Colony PCR E.coli PHT43 + Promotor grac + SdpABC
https://static.igem.org/mediawiki/2013/9/92/Reporter_5.tif
PHT43 + Promotor SdpRI + SdpABC Enzyme digestion

【References】

Parallel pathways of repression and antirepression governing the transition to stationary phase in Bacillus subtilis AV Banse, A Chastanet, L Rahn-Lee…,PNAS ,2008