One of the most essential problem urgent to solve in synthetic biology is the security problem. It has been a troublesome part whether we can pursue that the constructed bacteria cannot survive in the circumstance with some target genes imported by us. As for our project, though there is no safety worry, we come up with an original way to synthesis one kind of standard constructed bacteria, if slightly simplified by our project. In our previous project there are three cases when criminal can survive, but based on the safety consideration, we cannot let any bacteria alive in the wild, so we refer to this design because there is nearly no possibility for them to survive in the wild, for the living conditions have to reach several high levels, which is hard to achieve in the natural circumstance. What’s more, we have made mathematic model of it, which offers the theoretic evidence of it and make it more feasible. The design is like the figure below.

Just as the graph has shown, we make some modifications in our previous design. We remain the communication pair genes as the message access between them-LuxR and LasR, the products of which-6HSL and 12HSL still function as the signals between the two bacteria communities. It’s apparent that reporter genes are unrelated to the safety problem. So just leave them out. In the meantime, we cut off the tetA gene directly and import the cl gene exactly the same place of the zeo-R. The function of the cl gene is to reduce the expression of the ccdB gene. What is an obvious change is that we replace zeo-r gene that is resistant to the antibiotic zeocin in the culture with the toxin gene ccdB, whose product is a kind of poisonous protein and ccdB can express continuously and by itself. This requires us to switch the gene circuit, which means we change the poisonous protein gene and the mechanism of the toxin principle.

On the one hand, when the two types of bacteria are culture separately, each of them cannot survive because the ccdB gene expresses continuously and the toxin would result the death of the bacteria, what’s more, they cannot get the signal molecular from the partener-6HSL and 12HSL, so even though they are let out, they have no safety risk to our environment. On the other hand, when they are cultured together, they cannot survive as well, only when the concentration of the products of the cl gene reach a relative high level would the degree of the toxin ccdB product decease, but this kind of situation only occur in the laboratory circumstance because of human control. In the natural environment, due to the lack of products of the cl genes, the poisonous protein produced by the ccdB cannot disappear, which would lead to the death of the both sides.

Eventually, in order that the target genes we import would not lose efficacy for the sake of the loss of the plasmid by the bacteria. We proposal that all the target genes can be connected to the genome of the bacteria, then the condition would not happen.

Our model have imitated this idea, and there are the figures showing below:

Figure 1:When the concentration of these two bacteria are all very low (in nature)

Figure 2:When the concentration of these two bacteria are all very high (in lab)

Figure 3:Model for the mentioned figures

Our project is safe for the following reasons:

• The ''E-coli'' strains and genes we usd are already existed and are commercially available for years.
• We would not release the bacteria to the environment

Please click the pdf linked to get more details.

Media:saftysustc.pdf