The Coliguard: Our Model
We want our engineered E. coli to be able to recognize and destroy contaminants in culture medium. Since we want our bacteria to show maximum efficiency during the industrial process, we decided to create two different lineages of E. coli: the worker lineage – responsible for executing the industrial process – and the killer lineage – responsible for detecting and killing the contaminants. Both lineages are going to be simultaneously present in the culture medium, but their proportion will vary due to the presence or absence of contaminants. In the absence of contaminants, the amount of worker cells will be much higher than the number of killer ones, so that the industrial process will occur at its maximum efficiency. In the presence of contaminants, killer cells will induce surrounding worker cells to differentiate into more killer cells in a transient manner. After elimination of contaminants by the killer lineage, the proportion of worker and killer cells will return to its original rate.
Based on our model, we divided the project into three subparts.
- Recognition mechanism
- Differentiation mechanism
- Killing mechanism
1) Recognition mechanism
Our idea is based on the premise that the engineered E. coli most be able to recognize contaminants in the culture medium as non-self. As most bacterial species produces AI2 (auto inducer 2) as a secondary metabolite, we decided to use this compound as a recognition factor. Our E. coli will be an AI2- strain and won´t produce native AI2. The presence of AI2 in the culture medium indicates the presence of contaminants, which will be recognized by an AI2 sensitive promoter present in our E. coli.
In the absence of contaminants, the amount of worker cells will be much higher than the number of killer ones, so the industrial process will occur at maximum efficiency.
Contaminants in the culture medium are recognized by the presence of AI2.
2) Differentiation mechanism
The initial differentiation mechanism is based on a random slippage mechanism that will determine the expression of a CRE recombinase in a small percentage of cells. This device is an adaptation from the device presented by the Caltech 2008 iGEM team. When expressed, the CRE recombinase will remove a device from the genome – containing a gene involved in the cell cycle and a gene that represses conjugation – and thus lead to the differentiation into killer cells. Killer cells are unable to reproduce, but able to conjugate. This device is an adaptation of the device presented by the Paris 2007 iGEM team.
Moreover, the presence of AI2 in the culture medium will also trigger the expression of CRE recombinase and thus induce the differentiation of more worker cells into the killer cells, so the proportion of killer cells will be elevated during the decontamination process. After a certain number of generations, the proportion of killer and worker cells will return to its original state, due to the killer cells being unable to reproduce.
3) Killing mechanism
We decided to use conjugation as a sensor that will trigger the killing mechanisms. Our E. coli is going to be an F+ strain, carrying a modified version of the conjugative plasmid pPed100 containing a killing gene. Only the killer lineage will be able to conjugate because in the worker lineage the pPed100 plasmid will be repressed. Since most bacterial strains in nature are F- our F+ E. coli will be able to conjugate with most contaminants. There will be two killing mechanisms, one carried by the modified pPed100 plasmid into the contaminant and another triggered by the conjugation signal. This secondary mechanism is necessary to stall contaminant growth because conjugation may take some time to occur and may be impaired due to culture conditions. This secondary killing mechanism will be triggered by the presence of a diffusible conjugation signal and will be able to induce neighboring killer cells, even when not conjugating. Such diffusible signal and corresponding promoter have never been described, but we’ve found promising candidates and their characterization is the main challenge for this project.
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