Team:UCSF/Auxotroph Systems


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Tuning Symbiosis Using an Auxotroph System

In nature, we find that symbiosis is often utilized for simplifying complicated processes and facilitating many functions. For this part of project, we’ve decided to replicate and expand upon an experiment we’ve found in the paper A Programmable Escherichia coli Consortium via Tunable Symbiosis as a proof of concept that growth ratios can be controlled.

In the auxotroph experiment, two auxotrophic strains, a tyrosine auxotroph (now referenced to as tyr-) and a tryptophan auxotroph (now referenced as trp-), are used. They are forced to become interdependent through the use of two plasmids: pAK1 and pAK5. Each plasmid allows the export of one metabolite (pAK1 exports tyrosine while pAK5 exports tryptophan), allowing the strains to feed each other. With this set up, one strain cannot survive without the other.

The genes that export the metabolites are placed under inducible promoters to allow us to control the degree to which each metabolite is expressed based on the amount of inducers added. The more metabolites that are exported, the more the strain will grow. Using this method, we can create different ratios between the strains (example: 2 to 1 ratio between the tryptophan auxotroph and the tyrosine auxotroph).

The strains used in our experiments were kindly provided by the Lin Lab at the University of Michigan. Note: Strain Y3 (tyrosine auxotroph) has YFP integrated into its genome. This allows for the two strains to be distinguished during co-culturing.