Benjamin Holte

In nature, microbes often live together in communities. Many microbes cooperate through crossfeeding, which involves the exchange of essential metabolites. However, some members of the population can evolve to cease producing costly nutrients, which allows them to benefit from the cooperation without paying the cost of it. These members are called cheaters. Using a model system of crossfeeding microbes, we explored the evolution of cheating. We used a system of S. enterica and E. coli. S. enterica secretes methionine, which E. coli is unable to produce and is essential for growth, while the E. coli produces acetate, which S. enterica needs as a carbon source. Two groups of S. enterica were transferred 20 times, one group with the presence of methionine and the other without. Then both groups were grown with E. coli to test levels of methionine secretion. We hypothesize that methionine secretion will decrease the most in the group transferred with methionine, while the other group will decrease to a smaller degree. We found that the levels of methionine production had decreased the same amount for both groups after the 20 transfers in most replicates. The replicates were sequenced and most were found to have a mutation in the MetL gene. These results suggest that growth in the presence of methionine has little impact on the evolution of cheaters in this model system.