Non-additive species interactions determine the invasion success of a virulent phenotype in a microbial community

Previous work shows that a host's resident microbial community can provide resistance against an invading pathogen, however, this community is continuously changing over time due to adaptive mutations. How these changes affect the invasion resistance of these communities remains poorly understood. To address this knowledge gap, we used an experimental evolution approach in synthetic communities of Escherichia coli and Salmonella Typhimurium, to investigate how the invasion resistance of this community against a bacterium expressing a virulent phenotype, i.e. colicin secretion, changes over time. We show that evolved communities accumulate mutations that increase their resource utilization abilities, simultaneously making them less resistant to invasion. By investigating two-species competitions and generating a three-species competition model, we show that this outcome is due to non-additive species interactions. Our study demonstrates how adaptive changes in microbial communities can make them more prone to the detrimental effects of an invading species.