A stabilized microbial ecosystem of self-limiting bacteria using synthetic quorum-regulated lysis.

Microbial ecologists are increasingly turning to small, synthesized ecosystems as a reductionist tool to probe the complexity of native microbiomes. Concurrently, synthetic biologists have gone from single-cell gene circuits to controlling whole populations using intercellular signalling. The intersection of these fields is giving rise to new approaches in waste recycling, industrial fermentation, bioremediation and human health. These applications share a common challenge well-known in classical ecology-stability of an ecosystem cannot arise without mechanisms that prohibit the faster-growing species from eliminating the slower. Here, we combine orthogonal quorum-sensing systems and a population control circuit with diverse self-limiting growth dynamics to engineer two 'ortholysis' circuits capable of maintaining a stable co-culture of metabolically competitive Salmonella typhimurium strains in microfluidic devices. Although no successful co-cultures are observed in a two-strain ecology without synthetic population control, the 'ortholysis' design dramatically increases the co-culture rate from 0% to approximately 80%. Agent-based and deterministic modelling reveal that our system can be adjusted to yield different dynamics, including phase-shifted, antiphase or synchronized oscillations, as well as stable steady-state population densities. The 'ortholysis' approach establishes a paradigm for constructing synthetic ecologies by developing stable communities of competitive microorganisms without the need for engineered co-dependency.