Flipping DNA to generate and regulate microbial consortia.

Communities of interdependent microbes, found in diverse natural contexts, have recently attracted the attention of bioengineers. Such consortia have potential applications in biosynthesis, with metabolic tasks distributed over several phenotypes, and in live-cell microbicide therapies where phenotypic diversity might aid in immune evasion. Here we investigate one route to generate synthetic microbial consortia and to regulate their phenotypic diversity, through programmed genetic interconversions. In our theoretical model, genotypes involve ordered combinations of DNA elements representing promoters, protein-coding genes, and transcription terminators; genotypic interconversions are driven by a recombinase enzyme that inverts DNA segments; and selectable phenotypes correspond to distinct patterns of gene expression. We analyze the microbial population as it evolves along a graph whose nodes are distinct genotypes and whose edges are interconversions. We show that the steady-state proportion of each genotype depends on its own growth advantage, as well as on its connectivity to other genotypes. Multiple phenotypes with identical or distinct growth rates can be indefinitely maintained in the population, while their proportion can be regulated by varying the rate of DNA flipping. Recombinase-based synthetic constructs have already been implemented; the graph-theoretic framework developed here will be useful in adapting them to generate microbial consortia.