Experimental and Theoretical Studies of Artificial Microbial Symbiosis

Mutualistic symbiosis describes a cooperative relationship between two or more different organisms from which each partner benefits [4]. Symbiotic relationships exist widely in nature, for example, between plants and fungi in nutrient-poor soils, and between aphids and their endosymbiotic bacteria [3]. In this work, we are interested in investigating the evolutionary adaptation of a cross-feeding E. coli co-culture comprised of two different auxotrophic strains grown in serial batch culture on abundant glucose. We view this as a simplified experimental model of the much more complicated mutualistic ecosystems observed in nature that are often refractory to straightforward experimental analysis. One of the simplest manifestations of mutualistic growth is metabolite cross-feeding, whereby species A provides metabolite M required for the growth of species B, which in turn provides metabolite N required for the growth of species A. We artificially engineered such a system in vitro by co-culturing pairs of E. coli amino acid auxotrophs, each of which on its own is incapable of growth in minimal medium due to the absence of the required amino acid, but when grown with a partner can thereby obtain the needed nutrient. We hypothesized that upon evolving such a cross-feeding system by repeatedly passaging to fresh minimal medium, we could select for adaptive mutations in each strain that would increase the growth of the system as a whole, e.g. those enhancing amino acid export/import and/or biosynthesis. Apart from its significance to ecology and evolution, syntrophy (cross-feeding) is also of potential use for the engineering of microorganisms, for example, as an approach to select and/or maintain secretion of valuable biochemical products or biosensors.