title: Determinants and evolution of metabolic interactions in synthetic microbial communities
title: Determinanten und Evolution der metabolischen Interaktionen in synthetischen mikrobiellen Gemeinschaften
titleType: AlternativeTitle
Creators: name: Scarinci, Giovanni
Publisher: name: Philipps-Universität Marburg
Year: 2023
Subjects: mikrobielle Gemeinschaften
Life sciences
Biowissenschaften, Biologie
microbial communities
Evolution
Mikrobiologie
evolution
Synthetische Biologie
synthetic biology

description: Conversely to laboratory conditions, microorganisms often share space and resources with other organisms in their natural environments. This can result in the emergence of a plethora of interactions of different natures and diverse outcomes on fitness. Obligate mutualistic exchanges of metabolites represent a subset of these possible interactions. These lead to a strong entanglement between partners, resulting in the inevitable coexistence of the two trading organisms. The relevance of these interactions is exemplified by the fact that these trades might have played a crucial role in the early step of the evolution of eukaryotic cells. Nonetheless, a complete understanding of the evolutionary dynamics leading two originally autonomous organisms to become interdependent remains mostly elusive. Furthermore, since communities relying on metabolic mutualism face several challenges, including metabolite dispersal and the potential exploitation of the shared building blocks by non-cooperators, a deep investigation of the strategies adopted by these organisms to cope with such detrimental factors is required to have a complete overview of these systems and their evolution. In this work, we aimed to address these questions by engineering a mutualistic bipartite system obtained via the co-culture of E. coli and S. cerevisiae auxotrophic strains, which thus relies on a bidirectional and obligate cross-feeding of metabolites. We further proceed with iterative growth-dilution cycles resulting in a dramatic improvement in growth for one of the tested communities. Such improvement could be recapitulated to a good extent by the presence of four highly frequent mutations. Notably, these mutations seem to provide an advantage exclusively under cross-feeding, thus suggesting the existence of a different pool of beneficial mutations which emerge in the presence of partner interactions. Interestingly, some of these mutations caused a reduction in the ammonium assimilation ability by the yeast partner, potentially resulting in a higher degree of dependency on the bacterium. Even though additional experiments are required, if confirmed, this would prove that partners connected by obligate metabolic dependencies can increase their entanglement. Using another cross-feeding pair, we were able to investigate the role of aggregation, motility, and chemotaxis on obligate mutualistic communities grown under turbulence. Through competition experiments, we were able to demonstrate that cell-cell adhesion, and the ensuing aggregation, provide a fitness advantage to the bacterial partner. This advantage is further increased by the joint effect of motility. On the contrary, under our conditions, chemotaxis does not play a role. By introducing an engineer cheater strain, we were also able to prove the role of both aggregation and motility also as protective factors against a cheater by reducing its invasion success and thus delaying community collapse.