In vitro community synergy between bacterial soil isolates can be facilitated by pH stabilisation of the environment

Composition and development of naturally occurring microbial communities is defined by a complex interplay between the community and the surrounding environment and by interactions between community members. Intriguingly, these interactions can in some cases cause community synergies where the community is able to outperform it single species constituents. However, the underlying mechanisms driving community interactions are often unknown and difficult to identify due to high community complexity. Here we show how pH stabilisation of the environment through the metabolic activity of specific community members acts as a positive inter-species interaction driving in vitro community synergy in a model consortium of four co-isolated soil bacteria: Microbacterium oxydans, Xanthomonas retroflexus, Stenotrophomonas rhizophila and Paenibacillus amylolyticus. Using micro-sensor pH measurements to show how individual species change the local pH micro-environment, and how co-cultivation leads to a stabilised pH regime over time. Specifically, in vitro acid production from Paenibacillus amylolyticus and alkali production primarily from Xanthomonas retroflexus lead to an overall pH stabilisation of the local environment over time, which in turn resulted in enhanced community growth. This specific type of interspecies interaction was found to be highly dependent on media type and media concentration, however similar pH drift from the individual species could be observed across media variants.\n\nImportanceWe show that in vitro metabolic activity of individual members of a synthetic, co- isolated model community presenting community synergistic growth arises through the inter-species interaction of pH stabilization of the community micro-environment. The observed inter-species interaction is highly media specific and most pronounced under high nutrient availability. This adds to the growing diversity of identified community interactions leading to enhanced community growth.