Propionate production from carbon monoxide by synthetic co-cultures of Acetobacterium wieringae spp. and propionigenic bacteria.

Gas fermentation is a promising way for converting CO-rich gases to chemicals. We studied the use of synthetic co-cultures composed of carboxydotrophic and propionigenic bacteria to convert CO to propionate. So far isolated carboxydotrophs cannot directly ferment CO to propionate, and therefore this co-cultivation approach was investigated. Four distinct synthetic co-cultures were constructed, consisting of: Acetobacterium wieringae (DSM 1911T) and Pelobacter propionicus (DSM 2379T); Ac. wieringae (DSM 1911T) and Anaerotignum neopropionicum (DSM 3847T); Ac. wieringae strain JM and P. propionicus (DSM 2379T); Ac. wieringae strain JM and An. neopropionicum (DSM 3847T). Propionate was produced by all the co-cultures, with the highest titer (∼24 mM) measured in the co-culture composed of Ac. wieringae strain JM + An. neopropionicum, which also produced isovalerate (∼4 mM), butyrate (∼1 mM), and isobutyrate (0.3 mM). This co-culture was further studied using proteogenomics. As expected, enzymes involved in the Wood-Ljungdahl pathway in Ac. wieringae strain JM, which are responsible for the conversion of CO to ethanol and acetate, were detected; the proteome of An. neopropionicum confirmed the conversion of ethanol to propionate via the acrylate pathway. In addition, proteins related to amino acid metabolism and stress response were highly abundant during co-cultivation, which raises the hypothesis that amino acids are exchanged by the two microorganisms accompanied by isovalerate and isobutyrate production. This highlights the importance of explicitly looking at fortuitous microbial interactions during co-cultivation to fully understand co-cultures behavior.IMPORTANCESyngas fermentation has great potential for the sustainable production of chemicals from wastes (via prior gasification) and flue gases containing CO/CO2 Research efforts need to be driven to expanding the product portfolio of gas fermentation, which is currently limited to mainly acetate and ethanol. This study provides the basis for a microbial process to produce propionate from CO using synthetic co-cultures composed of acetogenic and propionigenic bacteria and elucidates the metabolic pathways involved. Furthermore, based on proteomics results, we hypothesize that the two bacterial species engage in an interaction that results in amino acid exchange, which subsequently promotes isovalerate and isobutyrate production. These findings provide a new understanding of gas fermentation and a co-culturing strategy for expanding the product spectrum of microbial conversion of CO/CO2.