P64: Acrylate metabolism by Megasphaera elsdenii

Megasphaera elsdenii is an anaerobic bacterium that metabolizes lactate into acetate and propionatem the latter using the acrylate pathway.  Batch fermentations were performed in presence of 0, 5, 10, 20, 30 mM acrylate.  The (dynamic) metabolic flux in response to acrylate demonstrated that acrylate takes the place of lactate in a reaction mediated by a CoA transferase leading to propionate.  This shift in metabolic flux allows M. elsdenii to direct more lactate toward acetate with the concomitant generation of ATP.  Steady-state growth (in chemostats) under nitrogen-limiting conditions resulted in behavior similar to batch observations, with propionate being the dominant product.  However, under carbon-limited conditions, M. elsdenii formatted principally butyrate, bypassing propionate formation.  Additional results with the acrylate analogue 2-methyl acrylate clarify the fluxes in this organism.  The reasons for this shift from propionate to butyrate production are discussed in the context of ATP generation and the requirement for the organism to maintain a redox balance.