Acetate utilization strategy in chain-elongating bacteria determines butyrate versus medium-chain carboxylate production.

Abstract

Chain-elongating bacteria (CEB) are a unique guild of anaerobes that upcycle organic waste into valuable short- and medium-chain carboxylic acids (MCCAs), enabling a circular bioeconomy. However, the metabolic rules that determine product chain length have remained elusive. Here we combine 13C isotope tracing, proteomics, enzyme assays and metabolic modelling to show that distinct acetate utilization strategies underlie the divergence between MCCA-producing CEB and those solely producing less valuable, short-chain butyrate. MCCA-producing strains recycle acetate to maximize lactate use under acetate limitation, but at the cost of slower growth. In contrast, butyrate-producing strains grow faster by favouring acetate assimilation, at the cost of restricted lactate utilization when acetate is scarce. These physiological trade-offs are encoded in the substrate specificity of coenzyme A transferase, the terminal enzyme in reverse β-oxidation. Our findings uncover a fundamental constraint shaping chain-length selectivity in CEB and offer strategies that could optimize MCCA production from organic waste streams.