Coupling Stickland fermentation with reverse β-oxidation represents a new mode of medium-chain carboxylate production

Biotechnologies aimed at producing medium-chain carboxylic acids (MCCAs) through chain elongating bacteria (CEBs) may transform the oleochemical industry by reducing reliance on land use, unreliable supply chains, and substantial environmental impacts. Achieving this potential necessitates a deeper comprehension of CEB physiology, which presently centers on lactate and ethanol consumption, although these bacteria flourish in protein-rich gut environments and organic waste bioreactors.

A comparative genomic analysis of established MCCA producers, coupled with physiological investigation of the newly discovered species Peptonella octanoica and other genus members as a model system, reveals that amino acid utilization is a wide-spread yet largely neglected trait among many CEB. Some lineages showing amino acid fermentation exhibit a conserved metabolic structure combining Stickland fermentation with reverse beta-oxidation (RβO), allowing MCCA generation when acetate or electron-accepting amino acids are scarce.

This coupling helps overcome thermodynamic barriers to lactate and amino acid oxidation by promoting the creation of longer-chain products as hydrogen partial pressure rises.

The discovery also indicates that amino acid-fermenting CEB are not only prevalent in gut microbiomes but also present in engineered bioreactors. Their unique physiology offers a mechanistic explanation for high-titer n-octanoate production from proteinaceous organic waste sources. Moreover, these findings broaden the understanding of CEB throughout anaerobic microbiomes, where amino acid fermentation and chain elongation coexist as intersecting ecological strategies.