Energy extraction optimisation strategies of Actinobacillus succinogenes

Abstract

Actinobacillus succinogenes is known to produce the acids of acetate and formate as co-products in conjunction with succinic acid. These compounds not only divert a portion of the carbon flux away from succinic acid production, but also necessitate separation techniques that further augment production costs and, as a result, render the bio-production of succinic acid unviable. To explore methods of reducing or eliminating the production of these co-products, one needs to understand the reason for their presence in the first place. This study aimed at defining the energy boundaries that describe the fermentative behaviour of the microbe. It was found that A. succinogenes displays a clear preference for routes with higher energy extraction efficiency in the early stages of its batch-operated lifespan, subsequently replacing the routes with others of lesser efficiency as fermentation progresses. This clear observation of diminishing energy extraction efficiency supports the idea of route hierarchy, i.e. routes that are more efficient at extracting energy from the available resources are favoured over those that are less efficient. Furthermore, it suggests that accumulated environmental stresses is a likely reason for the observed shift in metabolic strategies for energy extraction. This idea is further supported by the finding of co-activation between the pyruvate dehydrogenase and pyruvic acid excretion routes. Since these have vastly different energy extraction capabilities, it is postulated that the pyruvic excretory pathway is an inherent overflow response mechanism activated to limit the amount or rate of acetic acid production. This suggests a scenario in which the co-production of pyruvic acid is holistically energetically more favourable. The prevailing postulate is that the build-up of acetic acid (and formic acid), if left unchecked, might result in a runaway energy cost effect. By limiting the external pressure of passive back-diffusion, the organism is in an energetically more favourable position since less energy need be wasted on the active export of those components.