Sugar uptake by the solventogenic clostridia.

Wilfrid J Mitchell

doi:10.1007/s11274-015-1981-4

Abstract

The acetone–butanol–ethanol fermentation of solventogenic clostridia was operated as a successful, worldwide industrial process during the first half of the twentieth century, but went into decline for economic reasons. The recent resurgence in interest in the fermentation has been due principally to the recognised potential of butanol as a biofuel, and development of reliable molecular tools has encouraged realistic prospects of bacterial strains being engineered to optimise fermentation performance. In order to minimise costs, emphasis is being placed on waste feedstock streams containing a range of fermentable carbohydrates. It is therefore important to develop a detailed understanding of the mechanisms of carbohydrate uptake so that effective engineering strategies can be identified. This review surveys present knowledge of sugar uptake and its control in solventogenic clostridia. The major mechanism of sugar uptake is the PEP-dependent phosphotransferase system (PTS), which both transports and phosphorylates its sugar substrates and plays a central role in metabolic regulation. Clostridial genome sequences have indicated the presence of numerous phosphotransferase systems for uptake of hexose sugars, hexose derivatives and disaccharides. On the other hand, uptake of sugars such as pentoses occurs via non-PTS mechanisms. Progress in characterization of clostridial sugar transporters and manipulation of control mechanisms to optimise sugar fermentation is described.

Highlights

Two of the most important issues facing the world at the present time are energy security and climate change
Regulons associated with uptake and metabolism of xylose and arabinose have been identified in C. acetobutylicum and C. beijerinckii (Gu et al 2010; Zhang et al 2012), and in both organisms increased gene expression following inactivation of a XylR transcriptional repressor has been shown to be associated with increased utilization of xylose as a substrate (Hu et al 2011; Xiao et al 2012)
In terms of detailed molecular analysis, studies on sugar transport in the solventogenic clostridia are less well developed compared to investigations on some other bacteria

Summary

Introduction

Two of the most important issues facing the world at the present time are energy security and climate change. Around the globe, there is a widespread recognition that continued reliance on a finite energy source is unsustainable, resulting in increased emphasis on development of renewable and environmentally friendly alternatives. Biofuels produced by microbial fermentations are viewed as a viable option, with bioethanol and biodiesel already in widespread use as transportation fuel extenders. A more attractive biofuel is biobutanol, which is produced by the solventogenic clostridia in the acetone–butanol– ethanol (ABE) fermentation (Schiel-Bengelsdorf et al 2013). The ABE fermentation was introduced at the time of the First World War to produce acetone required for cordite manufacture, and subsequently the by-product butanol became important as a solvent in production of nitrocellulose lacquer sprays for the expanding automobile industry. The considerable importance of butanol as a bulk industrial chemical, coupled with its advantageous properties as a biofuel, has stimulated interest in re-establishment of the fermentation process (Durre 2011; Green 2011)

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Conclusion

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