Role of intra-cellular and extra-cellular cues in triggering the switch from acidogenesis to solventogenesis in Clostridium acetobutylicum

Abstract

Biobutanol can be used as efficient vehicle fuel as its properties are closer to gasoline and comparatively advantageous over other fermentation based biofuels. However, Clostridium acetobutylicum is well known organism for biobutanol production since the history of ABE (Acetone Butanol Ethanol) fermentation. The organism has two distinguished phases, namely acidogenesis and solventogenesis, in its metabolic pathway. The first phase, acidogenesis, incorporates accumulation of acetic and butyric acids, whereas in the second phase, solvenogenesis, sugars along with the acids produced in first phase are consumed simultaneously for production of organic solvents. From the point of veiw of productivity of butanol, the duration of the acetogenesis phase elays switching of metabolic network from acidogenesis to solventogenesis. However, on completion of the acidogenesis phase, the cell can either enter the solventogenesis phase, or trigger the onset of sporulation. The precise cues which trigger the switch from one phase to another, and the choice between sporulation and solventogenesis are not well understood. Greater understanding of and control over the switching mechanism would enable design of strains for enhanced butanol production. In present paper, we investigated the role of cellular and environmental cues which triggers the switch from acidogenesis to solventonesis. We demonstrate that C. acetobutylicum, employs a hierarchical sugar utilization strategy, with the bacterium preferentially utilizing glucose over arabinose over xylose. This hierarchy is consistent with other sugar utilization patters observed on other organisms. Our results also indicate that the cell utilizes intracellular cues (growth phase and growth rate) and extracellular cues (pH and organic acid) to determine the precise timing of the switch from the acidogenesis to the solventogenesis phase.