Production of Short Chain Butyl Esters By E. coli As a Strategy for Butanol Extraction

Interest in expanding our use of renewable energies has been gaining momentum over the last few decades. Biological production of transportation fuels, especially, has been of growing research interest, with ethanol fuel leading the way as a surrogate for gasoline. However, ethanol is not the most ideal bio-alcohol fuel because of its low energy density relative to gasoline, and water solubility. Compared to ethanol, butanol fuel is less hydroscopic, less volatile, less corrosive, and contains an energy density that is more similar to gasoline. Butanol is naturally produced by certain species of Clostridia through a metabolic pathway known as acetone, butanol and ethanol (ABE) fermentation, of which butanol constitutes the major product. This ABE pathway has been expressed in more tractable organisms such as Escherichia coli and Saccharomyces cerevisiae, and manipulation of these host microbes to maximize their butanol yield is a current stream of research, with one of its tributaries being the improvement of host butanol tolerance. Microbial butanol exposure is significantly toxic, with even native Clostridial producers only capable of accumulating it to approximately 2% (v/v) in culture, limiting final butanol titres.

In this research we investigate an alternative strategy to improve tolerance, and therefore yield, of butanol in microbial cultures. This strategy involves sequestering the endogenous butanol made by E. coli heterologously expressing the ABE fermentation pathway, into a more neutral ester molecule that can both accumulate to higher concentrations in culture with minimal toxic effect, as well as permeate into the media where it can be collected downstream for butanol recovery. Here we discuss the feasibility of such a strategy, from the initial selection of an appropriate candidate butyl ester through to engineering E. coli harbouring the ABE pathway for butyl ester synthesis.