Abstract
A major challenge when using microorganisms to produce bulk chemicals such as biofuels is that the production targets are often toxic to cells. Many biofuels are known to reduce cell viability through damage to the cell membrane and interference with essential physiological processes. Therefore, cells must trade off biofuel production and survival, reducing potential yields. Recently, there have been several efforts towards engineering strains for biofuel tolerance. Promising methods include engineering biofuel export systems, heat shock proteins, membrane modifications, more general stress responses, and approaches that integrate multiple tolerance strategies. In addition, in situ recovery methods and media supplements can help to ease the burden of end-product toxicity and may be used in combination with genetic approaches. Recent advances in systems and synthetic biology provide a framework for tolerance engineering. This review highlights recent targeted approaches towards improving microbial tolerance to next-generation biofuels with a particular emphasis on strategies that will improve production.
Highlights
Microbes can be engineered to produce biologicallyderived replacements for gasoline, diesel, and aviation fuel
Next-generation biofuels have many advantages, but the fuels are often toxic to microorganisms
Limonene tolerance in E. coli was improved by heterologously expressing an efflux pump and the corresponding strain showed a 64% improvement in limonene yield [7]
Summary
Microbes can be engineered to produce biologicallyderived replacements for gasoline, diesel, and aviation fuel. Genomic studies have repeatedly revealed heat shock proteins among those that are upregulated in response to solvent stress and there is evidence that they may be good engineering targets for improving biofuel tolerance and yield. Consistent themes from these studies indicate that respiration, general stress response mechanisms, and membrane proteins are altered in response to biofuel [12] These may serve as engineering targets, either for improving yield or by use as switches to activate expression of other tolerance mechanisms. C. acetobutylicum studies have shown that butanol tolerance can be increased by genetic changes that modify the fatty-acid composition of the membrane, but these effects inhibited solvent production [10,11]. Overcoming toxicity limitations will likely require a combination of genetic manipulations, advances in bioreactor recovery methods, and optimization of growth conditions
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