Microbes Stress Out During Conversion of Pretreated Biomass to Biofuels
Chemical pretreatment of plant biomass prior to enzymatic breakdown significantly improves the release of sugar molecules, which are subsequently converted to biofuel compounds by fermentative microbes. However, pretreatment also introduces a variety of stress factors that can interfere with these fermentative organisms, including residual chemicals, toxins released from the biomass, high concentrations of sugars, and production of biofuels themselves. Researchers at the DOE Great Lakes Bioenergy Research Center (GLBRC) describe the integration of gene expression and physiological stress responses in an ethanol-producing strain of Escherichia coli during growth on corn stover that had been pretreated using ammonia fiber expansion (AFEX) and enzymatic digestion. Their results indicate that osmotic pressure resulting from high sugar concentrations and toxicity due to ethanol production were the two most important stressors to E. coli under these conditions, and that the cells activated a cascade of carefully timed stress tolerance pathways in response to these factors. Identification of these pathways provides new targets for metabolic engineering to improve stress tolerances of biofuel-producing microbes, leading to the development of more sophisticated approaches to leverage microbes’ natural abilities to sense and respond to environmental stress.
Swalbach, M. S., et al. 2012. “Complex Physiology and Compound Stress Responses during Fermentation of Alkali-Pretreated Corn Stover Hydrolysate by an Escherichia coli Ethanologen,” Applied and Environmental Microbiology 78, 3442-57, DOI: 10.1128/AEM.07329-11.