Microbial removal of acetate selectively from sugar mixtures
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Acetic acid is an unavoidable constituent of the biomass hydrolysates generated from acetylated hemicellulose and lignin, and acetate affects the performance of microbes used to convert these hydrolysates into biofuels or other biochemicals. In this study, acetate was selectively removed from synthetic mixtures of glucose and xylose using metabolically engineered Escherichiacoli strains having mutations in the glucose phosphotransferase system (PTS) genes (ptsG, manZ, crr), glucokinase (glk), and xylose (xylA). In batch culture, ALS1060 (ptsG manZ glk xylA) consumed exclusively acetate to depletion, and then consumed the two sugars only at a very slow rate (a growth rate of about 0.01 h−1). We also examined the effects of an additional knockout of either malX, fruA, fruB, bglF, or crr, genes that are involved in other PTSs, and a batch process using KD840 (ptsG manZ glk crr xylA) demonstrated a further reduction in glucose or xylose consumption by E. coli. These results demonstrate the feasibility of using a substrate-selective approach for the pre-treatment of biomass hydrolysate for microbial processes.