Effects of ptb knockout on butyric acid fermentation by Clostridium tyrobutyricum

Clostridium tyrobutyricum ATCC 25755 is an anaerobic, rod-shaped, gram-positive bacterium that produces butyrate, acetate, hydrogen, and carbon dioxide from various saccharides, including glucose and xylose. Phosphotransbutyrylase (PTB) is a key enzyme in the butyric acid synthesis pathway. In this work, effects of ptb knockout by homologous recombination on metabolic flux and product distribution were investigated. When compared with the wild type, the activities of PTB and butyrate kinase in ptb knockout mutant decreased 76 and 42%, respectively; meanwhile, phosphotransacetylase and acetate kinase increased 7 and 29%, respectively. However, ptb knockout did not significantly reduce butyric acid production from glucose or xylose in batch fermentations. Instead, it increased acetic acid and hydrogen production 33.3-53.8% and ≈ 11%, respectively. Thus, the ptb knockout did increase the carbon flux toward acetate synthesis, resulting in a significant decrease (28-35% reduction) in the butyrate/acetate ratio in ptb mutant fermentations. In addition, the mutant displayed a higher specific growth rate (0.20 h(-1) vs. 0.15 h(-1) on glucose and 0.14 h(-1) vs. 0.10 h(-1) on xylose) and tolerance to butyric acid. Consequently, batch fermentation with the mutant gave higher fermentation rate and productivities (26-48% increase for butyrate, 81-100% increase for acetate, and 38-46% increase for hydrogen). This mutant thus can be used more efficiently than the parental strain in fermentations to produce butyrate, acetate, and hydrogen from glucose and xylose.