Effective utilization of xylose-rich substrates for butyric acid production by Clostridium tyrobutyricum ATCC 25755 despite carbon catabolite repression

Carbon catabolite repression (CCR) can restrict xylose utilization during butyric acid production from lignocellulosic hydrolysates by Clostridium tyrobutyricum ATCC 25755. This study evaluated CCR behavior using 2-deoxy-D-glucose (2-DG), a glucose analog, and examined the effects of glucose-xylose composition on mixed-sugar fermentation. 2-DG caused concentration-dependent growth inhibition in xylose-containing medium, indicating CCR-like behavior. However, xylose was partially co-consumed in the presence of glucose or 2-DG, suggesting incomplete inhibition. In batch fermentations with 80 g/L total sugars, the overall xylose consumption rate declined from 2.68 to 0.53 g/L/h as the initial glucose concentration increased from 20 to 60 g/L. Nevertheless, a xylose-rich mixture (20 g/L glucose and 60 g/L xylose) achieved an overall total sugar consumption rate of 3.54 g/L/h and a butyric acid productivity of 1.22 g/L/h, comparable to those with 80 g/L glucose alone (3.64 and 1.26 g/L/h, respectively). Detectable xylose isomerase activity during glucose-xylose co-utilization further supported incomplete repression of xylose metabolism. In xylose-rich rice straw hydrolysate (5.0 g/L glucose and 15.1 g/L xylose), both sugars were completely consumed, producing 7.68 g/L butyric acid. These findings suggest that xylose-rich mixtures can support effective sugar utilization and butyric acid production by C. tyrobutyricum despite its CCR-like behavior.