Investigation of the Carbon Catabolite Repression Mechanism in L-Lactic Acid Fermentation from Mixed Sugars by...

Lignocellulosic hydrolysate is rich in various fermentable sugars, such as glucose, xylose, and cellobiose. Utilizing these sugars for L-lactic acid fermentation represents a promising strategy for the high-value utilization of biomass. However, when mixed sugars serve as carbon sources, microorganisms typically undergo carbon catabolite repression (CCR) at the initial fermentation stage, which significantly compromises both the yield and productivity of L-lactic acid. To clarify CCR mechanisms and explore effective mitigation strategies, Bacillus coagulans DSM 2314 was used as the fermentative strain, the effects of pH and temperature on fermentation with single and mixed carbon sources were examined, and L-lactic acid yields, productivities, and key enzymatic activities across different fermentation systems were systematically compared. The results showed that in glucose-containing mixed-sugar systems, glucose imposed strong CCR effects on both cellobiose and xylose. Under optimal conditions (initial total sugar concentration of 50 g/L, pH 7.0, and 45 °C), L-lactic acid yields increased in the following order: glucose/xylose (15.58 g/L) < glucose/cellobiose (29.65 g/L) < glucose (31.87 g/L). In contrast, in the glucose-free cellobiose/xylose system, both sugars were nearly co-consumed by B. coagulans DSM 2314, and L-lactic acid production was not significantly diminished by the mixing of carbon sources (xylose (27.45 g/L) < cellobiose/xylose (28.64 g/L) < cellobiose (29.60 g/L)). Under replicated optimal condition experiments, analyses of sugar consumption rates and enzyme activities further confirmed that the CCR between cellobiose and xylose was significantly weaker than in other mixed-sugar systems, with the L-lactic acid yield in the cellobiose/xylose system 1.61-fold higher than in the glucose/xylose system. These findings demonstrate that substituting glucose with cellobiose in mixed-sugar fermentation is an effective approach to mitigating CCR, providing a theoretical basis for efficient L-lactic acid production from lignocellulosic hydrolysates.