Effect of carbon catabolite repression on lactose and galactose catabolism in Lacticaseibacillus paracasei

Previous research has identified phosphoenolpyruvate-dependent phosphotransferase system and tagatose-6-phosphate (T6P) pathway of lactose/galactose metabolism in Lacticaseibacillus casei groups, which led to no galactose accumulation in fermented milk produced by Lb. paracasei. However, Lb. paracasei preferred lactose to galactose when grown in a mixture of the two sugars, indicating possibility of carbon catabolite repression of galactose catabolism in the fermented milk. Therefore, galactose accumulation was still observed during co-culturing of Lb. paracasei with Streptococcus thermophilus in fermented milk. We demonstrated that the tran-scriptional regulator lacR acts as a transcriptional repressor of the T6P pathway genes (lacA, lacC and lacD) and lacT acts as a transcriptional activator for genes (lacE) encoding the lactose-transporting PTS in the mixed lactose/galactose CDM medium, indicating possible dual repressive effects of global or local regulation on the carbon catabolism of galactose in Lb. paracasei.