Streptococcus thermophilus capable of metabolizing galactose: mutagenesis and application in production of fermented milk

Abstract:

[Objective] Streptococcus thermophilus is widely used in the dairy industry as a common starter for fermented dairy products such as yogurt and cheese. Most strains of S. thermophilus are galactose-negative (Gal^-) and unable to metabolize galactose and excrete it extracellularly, which results in an increase in the galactose content in fermented milk. S. thermophilus can be treated by chemical mutagenesis to metabolize galactose and then used to develop the fermented milk products with low galactose content. [Methods] We used nitrosoguanidine (NTG) to induce the mutation of S. thermophilus IMAU80846. Furthermore, we measured the activities of β-galactosidase (β-Gal), galactokinase (GalK), pyruvate kinase (PK), and glucokinase (GK) in the wild-type and mutant strains of S. thermophilus IMAU80846 and analyzed the amino acid sequences encoding these enzymes. S. thermophilus IMAU80846Y that could metabolize galactose was obtained. We performed whole genome sequencing of the mutant strain and measured the content of lactic acid, lactose, galactose, and glucose in the fermented milk products produced with the wild-type strain and the mutant strain. We then compounded the wild-type strain and the mutant strain with Lactobacillus delbrueckii subsp. bulgaricus IMAU20450, respectively, and characterized the two groups of fermented milk products during fermentation and storage. Finally, we prepared a fermented milk product with low galactose content. [Results] S. thermophiles IMAU80846Y had higher activities of β-Gal and GalK and lower activities of PK and GK than the wild-type strain. The amino acid sequences and whole genome sequences showed that the mutant strain had mutations in the genes involved in carbohydrate metabolism. The HPLC results showed that the fermented milk produced with the mutant strain had lower content of lactose and galactose and higher content of lactic acid and glucose than that produced with the wild-type strain. Compared with the compound group with the wild-type strain, the compound group with the mutant strain improved the titration acidity, viable cell count, viscosity, and water holding capacity of fermented milk. [Conclusion] The mutagenesis with NTG changed the ability of S. thermophilus IMAU80846 to metabolize galactose, and the mutant strain could be used to produce the fermented milk with low galactose content.