Production and evaluation of a mutant galactose-utilizing strain of Streptococcus thermophilus for application in milk fermentation

Streptococcus thermophilus is a thermophilic gram-positive bacterium used in the production of fermented milk. Most S. thermophilus strains originating from milk can only use the glucose moiety of lactose as a carbon source, while the galactose moiety is secreted into the fermenting milk. In this study, one galactose-positive mutant (S. thermophilus IMAU80285Y) was produced by treating S. thermophilus IMAU80285 with the chemical mutagen, N-methyl-N' -nitro-N-nitrosoguanidine (NTG), and fermentation characteristics were evaluated. 13-Galactosidase activity was higher in the mutant strain while glucokinase and pyruvate kinase activities were significantly decreased compared with the wild type. Sequencing revealed that S. thermophilus IMAU80285Y had mutations in the 13-galactosidase, glucokinase and pyruvate genes. High performance liquid chromatography (HPLC) analysis demonstrated that accumulation of galactose in milk fermented by the mutant S. thermophilus IMAU80285Y was reduced by 55.4%, compared with the wild strain. Our results indicate that mutation enabled rerouting of the galactose metabolism pathways and provides new insights into using chemical mutagenesis to select strains with excellent characteristics for dairy application.