Galactose-fermenting mutants of Streptococcus thermophilus

Three galactose-positive (Gal+) mutants were isolated after treating Streptococcus thermophilus strain CNRZ 302 with N-methyl-N′-nitro-N-nitrosoguanidine; other physiological and biochemical characteristics were retained. In contrast with the wild type's inability to utilize galactose, the A5 mutant metabolized 70% of the galactose resulting from the hydrolysis of lactose. Phosphoenolpyruvate-phosphotransferase activity could not be detected, indicating that this system was not involved in galactose transport. Galactokinase activities were higher in induced galactose-fermenting strains than in the galactose-negative strain. The mutation seemed to affect a galactokinase regulation gene.