Selection of Galactose-Fermenting Streptococcus thermophilus in Lactose-Limited Chemostat Cultures

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Stock cultures of Streptococcus thermophilus are essentially galactose negative (Gal−). Although both galactose 1-phosphate uridyl transferase and uridine-5-diphospho-glucose 4-epimerase are present, suggesting that the genes for the Leloir pathway exist, cells cannot induce high levels of galactokinase. Therefore, galactose is largely excreted when cultures are grown on lactose, and most strains cannot be readily adapted to grow on free galactose. Gal− cultures were grown in a chemostat under lactose limitation in which high concentrations of residual galactose were present. Under this selection pressure, Gal+ organisms eventually took over the culture with all four strains examined. Gal+ cells had induced galactokinase, and three of the four strains grew on free galactose with doubling times of 40 to 50 min. When Gal+ organisms were grown on lactose in batch culture, the galactose moiety was only partially utilized while lactose was still present. As lactose was exhausted, and catabolite repression was lifted, the Leloir pathway enzymes (especially galactokinase) were induced and the residual galactose fermented. Neither phospho-β-galactosidase activity nor the enzymes of the d-tagatose 6-phosphate pathway were detected in S. thermophilus. In contrast to Streptococcus cremoris and Streptococcus lactis, fermentation was homolactic with galactose in batch cultures and with lactose limitation in the chemostat. When mixed Gal+-Gal− cultures were repeatedly transferred in milk, the Gal− cells became the dominant cell type. The Gal− phenotype of stock cultures probably reflects their prolonged maintenance in milk.

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