Mechanisms of Lactose Utilization by Lactic Acid Streptococci: Enzymatic and Genetic Analyses

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Abstract
The apparent instability of β-galactosidase in toluene-treated cells or cell-free extracts of lactic streptococci is explained by the fact that these organisms do not contain the expected enzyme. Instead, various strains of Streptococcus lactis, S. cremoris, and S. diacetilactis were shown to hydrolyze o-nitrophenyl-β-d-galactoside-6-phosphate (ONPG-6-P), indicating the presence of a different enzyme. In addition, lactose metabolism in S. lactis C2F was found to involve enzyme I (EI), enzyme II (EII), factor III (FIII), and a heat-stable protein (HPr) of a phosphoenolpyruvate (PEP)-dependent phosphotransferase system analogous to that of Staphylococcus aureus. Mutants of S. lactis C2F, defective in lactose metabolism, possessed the phenotype lac− gal−. These strains were unable to accumulate 14C-thiomethyl-β-d-galactoside, to hydrolyze ONPG, or to utilize lactose when grown in lactose or galactose broth. In addition, these mutants contained EI and HPr, but lacked EII, FIII, and the ability to hydrolyze ONPG-6-P. This suggested that the defect was in the phosphorylation step. Lactose-negative mutants of S. lactis 7962, a strain containing β-galactosidase, could be separated into several classes, which indicated that this organism is not dependent upon the PEP-phosphotransferase system for lactose metabolism.

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