Isolation and Partial Characterization of Lactose-Negative Mutants from Streptococcus lactis C2 Defective in the Phosphotransferase System

Development of strains of dairy streptococci that improve or control rate of lactic acid production would be of value in dairy fermentations. However, improvement of the ability of dairy starter cultures to metabolize lactose has been limited because of inability to examine and characterize these plasmidlinked genes. A study was initiated to define further the genetic and enzymatic basis of lactose utilization of Streptococcus lactis C2. Lactose-negative mutants were isolated by ultraviolet irradiation or ethyl methanesulfonate mutagenesis from a C2 strain in which genes for lactose utilization had become integrated into the chromosome. Mutants were examined for phospho43-galactosidase activity and for their ability to accumulate [carbon-14] thiomethyl-/3-D-galactoside. Selected mutants were tested for enzyme complementation by Staphylococcus aureus mutants with known specific defects in lactose-utilizing enzymes. Genetic complementation via transduction between lactose-negative derivatives to yield lactose-positive recombinants also was obtained. Lactose-negative mutants missing only the lactose-specific Enzyme IIlac (EII lac) or Factor III lac (FIII lac) were identified. A mutant that retained the lactose-negative phenotype, even though it possessed Ell lac, FIII lac, and phospho-/3-galactosidase activities, was