Isolation of Streptococcus lactis C2 Mutants with High Phosho-β-Galactosidase Activity

We performed experiments in search of a plating medium which would differentiate lactose constitutive mutants from wild-type cells of lactic streptococci and of a broth medium that would enrich for lactose constitutive mutants. No selective medium was applicable for selection of lactose constitutive mutants from Group N streptococci. Prolonged growth in a chemostat with lactose as the limiting growth factor, however, enriched for these mutants from Streptococcus lactis C2. The mutants synthesized phospho-flgalactosidase constitutively with an activity 1.5 to 4 times that of the parent culture. The slow growth of S. lactis C2 and the acquisition of ability to grow on lactobionic acid by the mutants confirmed their constitutive nature. In comparison to S. lactis C2, the constitutive mutant M10 grew rapidly in milk whereas the lag phase was 45 rain with the parent strain when the inoculum was low. During this study we observed a change in the specific activity of phospho-flgalactosidase in S. lactis C2 under different growth conditions. Stage of growth markedly influenced enzyme activity. In addition, in complex broth lactose utilizing enzymes were repressed by glucose. However, in a semi-synthetic medium containing glucose, the phospho-fl-galactosidase activity remained essentially constant for glucose concentrations rangin K from .5 to 10 mg per ml. S. lactis C2 did not exhibit a glucose-lactose growth Beeelved June 18, 1974.