The Nutritional Requirements of Lactic Streptococci Isolated from Starter Cultures. II. A Stimulatory Factor Required for Rapid Growth of Some Strains in Reconstituted Nonfat Milk Solids

Summary Liver fraction L, trypsinized skimmilk, or peptonized milk, when added in a low concentration to reconstituted nonfat milk solids, produced greater stimulation in the growth of mixed starter cultures and individual strains of S. lactis and S. cremoris isolated from them than did any individual amino acid or vitamin. Liver fraction L produced marked stimulation in the initial growth rate of many so-called slow strains. Some of the faster strains were inhibited or showed only a slightly increased response when this factor was added to the milk. Results suggested that the material responsible for such increased initial growth rate might be a peptide or peptide-like in nature, and that an enzymatic digestion of milk may tend to release these substances from the milk proteins in a readily available form for the organisms comprising a starter culture. When some slow strains were supplemented with such hydrolytic substances, the rate of growth approached that of faster strains.