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

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Abstract
Addition of liver fraction L, trypsinized skim milk, and peptonized milk produced marked stimulation of certain mixed starter cultures and individual strains of Streptococcus lactis and S. cremoris. The stimulatory effect of liver fraction L and enzyme protein hydrolysates was more pronounced than that of any individual amino acids, vitamins or of acid hydrolysate of casein and was particularly evident in the case of many characteristically slow strains. Some faster strains were inhibited when liver L was added to milk. Results suggested that the stimulatory material might be a peptide or peptide-like in nature.