SUMMARY: A strain of Lactobacillus bifidus was isolated from the faeces of a breastfedinfant and subcultivated in the minimal medium of Hassinen, Durbin, Tomarelli & Bernhart (1951). The cysteine requirement for maximal growth of the strain was c. 100 μg. cysteine/ml. Human and cow milk were found to contain another factor, besides cysteine, which was essential for optimal growth. The active material in milk could be replaced by enzymic hydrolysates of some commercial proteins; egg albumin, and blood albumin were the most potent sources. The active material was heat stable, but was completely inactivated by treatment with strong acid. All the properties of this active material studied showed a close resemblance to those of strepogenin.
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