The nature of the stimulation of the growth ofStreptococcus lactisby yeast extract

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Abstract
Summary: Yeast extract was fractionated on Sephadex G-25 into 7 fractions. The fraction most stimulatory to the growth ofStreptococcus lactisC10 contained over 70% of the amino N present in yeast extract and consisted of a wide variety of free amino acids and a small amount of peptide material. Examination of possible replacement factors for this fraction revealed that the amino-acid material present was largely responsible for the stimulation ofStr. lactisC10. Purine and pyrimidine bases and inorganic constituents also contributed to the stimulation. In addition, yeast extract contained a component which decomposed H2O2, a metabolite which accumulates in the growth medium under aerobic conditions and inhibits growth. The nature of the stimulation was studied by isolating slow and fast acid-producing colonies ofStr. lactisC10. It appeared that yeast extract and other amino-acid supplements prevented an observed inhibition of the growth of the slow variants below pH 6·0, apparently by satisfying a nutritional deficiency caused by a drop in pH.