Effects of nutritional characteristics of Streptococcus agalactiae on inhibition of growth by lactoperoxidase-thiocyanate-hydrogen peroxide in chemically defined culture medium.

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Five cultures of Streptococcus agalactiae have an absolute requirement for L-cystine to grow in a chemically defined medium. The L-cystine could be replaced with cysteine, glutathione, or the disulfide form of glutathione. Dithiothreitol could not substitute for the sulfur-containing amino acids of glutathione; hence, the growth requirement appears to be truly nutritional. Growth was maximum with 4 to 5 mug of L-cystine per ml. If the concentration of L-cystine was no greater than 4 to 5 mug/ml, complete growth inhibition could be obtained by the addition of lactoperoxidase, thiocyanate, and H2O2. The growth inhibition, however, was nullified by additions of L-cystine 10-fold or more in excess of the concentration needed for maximum growth. During the aerobic degradation of glucose by cell suspensions, H2O2 accumulation could be shown with cultures 317 and 11-13, the only cultures the growth of which was inhibited without addition of exogenous H2O2. All of the cultures had varying degrees of peroxidase activity. The balance between H2O2 generation and peroxidase activity of the culture evidently determined whether growth could be inhibited with lactoperoxidase and thiocyanate without H2O2 addition. The growth yeilds per 0.5 mol of the disulfide forms (cystine and oxidized glutathione) were 1.5 and 1.9 times greater than that per 1 mol of the sulfhydryl forms (cysteine and glutathione).

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