Abstract
The effects of various amino acids on the growth of Streptococcus salivarius subsp. thermophilus 510 were examined.
The rate of growth of this strain in skim milk was evaluated from the acid production determined by titrating with 0.1 N NaOH, and from the turbidity measured by the absorbance at 660 nm when grown in a peptone-yeast extract-glucose medium.
The addition to 100 ml of skim milk of 50 mg of L-aspartic acid, L-isoleucine, L-methionine, L-phenylalanine, L-serine, L-threonine, L-cysteine, and L-cystine each inhibited the growth of this strain. Only L-aspartic acid among these amino acids inhibited the growth to a large degree, and even the addition of 10 mg to 100 ml of skim milk inhibited growth to some extent.
The addition of 50 mg of each of these amino acid to 100 ml of skim milk to which 25 mg of L-aspartic acid had been added resulted in only L-glutamic acid removing this inhibiting effect of L-aspartic acid after 24 h of incubation. After 48 h, L-hydroxyproline, L-lysine, L-proline, and L-tryptophan partially eliminated this inhibiting effect of L-aspartic acid, but conversely, L-cysteine and L-cystine enhanced the inhibiting effect of L-aspartic acid.
The addition of more than 25 mg of L-aspartic acid to 100 ml of the peptone-yeast extract-glucose medium inhibited considerably the growth of this strain.
The inhibiting effect of L-aspartic acid on the growth of this strain in both skim milk and in the peptone-yeast extract-glucose medium could be removed by adding the same amount of L-glutamic acid.
