Microbiological Methods for the Determination of Amino Acids I. Aspartic Acid and Serine

Knowledge of the growth factor requirements of various lactic acid bacteria, especially Lactobacillus casei and L. arabinosus 17-5, has led to the development of useful microbiological methods for the quantitative determination of most-of the B vitamins. Recently attention has turned to the amino acid requirements of the lactobacilli. The amino acids necessary for growth of L. casei and L. arabinosus have been determined (14) and satisfactory methods have been developed with these two organisms for the assay of leucine (3-5), isoleucine (3), valine (3%6), arginine (6), phenylalanine (4), tryptophane (7), and glutamic acid (8) in purified proteins and to a more limited extent in natural products. Although Lactobacillus casei or L. arabinosus responds quantitatively to arginine, isoleucine, leucine, methionine, phenylalanine, threonine, tryptophane, saline, glutamic acid, tyrosine, and cystine (2-4, 6, 9), irregular results have been obtained with serine, aspartic acid, and lysine (9). Also neither organism requires histidine, proline, hydroxyproline, norleucine, glycine, or alanine for growth, presumably because these amino acids are synthesized by the bacteria. It seemed desirable, therefore, to investigate the amino acid requirements of other lactobacilli which might be used for the assay of those amino acids which cannot be satisfactorily measured with L. casei or L. arabinosus. The amino acids necessary for growth of L. de&ii&ii LD5l have been determined (to be published elsewhere) and a method has been developed for the assay of aspartic acid and serine in purified proteins. The latter is described in the present paper.