The Microbiological Determination of Amino Acids

After the development of methods for the determination of valine, leucine, and isoleucine (1) with LactobacilEus arabinosus 17-5 as the test organism, it was assumed that other amino acids essential for the growth of this organism could be determined by about the same procedure. However, it was soon discovered that glutamic acid is unique in its relationship to the growth of Lactobacillus arabinosus. Once this relationship was understood, the microbiological determination of glutamic acid became one of the most satisfactory assay methods which the authors have attempted so far. Glutamic acid has been reported to be an essential nutrient for Lactobacillus arabinosus 17-5 by a number of investigators (l-6). According to Pollack and Lindner (3) glutamine and glutamic acid have equal growthpromoting activity for this organism. Shankman et al. (6) have used Lactobacillus arabinosus to determine glutamic acid. In our hands unsatisfactory results were obtained by an analogous procedure. The consistently satisfactory standard curves which had been obtained with valine, leucine, and isoleucine were not obtained with glutamic acid. The frequent failure of the organism to respond at all at the lower test levels of glutamic acid was particularly troublesome. The data suggested that glutamic acid is converted to some other metabolite before being utilized, and that this conversion is readily performed by the organism when the concentration of glutamic acid is relatively high or when vigorous growth has once been initiated. Evidence is presented which indicates that glutamine may be the active metabolite. The analytical method reported *here employs a medium supplemented with glutamine.