Antagonism of amino acids in the growth of lactic acid bacteria.

Inhibition of microbial growth by compounds closely related to essential metabolites has been demonstrated by many investigators (l-3). Early workers (4-7) showed that inhibitory effects of single amino acids could be reversed by altering the concentration of structurally related amino acids. Gladstone (8), using Bacillus anthracis, showed such a relationship among leucine, valine, and isoleucine and between threonine and serine. Threonine exerts an antagonistic action on the utilization of serine by Lactobacillus arabinosus (9). Using Lactobacillus casei, Feeney and Strong (10) demonstrated an inhibitory effect of aspartic acid, reversible by glutamine, glut.amic acid, or asparagine. While investigat.ing a uniform medium for the microbiological determination of amino acids (II), certain difficulties were encountered with the glutamic acid and isoleucine assays with Lactobacillus arabinosus. A lag in the growth response curve was observed at the lower concentrations of these amino acids. The tubes containing samples, particularly in the case of isoleucine assays, did not show this lag to the same extent as the standard curves, resulting in marked downward drift of assay values. To alleviate the lag in the glutamic acid curve and to give a valid assay, a heavy inoculum and adjustment of the medium to pH 6 have been used (12). While this provided a satisfactory assay for glutamic acid, further work on the fundamental defect was indicated. As in previous studies (13, 12, 14, 15), when glutamic acid standard was replaced by glutamine (sterilized by filLration) the lag was absent. This indicated, as suggested previously (12, 14), that glutamic acid is utilized through glutamine and that the lag is the result of partial or complete failure of the small amounts of glutnmic acid present to be amidated to glut>smine. In this study the mutual antagonism of the members of two groups of