The effect of D-tryptophan on the utilization of the L isomer by some lactic acid bacteria.

The utilization of the isomers of tryptophan by Lactobacillus arabinosus 17-5 has been studied by several investigators. Greene and Black (1, 2) concluded that n-tryptophan was not utilized by this organism. The procedure outlined for the determination of tryptophan by these investigators was based on the interpretation that nn-tryptophan is exactly onehalf as active as the pure L isomer. Wooley and Sebrell (3) found two samples of DL-tryptophan which had exactly 50 per cent of the activity of their n-tryptophan, and one sample which exhibited 65 per cent activity. Baumgarten et al. (4) reported that twice the amount of DL-tryptophan was required to produce the same amount of acid as that produced by the L form. Greenhut et al. (5) found two samples of nn-tryptophan which possessed one-half the activity of samples of L-tryptophan. A sample of D-tryptophan showed no activity. The inactivity of n-tryptophan for Streptococcus jaecalis R is indicated in the reports by Stokes et al. (6), Baumgarten et al. (4), and Kuiken et al. (7). During tests on tryptophan samples carried out by the present authors for the purpose of selecting standards, it was found that when tested at high levels several samples of nn-tryptophan had approximately one-half the activity of the L isomer, but at low levels, the activity of the DL compound was considerably less. These experiments pointed to an inhibitory effect of the D compound on the utilization of the L form. More definite information was desired concerning this inhibition than could be obtained in experiments in which the activities of L and DL compounds were compared. Pure n-tryptophan was therefore prepared, and its effect on the utilization of L-tryptophan by several lactic acid bacteria was studied. The purpose of this paper is to report the results of these tests. An inhibitory effect of high concentrations of n-leucine and n-valine on the growth of Lactobacillus arabinosus (8, 9) and the inhibition of Escherichia coli (10) by n-alanine, n-leucine, and n-valine has been studied