While amino-acids in appropriate concentrations have long been known as valuable aids to bacterial growth, it does not appear to have been recognised that in relatively low concentrations they are often inhibitory.

Inhibition has been shown to occur with 10 out of 11 single amino-acids tested and with certain mixtures rich in amino-acids. The inhibitory concetration varies from 11 to 130 millimols per litre, or from 0·2 to 2%(wt./vol. ). It is of some intrest that the amino-acids inhibitory in the lowest concentration were the cyclic compounds—histidine, tyrosine, tryphane, phenylalamine. Of the chain compounds tested cystine was the most inhibitory.

Several organisus belonging to various groups of bacteria are susceptible of inhibition by amino-acids; certain intestinal organisms are not susceptible. The effect cannot be attributed to the physical effects of high concentration.

There is an apparent divergence between our results and those of workers who have recommended tryptic digests for stimulating the growth of some of the bacteria here shown to be readily inhibited by amino-acids. A suggested explanation of this divergence is that it is the products of partial protein digestion, the polypetides, which are mainly responsible for the effect of stimulating growth, not the amino-acids. A “polypeptide medium” might prove particularly valuable for bacterial growth.

A metabolism experiment with glycine in high but sub-inhibitory concentration showed that the amino-acid is broken down by Staphylococcus aureus, at this concentration.