A Modification of the Method for Determining the Production of Indol by Bacteria

The Dunham-Bujwid indol test, used originally to distinguish cholera spirilla from other spirilla, was applied by Kitasato to bacteria other than those of Asiatic cholera, notably the colon bacillus and the typhoid bacillus, and today the test for indol is, by common consent, used regularly in the study of bacteria. As a result of its application we now know of two classes of indol-producing bacteriawthose which produce, in addition, nitrites, and those which do not. The first includes the spirillum of Asiatic cholera; the second, most other indol-producing species. The fairly extensive literature deals mainly with the first class, because of the obviously important bearing of any method which will distinguish cholera from other spirilla, As is well known, the " cholera-red" reaction is brought out by adding sulphuric or chlorhydric acid free from nitrous acid to bouillon cultures, when the presence of indol plus nitrites will impart a violet-red color to the fluid. Dunham introduced the nutritive solution consisting of 1 per cent peptone and ½ per cent sodium chloride in water. This is now universally used in testing for indol. There have been not a few voices raised in criticism of this method for determining the cholera-red reaction. To these I shall refer only as they antedate my own efforts towards improving the indol test, since I have restricted myself to the purely indol-producing forms. In attempting for a number of years past to apply the indol test by using the simple peptone solution I have been forced to consider the method quite unsatisfactory for some groups of bacteria. I was therefore led recently to try dextrose-free bouillon, with a premonition that possibly the carbohydrate may inhibit the production of indol as it does that of toxins in some cases. The result was very satisfac: