Abstract
One of the bio-chemical reactions extensively used for the identification of bacterial types is the production of indol from peptone in a peptone-water solution. Of the various tests that might be used for the detection of that substance the one most widely adopted in this country is the so-called Salkowski test (H2SO4 + KNO2). In Germany Bohme (1905) and in England Marshall (1907) have found, after a comparative study, that the Ehrlich reaction (Paradimethylamidobenzaldehyde + HCl) gives more uniform and constant results.
This study was undertaken in order to obtain further light on the relative reliability of the two tests. Seventy-five organisms, representing the various members of the colon-typhoid group, were used. Duplicate peptone broth tubes of each strain were incubated at 37° C. for four days. The contents of the tubes was then divided in half and the half portions subjected respectively to the two tests.
Twenty-eight of the seventy-five strains were positive in both duplicate tubes with both tests. Thirty-seven were uniformly negative with both tests. Five were positive in both controls with the Ehrlich test and positive in one tube and negative in the other with the Salkowski. Finally five tubes were uniformly positive with the Salkowski and negative with the Ehrlich test. The five cases in which the Salkowski test was positive and the Ehrlich test negative illustrate the fact brought out by Bohme and Marshall that the Salkowski test may give erroneously positive results when indol is not present. This error seems to be due to some red substance other than nitroso-indol but which may easily be mistaken for it. If the test is carefully performed this reddening can, however, easily be distinguished from the nitroso-indol red by the fact that it diffuses rapidly throughout the entire tube instead of remaining as a ring between the two liquids.
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