VITAL PHENOMENA OF BACTERIA. 77 
Aromatic Products of Decomposition. Many bacteria 
produce aromatic substances as the result of their 
growth. The best known of these are indol, skatol, 
phenol, and tyrosin. Systematic investigations have 
only been made with regard to the occurrence of indol 
and phenol. 
Test for Indol. Toa bouillon culture, which should, 
if possible, be not under eight days old and free from 
sugar, is added half its volume of 10 per cent. sulphuric 
acid. If in heating to about 80° C. a pink or bluish- 
pink coloration is immediately produced it indicates the 
presence of both indol and nitrites, the above-described 
nitroso-indol reaction requiring the presence of both of 
these substances for its successful operation. This is 
the so-called ‘‘ cholera-red reaction,’’ but it may be 
applied to many other spirilla beside cholera. As a 
rule, however, the addition of sulphuric acid alone is 
not sufficient, and a little nitrite must be added; this 
may be done later, the culture being first warmed with- 
out nitrite, when if there is no reaction or a doubtful 
one, 1 to 2 c.c. of a 0.5 per cent. solution of sodium 
nitrite is added until the maximum reaction is obtained. 
The addition of strong solutions of nitrite colors the 
acid liquid brownish-yellow and ruins the test. 
Out of sixty species examined by Lehmann, twenty- 
three gave the indol reaction. Levandoosky states that 
the color group in general, glanders, diphtheria, proteus 
vulgaris, and most of the spirilla, are indol producers; 
with the exception of the spirilla, these bacteria also 
produce phenol. 
Decomposition of Fats. Pure melted butter is not a 
suitable culture medium for bacteria. The rancidity 
of butter is brought about (1) as the result of a purely 
