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 ty rosin. Systematic investigations have 

 only been made with regard to the occurrence of indol 

 and phenol. 



Test for Indol. To a 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) ak the result of a purely 



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