130 MANUAL OF BACTERIOLOGY. 



it is remembered that the bacillus coli communis produces 

 indol and the bacillus of typhoid fever usually does not. 

 The reaction depends upon the liberation of nitrous acid, 

 which, with indol, forms a red color. 



The change of organic substances into more stable ones 

 does not cease with the compounds mentioned above. Cer- 

 tain bacteria of the soil which will be discussed further on 

 are able to complete the conversion of ammonia into nitrous 

 acid (leading to the formation of nitrites) ; and others still 

 that of nitrites into nitric acid, which at once forms nitrates, 



Formation of Acids. In the course of their growth 

 many bacteria produce acids, especially from substances 

 containing sugar. The power of developing lactic acid is 

 possessed by a large number of species. Acetic acid is 

 another common by-product. Besides these, butyric acid, 

 formic acid, propionic acid and many more are formed by 

 different bacteria. 



Development of Gas. The evolution of gas from bac- 

 terial growths is of frequent occurrence. Carbon dioxide, 

 hydrogen sulphide and nitrogen are among the better 

 known gases that may be formed. The odors that arise 

 from cultures and that are so characteristic of putrefactive 

 processes depend upon the development of gases, or a mix- 

 ture of gases, of considerable complexity. The bacillus 

 aerogenes capsulatus leads sometimes to the formation of 

 gas in the organs of the human cadaver within a short time 

 after death. 



T. Smith considers the formation of gases in media con- 

 taining sugar of importance in discriminating between dif- 

 ferent species. Bouillon containing i per cent, of dextrose 

 (or lactose, etc.) is the culture-medium advised. The 

 test is best conducted in a U-shaped tube, closed at one end, 

 and at the other end provided with a bulb (Fig. 46). The 

 tube is stoppered with cotton, sterilized by dry heat, after- 



