PRODUCTS OF THE GROWTH OF BACTERIA. II9 



ened by warming slightly. The value of this reaction will be 

 understood when, to give one illustration, 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. Certain 

 bacteria of the soil which will be discussed further on are able 

 to complete the conversion of ammonia into nitrous acid, lead- 

 ing 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 bacterial 

 growths is of frequent occurrence. Carbon dioxide, hydrogen 

 sulphide and nitrogen are among the better known gases that 

 maybe formed. The odors that arise from cultures and that 

 are so characteristic of putrefactive processes depend upon the 

 development of gases, or a mixture of gases, of considerable 

 complexity. The bacillus aerogenes capsulatus leads some- 

 times to the formation of gas in the organs of the human cadaver 

 within a short time after death. Theobald Smith introduced 

 a valuable means of differentiating species of bacteria based 

 upon their power of forming gas in media containing different 

 sugars, or in their inability to do so. 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, afterward filled 



