PRODUCTS OF THE GROWTH OF BACTERIA. 



131 



Species, Society of American Bacteriologists, recommends that 

 ammonia and indol tests be made on cultures at the end of the 

 tenth day, nitrite tests at end of the fifth day (see Appendix I., 

 Note 5). 



The reaction may be hastened by warming slightly. The 

 value of this reaction will be understood when, to give one 

 illustration, it is remembered that the bacillus coli communis 

 usually, if not ahvays, produces indol and the bacillus of 

 typhoid fever usually does not, except in cultures kept for 

 some time— teh days or more. Morris* found that in cultures 

 of B. typhosus, cultivated at 37 ° C. for this length of time, 

 indol was always present, but not in cultures of this organism 

 which were one or two days old. The recommendation 

 of the Society of American Bacteriologists that the indol test 

 should be made in cultures ten days old would seem not to 

 apply for differentiation of B. typhosus. 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. Many bacteria have the 



*Cited from Giinther. Einfiihrung in das Studium der Bakteriologie. 6te 

 Auf., 1906, p. 526. 



