102 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



In studying the reduction of sulphates to sulphides, there- 

 fore, a medium should be chosen which does not contain 

 any organic sulphur. Hydrogen sulphide may be recog- 

 nized by heating the medium and exposing lead acetate 

 paper to the vapor. The presence of hydrogen sulphide 

 will be shown by the darkening or blackening of this paper. 



Reduction of Pigments — Certain pigments, such as litmus 

 and methylene blue, may be partially or completely decolor- 

 ized by the growth of organisms under anaerobic conditions. 

 This fact is made use of in litmus milk. Many bacteria will 

 completely reduce the litmus to its leuco base. The ability 

 of microorganisms in the. presence of organic matter to 

 reduce methylene blue has been made the basis for a test 

 to determine the amount of putrescible organic matter 

 present in solution. Raw sewage, for example, if shaken 

 up with methylene blue and corked tightly in a bottle, .vill 

 soon become decolorized. The amount of pollution in the 

 sewage or amount of organic material present in the sewage 

 is a function of the length of time which it will take the 

 methylene to become decolorized under standard condi- 

 tions. The time which passes before the blue color com- 

 pletely disappears is usually regarded as inversely propor- 

 tional to the amount of organic matter present which may 

 be decomposed readily. 



Determination of Indol. — The compound indol is pro- 

 duced by certain species of bacteria from the amino acid 

 tryptophane. This tryptophane in turn is a product of the 

 hydrolysis of certain proteins. Some organisms are capa- 

 ble of producing indol from proteins, that is, they are 

 capable of hydrolyzing the proteins, breaking them down 

 into their component amino acids, including tryptophane, 

 and then decomposing tryptophane with the development 

 of indol. Most of the indol-producing bacteria, however, 

 are incapable of attacking the proteins and must have 

 tryptophane present before they can produce .indol. The 



