METHODS OF STUDYING PHYSIOLOGIC CHARACTERS 103 



various peptones on the market differ considerably in their 

 content in amino acid and tryptophane. For demonstra- 

 tion of this property a peptone relatively high in trypto- 

 phane should be chosen. 



The development of indol from tryptophane is markedly 

 influenced by the presence of other substances. Most bac- 

 teria, for example, which can produce indol from trypto- 

 phane will not bring about this change in the presence of 

 carbohydrates. 



Indol has a characteristic, disagreeable, fecal odor. 

 Many dififerent chemical tests have been suggested for its 

 detection. The simplest of these is the nitroso-indol test, 

 which is carried out by adding a cubic centimeter of a 

 0.1 per cent solution of sodium nitrite together with a few 

 drops of strong acid such as sulphuric to the medium. The 

 nitrite is decomposed by the sulphuric acid, freeing nitrous 

 acid. This in turn combines with indol to form nitroso- 

 indol which has a bright red color. For other laboratory 

 tests for indol, and for methods of detection in very small 

 quantities the student should consult a laboratory manual. 



Some bacteria are capable, of producing the closely 

 related compounds skatol and phenol from tryptophane. 



Digestion of Starch and Other Insoluble Carbohydrates. 

 — Many bacteria when grown upon potato or in agar or 

 other media containing starch hydrolize the starch or fer- 

 ment it more or less completely. The ability to bring about 

 this change is termed the diastatic power of the microor- 

 ganism. It may be recognized by using an agar containing 

 enough starch to make it somewhat opaque. The organism 

 when grown upon this medium will produce a clear area 

 immediately about the colony. This may be emphasized by 

 the addition of a solution of iodine, when the undigested or 

 unfermented starch will turn blue, leaving a colorless zone 

 about the colony. Not all bacteria hydrolize the starch 

 with the production of sugars. 



