Cultivation and Identification of Bacteria 133 



pliane and releasing the part of this molecule known as the indole 

 fraction. Proteins are constructed of building blocks known as 

 amino acids, and tryptophane is one of these substances. 



Paul Ehrlich found that by adding an indicator substance, 

 which has the interesting designation of paradimethylamido- 

 benzaldehyde (Ehrlich's aldehyde for short!), the presence of free 

 indole could be detected in the medium. As long as the indole is 

 still attached to the parent tryptophane molecule, the aldehyde 

 indicator will not turn pink. If free indole is present, the Ehrlich's 

 aldehyde will turn a pink color. This is a positive indole test. 



In addition to aiding in the identificaton of a number of bac- 

 teria, the indole test has particular significance in water analysis 

 where sewage organisms (Escherichia coli) can be distinguished 

 from the closely related soil organisms (Aerobacter aerogenes) by 

 the indole reaction when it is used in conjunction with other tests. 



Methyl Red Test 



When the pH drops to 4.7 or below, methyl red indicator turns 

 red from the normal yellow color it exhibits above pH 4.7. By 

 cultivating bacteria in a highly buffered glucose broth (Clark and 

 Lubs broth), if the bacteria have generated enough acid to depress 

 the pH of this buffered broth to the critical level, the broth will 

 turn red when methyl red indicator is added to the tube. This 

 test also has significance in water analysis. 



Voges-Proskauer Test 



This is the third of four tests employed to separate Escherichia 

 coli from Aerobacter aerogenes. Without going into the intricate 

 chemistry involved in this reaction, suffice it to say that if bacteria 

 are cultivated in Clark and Lubs broth, Aerobacter aerogenes is 

 capable of forming an intermediate breakdown product of glucose. 

 When a strong alkali (sodium or potassium hydroxide) is added, 

 the intermediate breakdown product is converted into a pink- 

 colored compound. A positive Voges-Proskauer test means that 

 this series of reactions has occurred, whereas no change in color is 

 a negative V.-P. test. It happens that a typical Aerobacter aero- 



