94 



PHYSIOLOGY OF BACTERIA 



Thornton and Hastings (1929b) compared methylene blue with 

 janus green B both of which have been recommended for the reductase 

 test in milk, and found that janus green B indicates a potential about 

 0.1 volts lower than methylene blue. (See points A and B in Fig. 9.) 

 They also found that methylene blue in milk was reduced at a poten- 

 tial higher than that given by Clark, Cohen and associates. 



The indicators are greatly influenced by the acidity of 

 the medium, as can be seen from Fig. 6 illustrating the 

 influence of pH on methylene blue. 



(d) BUFFERING OR POISING OF THE POTENTIAL 



It has been stated before that the reduction potential 

 measures the intensity, but not the quantity of reduction. 

 The term quantity is now usually substituted by the 



Fig. 8. 



-Differences in potential between 4 water-logged soils and a calomel 

 electrode. 



more significant term ^^ capacity." A bacterial culture 

 will not only show a certain intensity of reduction (which 

 might be measured by reduction indicators) but it has a 

 definite capacity for reduction. We can imagine a 

 medium containing a large amount of a substance which 

 acts as hydrogen acceptor at a Eh of about +0.05 volts 

 or less. The potential of a bacterial culture will remain 

 near this point until all of this compound is reduced, 

 and then will fall to a lower potential. This corresponds 



