STAINING THE TUBERCLE EACILLUS. , 163 



received the contrast-color a great many other or- 

 ganisms will appear; these take on the second color 

 employed, while the tubercle bacilli retain their orig- 

 inal color. Before decolorization all organisms in the 

 preparation were of the same color, but during the appli- 

 cation of the decolorizing solution all except the tubercle 

 bacilli gave up their color. This micro-chemical charac- 

 teristic, together with other reactions to be described, 

 serves to differentiate the tubercle bacillus from other 

 organisms with which it might be confounded. A 

 number of different methods have been suggested for 

 the staining of tubercle bacilli, but the original method 

 as employed by Koch is so satisfactory in its results 

 that it is not advisable to substitute others for it. The 

 above differs from the original Koch-Ehrlich method 

 for the staining of tubercle bacilli in sputum only in the 

 occasional employment of ZiehPs carbol-fuchsin solution 

 and in the method of heating the preparation with the 

 stainiug-fluid upon it. 



As Nuttall has pointed out, however, the strong acid 

 decolorizer used in this method can, with advantage, 

 be replaced by much more dilute solutions, as a number 

 of the bacilli are entirely decolorized by the too energetic 

 action of the strong acids. He recommends the follow- 

 ing method of decolorization : after staining the slip or 

 section in the usual way, pass it through three alcohols ; 

 it is then to be washed in a solution composed of 



Water 150 c.c. 



Alcohol SO.c.c. 



Concentrated sulphuric acid 20 to 30 drops. 



From this it is removed to water and carefully rinsed. 



