350 Tuberculosis 



it is also tenacious of the color once assumed, resisting the 

 decolorizing power of strong mineral acids (Ehrlich). 



The peculiarity of staining the bacillus delayed its dis- 

 covery for a considerable time, but, now that we are familiar 

 with it, gives us a most valuable differential character, few 

 other organisms reacting in the same way. 



Koch* first stained the bacillus with a solution consisting 

 of i c.c. of a concentrated solution of methylene-blue mixed 

 with 200 c.c. of distilled water, well shaken, and then, 

 before using, receiving an addition of 2 c.c. of a 10 per cent. 



Fig. 105. Bacillus of tuberculosis showing branched forms with invo- 

 lution (Migula). 



solution of caustic potash. Cover-glasses were allowed to 

 remain in this for twenty-four hours and subsequently 

 counterstained with vesuvin. Ehrlich subsequently modified 

 Koch's method, showing that pure anilin was a better 

 mordant than potassium hydrate, and that the use of a 

 strong mineral acid would remove the color from everything 

 but the tubercle bacillus. This modification of Koch's 

 method given us by Ehrlich probably remains the best 

 method of staining the bacillus. 



Nearly all of the recent methods of staining are based 

 upon the impenetrability of the bacillary substance to 

 mineral acids which characterized the acid-fast or acid-proof 



* " Mittheilungen aus dem Gesundheitsamte," n, 1884- 





