Staining 



701 



true branches, a circumstance that has modified the present opinion 

 regarding its classification. It is probable that it has been errone- 

 ously placed among the bacilli, and really belongs among the higher 

 bacteria. 



The organism is not motile, does not possess flagella, and has 

 no spores. 



Staining.— The tubercle baciUus belongs to a group of organisms 

 which, because of their peculiar behavior toward stains, are known 

 as "saurefest" or acid-proof. Young organisms may stain quite 

 easily with ordinary solution of anilin dyes, but it is difficult to stain 

 after it has hved long enough to invest itself with a waxy capsule, 

 requiring that the dye used shall contain a mordant (Koch). It is 

 also tenacious of color once assumed, resisting the decolorizing 

 power of strong mineral acids (Ehrlich). 





Fig. 277. — Bacillus of tuberculosis, showing branched forms with involution 



(Migula). 



Koch* first stained the bacillus with a solution consisting of i cc. 

 of a concentrated solution of methylene blue mixed with 20 cc. 

 of distilled water, well shaken, and then, before using, receiving an 

 addition of 2 cc. of a 10 per cent, solution of caustic potash. Cover- 

 glasses were allowed to remain in this. for twenty-four hours and sub- 

 sequently counterstained with vesuvin. Ehrhch subsequently modi- 

 fied 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 by mineral acids which 

 * " Mittheilungen aus dem Kaiserlichen Gesundheitsamte," 1884, n. 



