Staining 713 



regarding the classification of the organism, which is prob- 

 ably erroneously placed among the bacilli, belonging more 

 properly to the higher bacteria. 



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



Staining. The tubercle bacillus belongs to a group of 

 organisms which because of their peculiar behavior toward 

 stains is known as " saurefest " or acid-proof. It is difficult 

 to stain after it has lived 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). 



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 20 c.c. of distilled water, well shaken, and then, before 

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

 tion 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 characterizes the acid-fast or acid-proof 

 (saurefest) micro-organisms. But it is not improbable that 

 we have been led into error by the assumption, upon inade- 

 quate grounds, that this is a constant and uniform quality 

 of the tubercle bacillus and similar micro-organisms. The 

 interesting observations of Muchf have shown that many of 

 the paradoxes of tuberculosis can be accounted for by the 

 fact that during certain stages, or in certain conditions, the 

 bacilli are not acid-proof at all. Thus, examinations of 

 caseous masses from the lungs of cattle show complete 



"Mittheilungen aus dem Gesundheitsamte," n, 1884. 

 t "Berliner klin. Wochenschrift," April 6, 1908, p. 691. 



