BACILLUS TUBERCULOSIS. 127 



Staining. it is difficult to stain by the usual staining 

 methods, and requires the use of special staining technic. 

 Koch's method of staining it consists in adding liquor potassa? 

 to the alkaline anilin dyes. 



Ehrlich's modification of Koch's method, which consists in 

 preparing anilin water and adding this to the solution of an 

 anilin dye, is perhaps the best method of bringing out the 

 tubercle bacillus. 



The mode of procedure for the staining of bacilli in secre- 

 tions, especially in sputum, has been described in the chapter 

 on staining, as the Koch-Ehrlich method, orthe Ziehl carbol- 

 fuchsin method, or, better still, as Gabbett's modification of 

 Ziehl' s method. 



In tissue the bacillus is stained best by an application of 

 either method, which will also be found described in the chap- 

 ter on staining. 



When so stained, the bacillus shows a number of unstained 

 places in the cell-body, somewhat resembling spores. They 

 have given rise to the opinion that the bacilli are spore-form- 

 ing, but the fact that when the usual method for staining 

 spores is applied these spots remain unstained seems to prove 

 that they are not spores, but are due possibly to some degen- 

 eration in the protoplasm of the bacillus. 



Bacillus tuberculosis is the prototype of the acid-fast bac- 

 teria. Although hard to stain, when once stained, these bacilli 

 resist the decolorizing action of strong mineral acids, even a 

 25 per cent, nitric acid. In addition, B. tuberculosis resists 

 decoloration with strong alcohol, a fact which differentiates it 

 from Bacillus smegmatis. 



Nature and Occurrence. As mentioned, the tubercle bacillus 

 is a strict parasite, and is found only in tuberculous tissues 

 and in the secretions from tuberculous patients, especially the 

 sputum. It is also found in substances that have been con- 

 taminated with those secretions, and occasionally is wafted in 

 the air in this manner. 



Pathogenesis. The tubercle bacillus is pathogenic for man 

 and for nearly all the lower animals, especially the herbivora, 

 though the carnivora and birds are alike susceptible to it, and 



