STAINING THE TUBERCLE BACILUUS. 167 



jority of other bacteria by the tenacity with which it 

 retains the color when treated in this way; it is an 

 organism difficult to stain, but when once stained is 

 equally difficult to rob of its color. 



Method of Staining the Tubercle Bacillus. 

 — Select from the sputum of a tuberculous subject one 

 of the small, white, cheesy masses which it is seen 

 to contain. Spread this upon a cover-slip, dry and 

 fix it in the usual way. The slip is now to be taken 

 by its edge with forceps and the film covered with a 

 few drops of either the solution of Koch-Ehrlich or 

 that of Ziehl. It is then held over a gas-flame, at first 

 some distance away, gradually being brought nearer 

 until the fluid begins to boil. After it has bubbled 

 once or twice it is removed from the flame, the excess 

 of stain washed away in a stream of water, then im- 

 mersed in a 30 per cent, solution of nitric acid in 

 water, and allowed to remain until all color has dis- 

 appeared. This takes longer in some cases than in 

 others. One can ahvays determine if decolorization is 

 complete by washing off the acid in a stream of water. 

 If the preparation is still distinctly colored, it should 

 be again immersed in the acid ; if of only a very faint 

 color, it may be dipped in alcohol, again washed in 

 water, and stained with some contrast-color. If, for 

 example, the tubercle bacilli have been stained with 

 fuchsin, methylene-blue forms a good contrast-stain. 

 In making the contrast-stain the steps in the process 

 are exactly those followed in the ordinary staining 

 of cover-slip preparations in general : the slip contain- 

 ing the stained tubercle bacilli is carefully rinsed in 

 water, and a few drops of the methylene-blue solution 

 placed upon it and allowed to remain for thirty or 



