46 EXAMINATION AND STAINING OF BACTERIA. 



of absolute alcohol and 10 parts of an alcoholic solution of 

 fuchsin, methylene-blue, or gentian-violet, are added, and the 

 whole thoroughly mixed and filtered. The preparation is 

 better when made fresh in small quantity at each time it is 

 needed, as it decomposes in a few days. 



In Koch-Ehrlich's method this anilin water (violet, blue, 

 or red) is used instead of the simple watery solution of the 

 dye. It possesses much more penetrating power, and this 

 again may be increased by gently heating the slide or cover- 

 glass, over a Bunsen burner, while it is being stained. 



It is applicable whenever it is desirable so to fix the color 

 in the bacteria that one may by means of decolorizing agents 

 remove the color from the surrounding objects and tissue and 

 fix it solely on the bacteria themselves. Some bacteria so 

 retain the color by this method that it serves to distinguish 

 them from others which they very much resemble, but which 

 do not possess the same persistent retention of the stain. 



The usual decolorizing agent employed is either ordinary 

 alcohol or diluted sulphuric or hydrochloric acid (1 : 4). 



3. Gram's Method. 



Treat the object to be colored with anilin-water gentian- 

 violet for about three minutes, after which immerse in Gram's 

 fluid. This consists of: 



Iodine, 1 part; 



Iodide of potassium, 2 parts ; 



Water, 300 " . 



Maintain this immersion for five minutes, then pass the 

 preparation through alcohol and rinse in water. If the 

 object is still of a violet color, treat it again with Gram's 

 fluid and alcohol until no violet is visible to the naked eye. 

 This method is applicable to a number of bacteria, and serves 

 as a mode of differentiation between some of them which 

 could not otherwise be distinguished under the microscope. 

 It serves also to color the capsule of bacteria, and, slightly 

 modified, is useful for the stain of bacteria in tissue. A 

 contrast-color should be given to the uncolored parts. 



