EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 35 



(c) Wash in water. 



(d) Gabbett's solution is applied for one minute. 



(e) Wash in water. The preparation should have a blue 

 color. It may be examined in water directly or after drying and 

 mounting in Canada balsam. 



Gabbett's method has the advantage of decolorizing the 

 preparation and staining the background with methylene-blue 

 at the same time. Tubercle bacilli are colored a brilliant red; 

 most other bacteria and the nuclei of cells are colored blue. 

 The acid-proof bacilli mentioned on page 32 also retain the red 

 stain in most cases, and might be confused with tubercle baciUi. 



Of the numerous methods of staining tubercle bacilH, only 

 a few others can be mentioned. Aniline-water fuchsin, aniline- 

 water gentian- violet, or carbol-fuchsin may be used. The 

 intensity of the stain must then be increased by warming 

 the preparation till it steams or boils, then allowing the warm 

 stain to act on the specimens for from three to five minutes; 

 the preparation may also be left in the cold stain over night. 

 Decolorization may be effected with a 25 per cent, solution of 

 sulphuric acid used till the red color disappears, or a 30 per 

 cent, solution of nitric acid, which operates very rapidly. If 

 the red color persists after washing in water, dip in the acid 

 again. After either acid the preparation is to be washed in 

 alcohol until the last trace of the stain has been removed. An 

 excellent decolorizing agent is a 3 per cent, solution of hydro- 

 chloric acid in alcohol, used for about a minute. With any of 

 these acid solutions the decolorization can be accomplished 

 more perfectly than with Gabbett's solution, where the operation 

 of the decolorizing agent is masked. The contrast-stain may 

 be omitted entirely if it is desired. A suitable contrast-stain 

 after fuchsin staining is a solution of methylene-blue; after 

 gentian-violet staining, Bismarck brown. 



Those who have had experience in staining tubercle bacilli 

 soon discover that the bacilli exhibit some differences in their 



