EXAMINATION AND STAINING OF BACTERIA 45 



the preparation of a saturated solution. A complete saturation 

 may be obtained by adding the powdered dye to the solvent until 

 no more will enter into solution, a slight residue remaining after 

 repeated shaking on several days being taken as an indication. 

 The following quantities of the most frequently used dyes are 

 approximately sufficient for saturation : 



Gentian violet : 1 to 5 per cent in distilled water or 4 to 8 per cent 



in 96 per cent alcohol 

 Fuchsin : 1 to 5 per cent in distilled water or 3 per cent 



in 96 per cent alcohol 

 Methylene blue : 6 to 7 per cent in distilled water or 7 per cent 



in 96 per cent alcohol 



Stains should always be filtered through paper before use, other- 

 wise sediment may be deposited on the slide which would spoil 

 the preparation. 



Mordants and Decolorizing Agents. Certain organisms are 

 stained with difficulty unless a mordant is employed, which not 

 only increases the intensity of the dye but tends to make the bac- 

 terial cell more permeable. Again in films of blood or pus and 

 more especially in sections of tissue, the tissue cells may be so 

 deeply stained as to obscure the bacteria lying within them. To 

 obviate this methods have been devised whereby a mordant may 

 be used to fix the dye in the bacteria while subsequent treatment 

 with a decolorizing agent will remove the dye to a greater or less 

 extent from the tissue cells. 



Staining properties may be increased by : 



(a) The addition of weak solutions of alkalies, such as caustic 



potash or ammonium carbonate. 

 (6) The addition of carbolic acid, aniline oil, metallic salts, 



etc. 



(c) Heat. 



(d) Prolonged staining. 



The decolorizing agents generally employed are weak solutions of 

 acids, alcohol, or a combination of both. 



