STAINING BACTERIA 97 



the staining fluid is poured on, and after remaining a few minutes 

 is poured off and the slide is washed, dried by blotting paper, 

 and examined. If a cover-slip has been used a drop of balsam is 

 put upon a clean slide and the cover, smeared with stained 

 bacteria, is inverted on the balsam. Upon the stained bacteria 

 themselves (if a cover-glass has not been used) or upon the cover- 

 slip a drop of cedar oil may be placed, and the preparation 

 examined with a one-twelfth objective. This is one of the 

 simplest staining procedures practised in bacteriology. Other 

 more complicated methods will now be described. 



Besides heat, absolute alcohol, methyl alcohol, or formalin may 

 be used as fixatives. Some stains are made up with methyl 

 alcohol, and instead of fixing by heat, the stain is merely dropped 

 upon the dried film, and the bacteria are fixed and stained by the 

 same solution at the same time, water being added for differentia- 

 tion at the end. 



Aniline dyes are almost entirely used as stains in bacteriology 

 and these are divided into two classes, the basic and acid stains, 

 according as their staining properties depend upon the basic or 

 acid part of the molecule. Basic dyes stain nuclear tissues of cells 

 and bacteria. The acid are used as contrast stains and do not 

 color bacteria, but tissues in which they may be imbedded. 



The common basic stains are methyl violet, and gentian violet, 

 methyl green, methyl blue, and methylene blue, thionin blue, Bis- 

 marck brown, fuchsin, and saffranin. These are used for staining 

 different bacteria under different conditions. The most useful 

 stain is methylene blue, since it is difficult to overstain with it, and 

 it is very easily applied. It has been found that certain physical 

 and chemical conditions are necessary for successful staining with 

 aniline dyes. Alcoholic solution of dyes entirely devoid of water 

 do not stain; absolute alcohol does not decolorize bacteria after 

 staining with aniline colors, while diluted alcohol decolorizes 

 readily. The more completely a dye is dissolved, the weaker is its 

 staining power. A dyestuff unites, as a whole, with the bacterial 



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