STAINING BACTERIA 85 



fixing and thorough cooling, 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 differentiation 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, accord- 

 ing 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 too verstain with it, and it is 

 very easily applied. It has been found that certain physical and 

 chemical conditions are necessary for successful staining with ani- 

 line dyes. Alcoholic solution of dyes entirely devoid of water do 

 not stain, absolute alcohol does not decolorize bacteria after stain- 

 ing with aniline colors, while diluted alcohol decolorizes readily. 

 The more completely a dye is dissolved, the weaker is its staining 

 power. A dye stuff unites, as a whole, with the bacterial plasma, 



