CHAPTER IV 

 PREPARATION OF STAINED SMEARS 



IN preparing bacteria for microscopic examination a 

 tiny bit of the material (pus ; sputum, exudate, culture, 

 etc.) is thinly spread on a glass slide and allowed to dry. 

 Then the slide is passed several times through the flame 

 in order to "fix" the preparation. By this is meant the 

 drying, killing, and coagulating of the material, so that 

 it will remain fixed to the slide and not wash off in the 

 staining fluid. For the ordinary examination the 

 preparation is next flooded with the staining fluid e. g., 

 watery solution of methylene-blue and allowed to 

 stain for several minutes. The stain is poured off, the 

 slide washed in water, dried with blotting-paper and in 

 air, and is then ready to be examined. The simple 

 stains in common use are watery solutions of methylene- 

 blue, of gentian- violet, or of fuchsin. Some bacteria, 

 however, do not take these simple stains readily, and 

 it is necessary to add something to the staining fluid to 

 cause the stain to "bite in." The substance thus added 

 is called a mordant. Carbolic acid is an excellent 

 mordant, a solution of carbolic acid and fuchsin being 

 extensively used to stain tubercle bacilli. 



It was said above that stains were also used in identi- 

 fying bacteria. Most bacteria, for example, when stained 

 and then treated with acids quickly lose their color. 

 Some, however, withstand the action of acids, retain- 



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