THE STAINING OF MICROORGANISMS 87 



EXERCISE 28. THE STAINING OF MICROORGANISMS 



Microorganisms are devoid of color as a rule and are 

 stained for the purpose of observing their morphology to 

 better advantage than in a hanging drop. Staining also 

 often serves to bring out certain morphological character- 

 istics which are otherwise not evident, such as the presence 

 of metachromatic granules or a peculiar arrangement of 

 the protoplasm, resulting in what are known as " beaded 

 forms." 



The stains best suited to bacteria are the basic anilin 

 dyes which are derived from the coal-tar product anilin 

 (CeHsNH^). Many of them have the constitution of salts. 



Such compounds are divided into two groups, according 

 as the staining action depends on the basic or the acid 

 portion of the molecule. Fuchsin, gentian violet and methy- 

 len blue are basic dyes, while eosin, picric acid and acid 

 fuchsin are acid dyes. 



These groups have affinities for different parts of the liv- 

 ing cells. The basic stains have a special affinity for the 

 nuclei of tissues and for bacteria, the acid for the proto- 

 plasm and not for bacteria. The violet and the red anilin 

 dyes In order, are the most intense in action, easily over- 

 staining the specimen. It is difficult to overstain with 

 methylen blue. For this reason this stain is to be pre- 

 ferred where the bacteria occur in thick or viscid substances, 

 like pus, mucus or milk. In the presence of alkali, how- 

 ever, the stain acts more energetically. 



Stock solutions of the ordinary dyes are commonly 

 used. These are prepared by making a saturated solution 

 of the dye in absolute alcohol; this is diluted with water 

 as needed. 



Saturated alcoholic solutions of dyes will stain bacteria 

 with difficulty. The best results are obtained with the 

 diluted stain, spoken of here as an " aqueous-alcoholic " 

 stain. 



