44 BACTERIOLOGY FOR NURSES 



flagella are also stained with difficulty. Those organisms that do 

 not stain readily as a rule retain the dye and are not easily de- 

 colorized. Two explanations are advanced for this resistance to 

 take on and to part with stains: one hypothesis is that such 

 organisms, or parts, are of different chemical composition; this 

 assumption is. probably true in the case of spores and flagella. 

 The other theory supposes the presence of a waxy and therefore 

 difficultly permeable envelope surrounding the bacteria may be 

 the cause. The latter view is probably correct in the case of the 

 tubercle bacilli. The presence of a waxy or fatty material has 

 been demonstrated in certain bacteria ; moreover, when this waxy 

 substance has been extracted with ether the dye-resistant qualities 

 of the bacteria have disappeared also. It may be in certain cases 

 that both factors combine to produce the result. 



The best bacterial stains are derived from the coal-tar product 

 aniline. Many of the dyes have the constitution of salts and are 

 divided into two groups according as the staining depends on the 

 basic or acid part of the molecule. The basic stains have a special 

 affinity for nuclear material and the acid for cytoplasm. For 

 this reason the basic dyes are especially bacterial stains. 



The most frequently used stains are : 



Violet Methyl violet, gentian violet, crystal violet 

 Blue Methylene blue, thionin blue 

 Red Basic fuchsin 

 Brown Bismarck brown 



The violet dyes have the most intense action, consequently 

 care should be taken when using them not to overstain the speci- 

 men. Methylene blue is perhaps the most generally employed ; 

 it gives a good differentiation of structure and it is not easy to 

 overstain with it. Bismarck brown and eosin are weak dyes and 

 are used generally as counterstains. 



Saturated Solutions. It is a convenient arrangement to keep 

 in stock saturated solutions of the dyes most frequently employed 

 and from them to make dilutions for use as required. Since great 

 variations occur between different samples of dyes bearing the 

 same name no definite amount can be quoted as the minimum for 



