100 METHODS USED IN THE STUDY OF BACTERIA 



necessary to add mordants to the stain. The principle in- 

 volved here is the same as that used in dyeing cloth. For 

 staining purposes the mordants usually used are the iron salts, 

 weak alkalies, carbolic acid, anilin oil, and tannin. These 

 mordants enable one to stain even the most resistant bacteria. 

 When they are once stained it is possible to remove the stain 

 from other objects in the specimen, less acid-fast than them- 

 selves, by means of some decolorizing agent, such as the mineral 

 acids, organic acids, alcohols, etc. In this way it is possible 

 to secure double stains, in which one kind of bacteria is stained 

 one color and other bacteria another color. Such a double 

 stain is used for the differential diagnosis of certain bacteria. 

 The bacillus of tuberculosis, for instance, is stained in sputum 

 by using a double stain, by means of which the Bact. tubercu- 

 losis is colored a deep crimson, while the rest of the preparation 

 is blue or green in color. The Bact. tuberculosis has this acid- 

 fast property in common with a number of other bacteria, but 

 it is usually possible, by means of slight variations in the 

 technique, or by some other means, to differentiate this germ 

 from the other acid-fast bacilli. Depending upon principles 

 similar to those noted above, a double stain for endospores 

 has been devised and is frequently used. 



There are certain other stains which may or may not indicate 

 that the process of staining is a chemical one, but they do 

 indicate at any rate that there is some differentiation in 

 structure in the various parts of the cell. Some bacteria when 

 stained with a weak solution of methylene blue give the bi- 



