112 AGRICULTURAL AND INDUSTRIAL BACTPRIOLOGY 



Gram's Stain. — Gram's staining method is one very fre- 

 quently used for the differentiation of bacteria. It was 

 originally used for the demonstration of bacteria in tis- 

 sues. Essentially it consists in staining bacteria with one 

 of the aniline dyes, usually gentian violet or thionin, add- 

 ing a solution of iodine as a mordant, then decolorizing 

 with alcohol. Bacteria may be divided into two groups, 

 those which are Gram-positive, that is, retain the color, and 

 those which are Gram-negative. 



Staining of Flagella. — It was noted above that the flagella 

 of bacteria are usually so slender and so difficult to stain 

 that they are not seen in the ordinary stained preparation. 

 Many methods of mordanting the flagella so that they will 

 stain more readily have been suggested and used in prac- 

 tice. One of the most common methods is to use iron tan- 

 nate as a mordant, followed by carbol fuchsin as a stain. 

 It is important in making such preparation that only young 

 actively motile cells be used. It is best not to employ 

 cultures more than twelve to eighteen hours old. 



