MICROSCOPICAL EXAMINATION OF BACTERIA 97 



general purposes. Special stains have been devised for special 

 purposes. Loeffler's alkaline methylene blue is used for stain- 

 ing the Bact. diphtherias, Ziehl's carbol fuchsin for the Bact. 

 tuberculosis, and many other special stains are used for special 

 purposes. Processes for double staining have been devised, 

 e.g. those for differentiating a spore from the mother cell, 

 the metachromatic bodies from the rest of the protoplasm, the 

 capsule from the cell, etc. Certain selective stains are also 

 used. In particular, Gram's stain, which consists, essentially, 

 of treating the bacteria with a particular dye, i.e. anilin oil 

 gentian violet ; with Gram's iodine solution (iodine and potas- 

 sium iodide); then washing in alcohol. With some bacteria 

 the dye and the iodine solution form a compound with the 

 protoplasm of the cell which is insoluble in alcohol. Such 

 bacteria, when put through this process, retain the violet 

 color. Other bacteria, when treated with the stain and the 

 iodine solution, fail to effect a compound with the protoplasm 

 insoluble in alcohol (iodine-pararosanilin-protein). Such 

 bacteria, when treated by this process, are colorless ; or, if a 

 counter stain has been used, are of an entirely different color. 

 By means of Gram's stain it is possible to differentiate bac- 

 teria that are morphologically similar, and this stain is one of 

 the bacteriologists' most valuable assets. 



Most pathogenic micrococci retain the violet color when 

 treated by this method and are, therefore, said to be positive 

 to Gram. Many of the disease-producing bacilli do not take 

 the Gram stain, are negative to it, while others are positive. 



