42 MANUAL OF BACTERIOLOGY. 



Iodine I gram. 



Potassium iodide 2 grams. 



Water 300 c.c. 



In this solution the preparation becomes nearly black. 



(c) Wash in alcohol repeatedly; the alcohol becomes 

 stained with clouds of violet coloring matter; the alcohol 

 is used as long as the violet color continues to come away, 

 and until the preparation is decolorized or has only a faint 

 steel-blue color. 



(d) When desired, the specimens may be stained, by way 

 of contrast, with a watery solution of Bismarck brown or 

 eosin. 



(e) Wash in water, and examine either in water directly 

 or after drying and mounting in Canada balsam. [A modi- 

 fication of this method, sometimes called the Gram-Gunther 

 method, differs from the preceding by using a 3 per cent, 

 solution of hydrochloric acid in alcohol for ten seconds to 

 hasten decolorization, washing in pure alcohol before and 

 after the acid alcohol. Decolorization is more intense than 

 by the Gram method; the diphtheria bacillus, which is 

 stained by Gram's method, is decolorized by the Gram- 

 Gunther (Kruse).] The advantages of Gram's method are 

 that with it certain bacteria are stained a violet color witli 

 more or less intensity and other bacteria are not stained at 

 all. To some extent, then, it furnishes a means of diagnosis. 



List of some of the important bacteria that are stained 

 by Gram's method : 



Staphylococcus pyogenes aureus. 



Streptococcus pyogenes, 



Micrococcus lanceolatus (of pneumonia), 



Micrococcus tetragenus, 



Bacillus of diphtheria. 



Bacillus of tuberculosis, 



Bacillus of leprosy, 



