STAINING TUBERCLE BACTERIA (BACILLI) 63 
EXERCISE XVII 
STAINING TUBERCLE BACTERIA (BACILLI) 
94. The stained tubercle bacteria possess, because of the 
layer of fatty acids covering them, the power of retaining 
the coloring matter even when treated with a strong decolor- 
izer, such as a solution of sulphuric or nitric acid. On this 
account staining has a high differential value which is made 
use of in identifying this organism. Thus in the examination 
of sputum in cases of suspected tuberculosis the object is to 
determine the presence of tubercle bacteria. As this organism 
is not easily cultivated, the staining process is largely depended 
upon in making a differential diagnosis. 
95. Work for this exercise. Make 4 cover-glass prepara- 
tions from a culture of tubercle (furnished). Stain 2 of them 
for tubercle bacteria and carefully describe the appearance of 
the bacteria and illustrate with a few drawings. 
Counterstain 2 of the preparations after Gabbett’s method. 
Stain a cover-glass preparation of tubercular sputum (fur- 
nished). 
For the next exercise liquefy 2 large tubes of agar and 2 of 
gelatin and pour them into Petri dishes. After the medium 
has solidified remove the covers of the Petri dishes and expose 
one of each to the air for 5 minutes and one of each for 
1o minutes. Return the covers and place the agar plates in 
the incubator and the gelatin ones in the locker. When these 
plates are examined in the next and subsequent exercises there 
will be a colony for each bacterium that fell upon the medium 
from the air. It will be necessary to look out for impure or 
mixed colonies, as two or more organisms may have fallen 
together. 
Read the directions in the text-books for staining tubercle 
bacteria (bacilli). 
