64 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



Recognition of the Tubercle Bacillus. 



The tubercle bacillus is about half as long as a red blood- 

 corpuscle is wide, or rather longer, and is very slender. It is 

 straight or slightly curved, and is variable both in shape and in 

 size (Plate II., Fig. 2). 



We recognize it by means of a staining reaction. Tubercle 

 bacilli contain a considerable amount of fat, and this prevents 

 them from staining readily with ordinary stains. In the process 

 described above we used fuchsin, which is a very powerful stain, 

 and added a mordant (carbolic acid), which increases its penetrative 

 properties. Even with this staining is very slow, so that we 

 heated the specimen. 



The fat which prevents the bacilli from staining also prevents 

 the stain from being removed by such substances as acids and 

 alcohol. In stage 3 of the above process we aim at allowing the 

 acid to act until it has removed the fuchsin from everything 

 except the tubercle bacilli. The methylene blue is a counter- 

 stain, and colours all organisms, pus cells (especially their nuclei), 

 epithelial cells, and shreds of lung-tissue — in fact, everything 

 except the tubercle bacilli. The latter appear as lender red rods, 

 which often show the irregular staining which has been described 

 as occurring in the diphtheria bacillus. 



Now, " acid-fast " bacilli are rare, though they have been 

 found in unexpected situations of late years. Only three such 

 bacilli need to be taken into consideration in dealing with human 

 pathology. These are the tubercle bacillus, the leprosy bacillus, 

 and the smegma bacillus. The bacillus of leprosy would rarely 

 lead to mistakes in this country ; it is recognized by the fact that 

 it is straighter and more uniform than the tubercle bacillus, and 

 by the fact that it resists decolorization more powerfully than 

 the tubercle bacillus. The smegma bacillus may occur in the 

 urine, and lead to mistakes unless the sample examined was drawn 

 oS per catheter. It is distinguished by, the fact that it is readily 

 decolorized by alcohol (absolute alcohol or methylated spirit), 

 while the tubercle bacillus is not. In staining a film from the 

 urine, we decolorize in spirit for a quarter of an hour after the 

 acid and before the methylene blue — i.e., between stages 3 and 4 

 in the above description. 



In searching for the tubercle bacillus, the ^-inch lens will serve, 

 though an oil-immersion lens is an advantage. 



