74 BACTERIOLOGICAL DIAGNOSIS. 



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 recognise 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 penetra- 

 tive 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 sub- 

 stances 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 every- 

 thing except the tubercle bacilli. The latter appear as 

 slender red rods which often show the irregular staining 

 which has been described as occurring in the diphtheria 

 bacillus. 



Now " acid-fast " bacilli are very rare, though they 

 have been found in unexpected situations of late years. 

 Only three such bacilli need 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 



