68 CLINICAL BACTERIOLOGY AND H/EMATOLOGY 



red; if you leave the preparation in the acid for a short time, 

 you are more likely to get crystals of carbol fuchsin, stain 

 retained in deep scratches of the* glass, etc., all of which a 

 beginner may easily mistake for bacilli, with disastrous results. 

 The counterstaining with methylene blue may be shortened 

 or omitted altogether, though this is not advisable, as it is 

 then more difficult to focus the film. 



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 pre- 

 vents them from staining readily with ordinary stains. In 

 the process described above we used fuchsin, w r hich 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 pre- 

 vents 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 counterstain, 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 slender 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 is usually much more abun- 

 dant. The smegma bacillus may occur in the urine, and lead 



