EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 33 



Staining the Bacillus of Tuberculosis.— Since the tu- 

 bercle bacillus does not take the ordinary stains readily, a very 

 large number of methods have been proposed for staining it, 

 all of which depend upon the principle that, after adding to 

 solutions of aniline dyes certain substances, like aniline-water, 

 carbolic acid, or solutions of ammonia or soda, the bacillus 

 tuberculosis is stained with g^eat intensity, and gives up its 

 stain with difficulty. Solutions of acids will remove the stain 

 from all parts of the preparation excepting from the tubercle 

 bacilli, which retain the dye, once having acquired it. The 

 rest of the preparation may now be given a different color — 

 contrast-stain. 



Bacilli that resist decolorization by acids are called acid- 

 proof or acid-fast. The most important are tubercle and 

 leprosy bacilli. There are various other species, however, most 

 of which are less resistant to acids and alcohol than tubercle 

 bacilli. They are discussed in the article on the bacillus 

 tuberculosis in Part IV. 



•Occasionally spores of other bacteria, micrococci and horny 

 epithelial cells are imperfectly decolorized, but their forms dis- 

 tinguish them from tubercle bacilli. Minute crystalline 

 needles which have a shape like that of bacilli are often en- 

 countered in sputum, but their nature will be recognized after 

 a little practice. 



The stain for tubercle bacilli is most frequently used for 

 specimens of sputum from cases o f suspected pulmonary tu- 

 berculosis; it may be applied to other fluids and secretions 

 equally well. It is not reliable, however, when applied to 

 milk, as the oil present in milk interferes with its operation, 

 and milk and its products quite often contain other acid-proof 

 bacilli. The smegma of the external genitals also frequently 

 contains acid-proof bacilli that are not tubercle bacilli. On 

 this account all fluids and discharges from the genito-urinary 

 tract need to be examined with particular care not to confuse 

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