264 BACTERIOLOGY. 



It possesses some peculiarity in its composition that 

 renders it more or less proof against the simpler 

 dyes. It is therefore necessary that more energetic 

 and penetrating reagents than the ordinary watery 

 solutions should be employed. Experience has taught 

 us that certain substances not only increase the solu- 

 bility of the aniline coloring substances, but by their 

 presence the penetration of the coloring agents is very 

 much increased. Two of these substances are aniline 

 oil and carbolic acid. They are employed in the 

 solutions to about the point of saturation. (For the 

 exact proportions see chapter on Staining Reagents.) 



Under the influence of heat these solutions are seen 

 to stain all bacteria very intensely — the tubercle bacilli 

 as well as the ordinary forms. If we subject our prep- 

 aration, which may contain a mixture of tubercle 

 bacilli and other forms, to the action of decolorizing 

 agents, another peculiarity of the tubercle bacillus will 

 be observed. While all other organisms in the prep- 

 aration will give up their color and become invisible, 

 the tubercle bacillus retains it with marked tenacity. 

 It stains with great difficulty, but once stained it 

 retains the color even under the influence of strong 

 decolorizing agents. 



(Organisms with which the bacillus tubercu- 

 losis MAY BE CONFUSED. 



Differential Diagnosis. — While this peculiar 

 micro-chemical reaction is usually considered to be 

 diagnostic of the bacillus tuberculosis, it is well to 

 remember that there are at least three other species of 

 bacilli which, when similarly treated, react in the same 

 way. It is of importance to bear this point in mind. 



