Microbial Structures and Staining Reactions 113 



difficult. By applying heated dye, such as carbol fuchsin (com- 

 posed of basic fuchsin dye, alcohol, and phenol), the permeability 

 of the capsule is increased, possibly due to the softening effect 

 of the heat on the capsule. A common procedure employed in 

 staining acid-fast material is that method proposed by Ziehl-Neel- 

 sen in which hot carbol fuchsin is applied to the dried smear for 

 about 5 minutes. Decolorization is carried out using a mixture 

 of ethyl alcohol and 2 or 3% hydrochloric acid. This is called acid 

 alcohol. The length of exposure to this decolorizer depends upon 

 the thickness of the preparation, but in general a few seconds are 

 sufficient to remove the color from everything except the acid-fast 

 organisms. The decolorization is stopped by plunging the slide 

 into water, and the preparation is then counterstained in methylene 

 blue or some similar dye. The blue background facilitates locating 

 the pink acid-fast organisms. 



Paul Ehrlich noted this acid-fast phenomenon in 1882 when 

 he was studying the tuberculosis organisms. Recent evidence 

 points to mycolic acid, a constituent of the waxy material, as the 

 cause of acid-fastness. When mycolic acid is in combination with 

 complex sugars (polysaccharides), it is even more acid-fast than 

 when it is tested alone. However, just to make the problem more 

 interesting, some workers have shown that not all acid-fast bacteria 

 possess mycolic acid. Undoubtedly, more than one explanation 

 of acid-fastness probably exists, and the full story is yet to be 

 revealed. 



In recent years acid-fast bacteria have been stained with a 

 dyestuff called aurmnine, which possesses properties of fluorescence. 

 When such stained organisms are examined with ultaviolet light, 

 using a yellow filter to block out the blue light, the field is dark 

 but the auramine-stained organisms stand out as luminous yellow 

 bodies. This technic has value in the diagnosis of tuberculosis. 



Persons suffering from leprosy harbor acid-fast bacteria within 

 their diseased tissues, and these organisms resemble the tubercu- 

 losis microbes. Mi/cobacteriwn leprae is the name given to the 

 leprosy acid-fast cells. To date these organisms have not been 

 isolated and grown on artificial media, but the fact that they are 



