252 CLINICAL BACTERIOLOGY. 



thick, and, on the average, from 3 to 4 jj. long. They are slightly 

 curved, nonmotile, and as a rule lie singly, but in cultures at 

 times in small chains of from 4 to 6. In rare cases they exhibit 

 bulbous terminal enlargement and bifurcation, in consequence 

 of which a certain relationship with the actinomyces-group 

 (p. 25) is suggested. Tubercle-bacilli are distinguished from all 

 other bacteria by the fact that they stain with extreme difficulty, 

 but, having once taken up the stain, they retain it with 

 great tenacity. The simple solutions of aniline dyes ordinarily 

 employed for staining other bacteria do not suffice for tubercle- 

 bacilli unless the exposure be very prolonged. For this reason 

 tubercle-bacilli are stained with aniline-water staining solutions 

 (Koch-Ehrlich stain) or with the more commonly employed 

 carbolfuchsin (Ziehl stain). 



Upon cover-slip preparations, made in the usual way from 

 pure cultures and fixed, freshly prepared aniline-water fuchsin 



Fig- 55. Tubercle-bacilli : i, Forms suggesting sporulation ; 2, forms described 

 as beaded (the open spaces in the fragmented rods are sometimes mistaken for 

 spores). 



(or aniline-water gentian-violet or aniline-water methyl-violet), 

 or carbolfuchsin is dropped ; then heat is applied by means of a 

 small gas-flame or spirit-flame until vapor of steam arises, and 

 after a minute the excess of stain is washed off with water. The 

 tubercle-bacilli are now stained, and if the specimen is treated 

 for several seconds with dilute acid for instance, fifteen or 

 twenty per cent, nitric acid and alcohol from 60 to 70 per 

 cent. they will not yield up their color. Stained in this way 

 the tubercle-bacilli frequently exhibit bright deficiencies that 

 have not taken the stain. These were at first looked upon as 

 spores, and, later, as degenerative manifestations, but both views 

 are incorrect. 



Originally, the tubercle-bacilli were stained in cold solutions, 

 in which they were permitted to remain for several hours ; but 

 by application of heat the duration of the exposure can be re- 

 duced to a few minutes. 



The cause of this characteristic behavior in staining was con- 



