THE BACTERIUM TUBERCULOSIS 425 



haps only two, species are really represented, and that the 

 remainder may fairly be regarded as varieties. As said, 

 the characteristic common to all the members of this group 

 is that they are to greater or less extent acid-proof — i. e., 

 when once stained by the Koch-Ehrlich or Ziehl process 

 the color is not in all cases removed by the ordinary acid 

 decolorizers. In this particular, however, there is such a 

 striking difference between the degree of their resistance to 

 acid decolorizers and that of the tubercle bacillus as to 

 render this an important differential aid; for instance, the 

 tubercle bacillus, when stained, may be treated for several 

 minutes with so strong a decolorizer as 30 per cent, nitric 

 acid without losing its color; whereas, none of the members 

 of this group retain their color after a few seconds of such 

 treatment, and particularly if it be followed by washing 

 in alcohol. In morphology some of them might readily be 

 mistaken for bacillus tuberculosis, though even these are 

 usually a trifle larger and less delicately formed than that 

 organism; others are at once differentiated from normal 

 tubercle bacilli, but have somewhat their appearance when 

 degenerated or involuted; still others have nothing in their 

 general appearance to lead to confusion. 



When mixed with other bacteria, as is the case in the soil,' 

 in manure, in intestinal contents, etc., their isolation in 

 pure culture is a matter of difficulty, and this is by no means 

 lessened by the fact that under these circumstances they 

 are always numerically in the minority. When present 

 in butter, their isolation offers fewer difficulties, for by the 

 injection of the butter containing them into the peritoneal 

 cavity of guinea-pigs conditions are created that favor their 

 development, and from animals so treated they may usually 

 be recovered in pure culture. 



