358 BACTERIOLOGY. 



odd species (?) that have been described by different in-' 

 vestigators. It will suffice to say, from personal study 

 of the group, that in all probability not more than three ? 

 perhaps only two, species are really represented, and 

 that the remainder may fairly be regarded as varieties. 

 As said, the characteristic common to all the mem- 

 bers of this group is that they are to greater or less 

 extent acid-proof i. 6., 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 par- 

 ticular, however, there is such a striking difference be- 

 tween 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 min- 

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

 acid without losing its color ; whereas, none of the mem- 

 bers 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 delicate 

 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 con- 

 fusion. 



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

 soil, in manure, in intestinal contents, etc., their isola- 

 tion in pure culture is a matter of difficulty, and this is 

 by no means lessened by the fact that under these cir- 

 cumstances they are always numerically in the minority. 

 When present in butter, their isolation offers fewer diffi- 

 culties, for by the injection of the butter containing 



