232 TEXT-BOOK OF BACTERIOLOGY. 



between the tubercle bacilli and other kinds of bacteria. The reli>- 

 tive, gradual, or quantitative differences are, on the other hand, 

 great enough, at any rate, to impart to the specific staining of the 

 tubercle bacilli the value of a chemical reaction which enables us to 

 discriminate substances that can be separated only with.diiHculty. 



The tubercle bacilli may likewise be prepared according to 

 Gram's method. The protoplasmic contents of the rod-cells are 

 frequently contracted into small globules under the influence of 

 iodine; they lie one behind the other in a row, like a chain of cocci; 

 inexperienced investigators having, in fact, pronounced them as 

 such. 



Gram's method will, however, be used only in exceptional cases, 

 since by its application we lose the important advantage of being 

 able to recognize the tubercle bacilli as such even by their reaction 

 to staining alone. 



The question as to how specific staining is brought about and 

 what are the delicate processes which distinguish this variety of 

 bacteria cannot as yet be answered with certainty. But it is at 

 any rate very probable (as stated above) that this is (according to 

 Ehrlich's opinion) to be attributed to the existence of a particular 

 covering surrounding the rods and offering a strong resistance to 

 pigments. It is said to become more permeable under the influence 

 of various additions to the solution, of alkalies, phenol, and anilin ; 

 but the acids (decoloring everything else) are unable to penetrate the 

 skin later on ; hence the coloring matter once absorbed by the rod- 

 cell is surely and permanently retained in it. We may in this case, 

 therefore, on the whole have to deal with the same conditions we 

 have already become acquainted with in the double staining of the 

 spores. 



By the aid of special staining and microscopic investigation, 

 Koch has been enabled to establish the regular occurrence of bacilli 

 in all cases of tuberculosis, and only in this disease, and thus render 

 their specific significance very probable. In order to prove and 

 place it bej'ond doubt he undertook to cultivate the micro-organ- 

 isms artificially with the view to transmission. 



Many attempts to accomplish this in the ordinary manner had 

 failed, and Koch became convinced that the tubercle bacillus re- 

 quired unusual conditions before it could be successfully cultivated. 



We have seen that in this case we have to deal with a strictly- 

 parasitic bacterium, dependent for its development upon the ani- 

 mal body and highly sensitive to changes in its surroundings. 

 Koch thus found that our ordinary food media, the meat-peptone- 

 gelatin and agar, and the bouillon did not suffice, and that only tl>e 



