THE BACILLUS OF BUBONIC PLAGUE. 321 



source. Virulence is said by Yersin, Calmette, and 

 Borrel 1 to be accentuated by passing the organism 

 through a series of susceptible animals. 



In man the bacilli are most numerous in the en- 

 larged, suppurating lymphatics. They are present, but 

 in smaller numbers, in the blood and the internal 

 organs. 



It has been observed that in the suppurating lym- 

 phatic glands of man a variety of organisms may be 

 present, but among them are always the plague bacilli. 

 Occasionally, micrococci predominate. In these cases 

 of mixed infection the pest bacilli are said to stain less 

 intensely with alkaline methylene-blue than do the 

 streptococci, and more intensely than do the staphylo- 

 cocci that are present. Also, in this event, the strepto- 

 cocci retain the Gram stain, while the pest bacilli do not 

 and the staphylococci may or may not. It has been 

 suggested that possibly the organisms found by Kitasato 

 in the blood, and which he describes as pest bacilli, 

 that retained the color when treated by the method of 

 Gram, were pairs of micrococci, and not bacilli at all. 



It is the opinion of Aoyama that the suppuration of 

 the glands is not caused by the plague bacillus, but is 

 rather the result of the action of the pyogenic cocci 

 with which it is so often associated. It is also his 

 belief that the most important and frequent mode of 

 infection in man is through wounds of the skin. He 

 does not regard either the air-passages or the alimentary 

 tract as frequent portals of infection. Wilm, on the 

 contrary, is inclined to regard the alimentary tract as a 

 frequent portal of infection ; 2 and subsequent investiga- 



1 Annales de 1'Institut Pasteur, 1895, p. 589. 



2 Wilm : loc. cit. 

 21 



