ii CHARACTERS OF THE B. PESTIS 19 



suppurating bubo stained after this method have, in 

 several instances, yielded instructive specimens : Staphy- 

 lococcus aureus, Streptococcus pyogenes, diphtheroid 

 (xerosis) bacilli appeared deep violet ; B. pestis, proteus, 

 eoli-like microbes, bright pink. 



Plague bacilli are described by some observers as 

 being possessed of a capsule. This I think is based on 

 a misunderstanding : when film specimens, say of bubo 

 or other diseased tissues, are fixed and then stained, 

 and if the ground substance, owing to the thickness 

 of the film, happens to be present in too thick a layer, 

 or if the film has been insufficiently heated, or if 

 the specimen is insufficiently washed after staining, the 

 deeply stained bacilli or cocci, as the case may be, appear 

 surrounded by a clear space ; this is owing to the fact 

 that the ground substance, which is also more or less 

 stained, has during the drying process shrunk away from 

 the bacilli, therefore the latter appear surrounded by an 

 unstained clear area, which might be thus taken for a 

 capsule surrounding each bacillus. But this is, of course, 

 not a real capsule, such as surrounds the pneumococcus for 

 instance, for this capsule can be actually stained ; whereas 

 the other is merely a space, and cannot be stained. When 

 the film specimen is thin, well heated, and the preparation 

 after staining well washed, no capsule can be observed on 

 the plague bacilli. I have paid particular attention to 

 this point, and can speak confidently about film specimens 

 made of bubo materials, of the spleen, the lung, and blood 

 of human plague cases or of plague animals. Nor is there 

 any trace of a capsule around the individual bacilli or the 

 chains of bacilli which make up the characteristic powdery 

 or floccular sediment of broth-cultures of B. pestis. 



