Morphology 



S8S 



Morphology. — The bacillus is quite variable. Usually it is 

 short and thick — a "coccorbacillus," as some call it — with rounded 

 ends. Its size is smaU (1.5 to 2 ^ in length) and 0.5 to 0.75 n in 

 breadth. It not infrequently occurs in chains of four or six or even 

 more, and is occasionally encapsulated. It shows active Brownian 

 movements, which probably led Kitasato to consider it motile. 

 Yersin did not regard it as motile, and was correct. It has no 

 flageUa. No spores are formed. 



Staining. — It stains by the usual methods; not by Gram's method. 

 When stained, the organism rarely appears uniformly colored, be- 

 ing darker at the ends than at the center, so as to resemble a dumb- 

 bell or diplococcus. The bacilli sometimes appear vacuolated, 

 and nearly all cultures show a variety of involution forms. Kitasato 



^ 



-.•* 



M' 







# 



Fig. 230. — Bacilli of plague and phagocytes, from human lymphatic gland X 800 



(Aoyama). 



has compared the general appearance of the bacillus to that of 

 chicken-cholera. 



Involution forms on partly desiccated agar-agar not containing 

 glycerin are said by Haffkine to be characteristic. The microbes 

 swell and form large, round, oval, pea-shaped, spindle-shaped or 

 biscuit-like bodies which may attain twenty times the normal 

 size, and gradually lose the ability to take the strain. Such involu- 

 tion forms are not seen in hquid culture. 



Cultivation.' — Pure cultures may be obtained from the blood or from 

 the softened contents of the buboes, and develop well upon artificial 

 media The optimum temperature is about 3o°C. The extremes 

 at which growth occurs are 20° and 38°C. 



Bouillon. — In bouillon a diffuse cloudiness was observed by 

 Kitasato, though Yersin observed that the cultures resembled ery- 



