200 CLINICAL BACTERIOLOGY. 



They occur only in small number and change the appear- 

 ance and the odor of the milk to such a degree that it will 

 probably not be used. 



PLAGUE. 



The exciting agent of plague was discovered by Kitasato 

 and Yersin during an epidemic of cholera in China in the 

 year 1894. 



Morphology. The plague-bacillus is a small, nonmotile rod, 

 with rounded extremities. It is believed to possess a capsule, 

 which, however, is not readily demonstrated. Plague-bacilli 

 vary extraordinarily in form, both in cultures and in the 

 products of the disease in human beings and animals. At 



Fig. 50. Bacillus of bubonic plague (Yersin). 



times they appear as short, compact bacteria, at other times they 

 are suggestive of diplococci, distinct rods, short, sharply bent 

 threads, and, finally, in older cultures, degeneration-products 

 occur in the form of swollen spheres and clubs. The plague- 

 bacillus does not stain by Gram's method, but, on the other 

 hand, readily with all aniline dyes, and most strikingly with 

 methylene-blue. Under these circumstances it exhibits distinct 

 polar staining. The extremities are more deeply stained than 

 the central portion, which after feeble action of the staining 

 solution appears to be a deficiency. 



Spores are not present. 



The temperature-optimum is that of the body, although the 

 plague-bacillus thrives well at room-temperature. 



