BACTERIA AND DISEASE 307 



even infecting man. It is clear that rats are not the only- 

 agency acting in this way. Nevertheless it is true that rats 

 contract the disease more readily than any other animals, 

 and that when suffering from it they may spread the in- 

 fection. How it is thus spread it is not known. Drs. 

 Cantlie and Yersin have pointed out that previously to an 

 epidemic of plague rats die in enormous numbers. 



The bacteriology of plague is almost the latest addition 

 to the science. Kitasato, of Tokio, demonstrated the cause 

 of plague to be a bacillus during the Hong Kong epidemic 

 in 1894. This was immediately confirmed by Yersin, and 

 further proved by the isolation in artificial media of a pure 

 culture of a bacillus able to cause the specific disease of 

 bubonic plague. 



The bacillus was first detected in the blood of patients 

 suffering from the disease. It takes the form of a small, 

 round-ended, oval cell, with marked polar staining, and 

 hence having an appearance not unlike a diplococcus. In 

 the middle there is a clear interspace, and the whole is sur- 

 rounded with a thick capsule, stained only with difficulty. 

 The organisms are often linked together in pairs or even 

 chains, and exhibit involution forms. In culture the bacil- 

 lus is even more coccal in form than in the body. 



The plague bacillus grows readily on the ordinary media 

 at blood-heat, producing circular cream-coloured colonies 

 with a wavy outline, which eventually coalesce to form a 

 greyish film. The following negative characters help to 

 distinguish it : No growth occurs on potato, milk is not 

 coagulated, and gelatine is not liquefied; Gram's method 

 does not stain the bacillus, and there are no spores; the 

 bacillus is readily killed by heat and by desiccation over 

 sulphuric acid at 30° C. Both in cultures and outside the 

 body the bacillus loses virulence. To this may be attributed 

 possibly the variety of forms of the plague bacillus which 

 differ in virulence. On gaining entrance to the human body 



