THE BACILLUS OF PLAGUE. 109 



the larger ones according to Yersin, show a diminution in 

 virulence similar to that which is seen when examining a 

 growth obtained after successive cultivation. On potatoes the 

 bacillus grows feebly at a temperature of t,7° C., and in milk 

 a slight increase can be seen, but no curdling occurs (Abel). 

 No growth at all was seen in milk by Kolle (14), though 

 Wilm has described one accompanied by coagulation. The 

 plague-bacillus grows under both aerobic and anaerobic 

 conditions, develops no gas in media containing sugar, and 

 no indol reaction can be obtained from bouillon cultures. 

 An acid is developed in litmus-bouillon in twenty-four 

 hours, and this in no way can hinder growth, since a degree 

 of acidity such that 10.5 c.c. of decinormal soda was re- 

 quired to neutralise 100 c.c. of an acid bouillon that still 

 remained a good culture- medium. A study of the respira- 

 tion of the bacillus has been made by Hesse (15). Both 

 the assumption of oxygen and evolution of carbon dioxide 

 show variations, the maximum of which corresponds to the 

 period of most energetic growth and is reached on the fourth 

 or fifth day. A larger volume of oxygen is taken up than 

 the carbon-dioxide liberated, the two processes, like the 

 respiration of all protoplasm, being quite independent of 

 each other. Anaerobic cultures in an atmosphere of hydro- 

 gen did not succeed at all on agar, and only to a limited 

 extent on gelatine, showed that a diminished amount of 

 C0 2 was evolved. 



When compared with most other pathogenic micro- 

 organisms the bacillus of plague yields rather scanty 

 cultures which require some care for preservation, and it is 

 easily killed by weak solutions of disinfectants like carbolic 

 acid, lyssol, mercury perchloride and milk of lime. The 

 last of these as has been shown by Kitasato, Wilm and 

 others to effectively kill a culture in ten minutes to three 

 hours, according as the added lime varies from .5-5 per 

 cent, in a bouillon culture. Early experiments made by 

 Kitasato have proved that the Pest-bacillus, like many other 

 non-sporing bacteria, is easily killed by drying. The 

 microbes or pieces of tissue from plague-infected animals 

 become non-infective by the removal of water, and the 



