118 THE PLAGUE 



idly and abundantly some degrees lower. It grows, but Mess rapidly 

 still, at febrile temperatures. It develops on neutral and feebly alka- 

 line media and is an aerobe. On gelatin plates at 22 C. (71.6 F.) 

 it develops within three days fairly characteristic colonies. The super- 

 ficial ones project above the surface, are coarsely granular and par- 

 tially transparent. They do not liquefy the gelatin and the deep 

 colonies are in no way characteristic. In gelatin stick cultures, this 

 bacillus grows slowly, but uniformly along the line. In undisturbed 

 bouillon, a deposit forms; later the surface is covered, and stalactites 

 and stalagmites may develop. However, these formations, although 

 striking, are not confined to cultures of the plague bacillus. Hankin 

 uses for diagnostic purposes an agar containing from 2.5 to 3.5 per 

 cent, of salt. On this medium well-marked involution forms develop. 

 Swollen, spindle-shaped, and ovoid forms appear instead of the normal 

 rods. Other investigators claim that other bacteria grown on this 

 medium develop similar forms and that this test should not be relied 

 on for diagnostic purposes. However, this is largely a matter of 

 expert training. Hankin in the midst of the plague in India has in 

 all probability acquired an expertness which a European laboratory 

 worker with only old cultures at hand is not likely to secure after a 

 short series of tests. The plague bacillus grows abundantly on coagu- 

 lated serum, but offers nothing distinctive in such growths. It multi- 

 plies slowly in sterilized milk without coagulation, although there is 

 a slight development of acid. In peptone solution and on potato its 

 growth is slow and non-distinctive. 



The bacillus may retain both vitality and virulence for a long time 

 in men and rats recovered from the disease. The organism carried by 

 these hosts, however, is not always virulent. Moreover, as a rule, men 

 and animals, recovered from the disease, are free from the bacillus. 

 Strong found that avirulent bacilli injected subcutaneously in apes 

 are wholly destroyed within twenty-four hours. As has been said, 

 cultures protected from drying and from the light may remain viable 

 after four years and, indeed, they have been reported as still virulent. 

 Old subcultures after many years become quite inert. While complete 

 desiccation soon destroys the bacillus, so long as moisture persists, its 

 destruction within a reasonable time is improbable. From this we 

 may deduce two practical lessons. The first is that plague pus or 



