478 PLAGUE 



such as to suggest the presence of impurities in the cultures. In 

 stab cultures in peptone gelatin, growth takes place along the 

 needle track as a white line, composed of small spherical colonies. 

 On the surface of the gelatin a thin, semi-transparent layer may 

 be formed, which is usually restricted to the region of puncture, 

 though sometimes it may spread to the wall of the tube ; some- 

 times, however, there is practically no surface growth. There is 

 no liquefaction of the medium. In gelatin plates the superficial 

 colonies develop first and form slightly raised semi-transparent 

 discs with somewhat crenated margins ; the deeper colonies are 



smaller and of spherical 

 shape, with smooth out- 

 line. In bouillon the 



^ ' v growth usually forms a 



%"" slightly granular or 

 v powdery deposit at the 



* ^* anc ^ sides of the 



flask, somewhat resem- 

 bling that of a strepto- 

 coccus. If oil or melted 



-i, *m+ butter is added to the 



** t % ^" V-*V"4?I ' \S bouillon so that drops 

 -* **** ^ ? ** : V*' float on the surface, then 



a striking mode of growth 

 may result, to which the 

 term "stalactite" has been 



FIG. 144. Culture of the bacillus of plague applied. This consists in 

 on 4 per cent, salt agar, showing involution the growth starting from 

 forms of great variety of size and shape. ,, 11TU 3 PP a llr fo PP O f flip 

 See also Plate IV., Fig. 17. tne U i ncler n & ce 01 



Stained with carbol-thionin-blue. xlOOO. fat globules and extend- 

 ing downwards in the 



form of pendulous, string-like masses. These masses are 

 exceedingly delicate, and readily break off on the slightest 

 shaking of the flask; accordingly during their formation the 

 culture must be kept absolutely at rest. This manner of 

 growth constitutes an important but not absolutely specific 

 character of the organism ; unfortunately it is not supplied by 

 all races of the organism, and varies from time to time with 

 the same race. The organism flourishes best in an abundant 

 supply of oxygen ; in strictly anaerobic conditions almost no 

 growth takes place. 



The organism in its powers of resistance corresponds with 

 other spore-free bacilli, and is readily killed by heat, an exposure 

 for an hour at 58 C. being fatal. On the other hand, it has 



