490 



PLAGUE 



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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; 

 sometimes, however, there is practically no surface growth. 

 There is no liquefaction of the medium. In bouillon the 

 growth usually forms a slightly granular or powdery deposit at 

 the foot and sides of the flask, somewhat resembling that of 



a streptococcus. If oil or 

 melted butter is added to 

 the bouillon so that drops 

 float on the surface, then 

 a striking mode of growth 

 may result, to which the 

 term "stalactite" has 

 been applied. This con- 

 sists in the growth start- 

 ing from the under surface 

 of the fat globules and 

 extending downwards in 

 the form of pendulous, 

 string-like masses. These 

 masses are exceedingly 

 -"" delicate, and readily break 



Fig. 148.— Culture of the bacillus of plague off on the slightest shak- 

 os per cent, salt agar, showing involution • f t , floe,],, accord- 

 forms of great variety of size and shape. ] n S 0I , m . e na ° K / accord 

 See also Plate IV., Fig. 17. m gly during their forma- 

 Stained with carbol-thionin^blue. x 1000. tion the culture must be 



kept absolutely at rest. 

 This manner of growth constitutes an important but not ab- 

 solutely specific character of the organism ; unfortunately it is 

 not supplied by all strains of the organism, and varies from 

 time to time with the same strain. 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 

 remarkable powers of resistance against cold ; it has been exposed 

 to a temperature several degrees, below freezing-point without 

 being killed. Experiments on the effects of drying have given 

 somewhat diverse results, but as a rule the organism has been 



