122 AIDS TO BACTERIOLOGY 



1-7 /JL; but longer forms are seen, measuring 5 JLI. As 

 convalescence approaches, round and ovoid involution 

 forms, totally unlike the bacillus, appear. In cultivation 

 the young bacilli are so short as to be almost coccoid or 

 slightly oval, but in older cultures rod, thread, and involu- 

 tion forms occur. In broth culture the organism forms 

 chains (streptobacillus). 



In a film made from an agar culture the bacilli are 

 swollen and yeast-like, but it is questioned whether a 

 capsule exists. 



Staining Reactions. B. pestis does not stain by Gram's 

 method, but with ordinary dyes shows marked bipolar 

 staining. 



Cultural Characters. B. pestis is an aerobe and a 

 facultative anaerobe. Growth is slow at 18 to 20 C., 

 rapid at 37 C., but 30 C. is the optimum tempera- 

 ture. 



In broth a characteristic flocculent, wavy deposit is 

 formed, which settles to the bottom, leaving a clear 

 medium above. 



In broth containing a little butter or coco-nut oil, and 

 kept absolutely at rest, flocculent, tapering masses of 

 growth depend from the droplets of oil floating on the 

 surface (Haffkine's stalactite growth). 



On the surface of gelatin it forms a thin, whitish, 

 punctate growth, which is confined to the inoculation 

 streak ; the medium is not liquefied. On the surface 

 of agar and of serum it forms a thick, cream-coloured, 

 very sticky growth. 



On agar containing 2 - 5 to 3 - 5 per cent, of common salt 

 pear-shaped and spherical involution forms are so common 

 that Hankin recommends this salt agar as a diagnostic 

 medium. It grows in milk without coagulation. On 

 potato little or no growth takes place. It grows well in 

 bile-salt media. While the virulence of some strains 

 is retained in cultures for a long while, that of other 

 races quickly diminishes. 



Resistance. The organism is very easily killed by 

 disinfectants. The thermal death -point appears to be 

 between 58 and 70 C. While cold has little effect, 

 complete desiccation kills the bacillus. In sterilised 

 water the bacillus has remained alive for fifteen days at 

 room - temperature . 



