THE PLAGUE BACILLUS 393 



The typical form of the organism, the bi-polar staining, 

 short, stumpy bacillus, is met with in smears from the 

 buboes, in the sputum in the pneumonic form, and in the 

 blood in the septicsemic variety, but only in the earlier 

 stages of the disease. Later the typical forms tend to 

 disappear, their place being taken by a few large, rounded, 

 ovoid, or pear-shaped involution forms. Under cultiva- 

 tion the bacilli in young cultures (twenty-four to forty- 

 eight hours) are so short as to be almost coccoid or slightly 

 ovoid ; on agar their size is about the same as that in the 

 animal body, on gelatin they are somewhat smaller, but 

 a few well-marked rods and even threads are always present. 

 In older cultures, rod, thread and involution forms occur 

 more numerously ; on agar containing 2-3 per cent, of 

 salt the latter are swollen and yeast-like. 



In broth chains of slightly ovoid organisms occur 

 resembling streptococci (Plate XV. a). 



The organism is non-sporing and non-motile, although 

 Gordon described the presence of one or two fine spiral 

 terminal flagella (others have not found flagella). 



Sometimes in hanging- drop cultivations a capsule is 

 apparently present, but the writer has failed to verify this 

 by staining methods. 



The B. pestis stains well with Loffler's blue and anilin- 

 gentian violet, polar-staining being a marked feature, 

 especially in smear preparations. It does not stain by 

 Gram's method. With old laboratory strains polar stain- 

 ing may be completely absent, but in such cases may 

 sometimes be obtained by first treating the preparations 

 with alcohol or by the Gram method, and subsequently 

 staining with Loffler's blue or weak gentian violet. Sections 

 are best stained with carbol methylene or thionine blue. 



Cultural characters. The B. pestis is aerobic and facul- 

 tatively anaerobic. On blood-serum it forms moist, 

 smooth, shining, cream-coloured colonies or growths, 



