MORPHOLOGY 441 
invaded Europe. The great epidemics of the third and the fourteenth 
centuries caused widespread death ;^ Uterally milHons perished, and 
the effect upon the resi(hial population was most distressing.- The 
disease has become endemic on the western coast of the United States, 
within recent years, the reser^■oi^ of infection being certain rodents. 
The causative organism, B. pestis, was isolated and described almost 
simultaneously by Kitasato^ and Yersin^ from the purulent contents 
of buboes, the lymph glands, the blood and the cerebrospinal fluid. 
Later the specificity of the organism was established by laboratory 
accidents and by the comprehensive studies of the British Indian 
Plague Commission. 
Fig. 63.— Plague bacillus, bouillon culture, methylene blue stain showing bipolar 
staining. X 1000. (KoUe and Hetsch.) 
Morphology.— B. pestis is a small thick bacillus with rounded ends, 
which occurs singly or in pairs as a rule, although short chains of three 
to six elements are occasionally seen. The organism is not charac- 
teristically rod-shaped, rather it approaches in outline a somewhat 
ovoid cell. The size of the typical organism varies within the compara- 
tively narrow limits of 0.5 to 0.7 micron in diameter at the widest part 
and 1.5 to 1.8 microns in length. The bacilli, however, are very 
pleomorphic, and exhibit great variation in size and shape according to 
the medium and age of the culture. In young cultures and fresh lesions 
the typical ovoid shape predominates, but in older cultures and lesions 
considerable variation in size and outline is very common. The addi- 
tion of 2 to 3 per cent of salt to artificial media greatly increases the 
proportion of involution forms. B. pestis is non-motile and possesses 
no flagella. Spores are not produced. Zettnow^ and Albrecht and 
Ghon^ state that the organism forms a capsule. The organism stains 
readily with anilin dyes, and it is Gram-negati\'e. Dilute methylene 
' A very graphic contemporary picture of the plague is in the introduction to the 
"Decameron," by Boccaccio. 
■^ See Hankin: Jour. Hyg., 1905, 5, 4S for a history of plague epidemics. 
3 Lancet, 1894, ii, 428. * Ann. Inst. Pasteur, 1894, 8, 662. 
^ Ztschr. f. Hyg., 1896, 21, 165. ^ Centralbl. f. Bakteriol., 1899, 26, 302. 
