BACILLUS PEStIS 555 



Kitasato ^ and by Yersin,^ independently of each other. By both ob- 

 servers the bacillus could invariably be found in the pus from the buboes 

 of afflicted persons. It could be demonstrated in enormous numbers 

 in the cadavers of victims. The constancy of the occurrence of the 

 bacillus in patients, shown in the innumerable researches of many 

 bacteriologists, would alone be sufficient evidence of its etiological 

 relationship to the disease. This evidence is strengthened, moreover ,^ 

 by accidental infections which occurred in Vienna in 189S, with labora- 

 tory cultures. 



Morphology and Staining. — Bacillus pestis is a short, thick bacillus 

 with well-rounded ends. Its length is barely two or two and a half times 



Pig. 118. — Bacillus pestis. (After Mallory and Wright.) 



its breadth (1.5 to 1.75 micra by 0.5 to 0.7 micron). The bacilli appear 

 singly, in pairs, or, more rarely, in short chains of three or more. They 

 show distinct polar staining. In size and shape these bacilli are sub- 

 ject to a greater degree of variation than are most other microorganisms. 

 In old lesions or in old cultures the bacilli show involution forms which 

 may appear either as swollen coccoid forms or as longer, club-shaped, 

 diphtheroid bacilli. Degenerating individuals appear often as swollen, 

 oval vacuoles. All these involution forms, by their very irregularity, 

 are of diagnostic importance. They appear more numerous in artificial 

 cultures than in human lesions. 



According to Albrecht and Ghon,^ the plague bacillus may, by 



• Kitasato, Lancet, 1894. 2 Yersin, Ann. de I'inst. Pasteur, 1894. 



s Albrecht und Ghon, Wien, 1898. 



