808 PATHOGENIC MICROORGANISMS 



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 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 subject 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. De- 

 generating 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 



FIG. 85. BACILLUS PESTIS. (After Mallory and Wright.) 



in human lesions. A very important property of the plague bacillus 

 in this connection is the formation within twenty-four to forty-eight 

 hours of vacuolated and swollen involution forms upon salt agar, 

 that is, agar to which 3 to 5 per cent of salt is added. Such a 

 medium is of great value in diagnostic work. 



According to Albrecht and Ghon, 4 the plague bacillus may, by 

 special methods, be shown to possess a gelatinous capsule. It does 

 not possess flagella and does not form spores. 



The plague bacillus is easily stained with all the usual anilin 

 dyes. Diluted aqueous fuchsin and methylene-blue are most fre- 

 quently employed. With these stains the characteristically deeper 



* Albrecht und Ghon, Wien, 1898. 



