BACILLUS PESTIS 409 



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 addition of 2 to 

 3 per cent, of salt to artificial media greatly increases the proportion 

 of involution forms. Bacillus pest is is non-motile and possesses no 

 flagella. Spores are not produced. Zettnow 1 and Albrecht and Ghon 2 

 state that the organism forms a capsule. The organism stains readily 

 with anilin dyes, and it is Gram-negative. Dilute methylene blue 

 colors the bacilli in a characteristic manner. This is best observed 

 when the bacilli are fixed with absolute alcohol for thirty minutes in 

 place of heating. 3 The centre of the cell is practically uncolored and 

 the stain able substance is seen as a deeply colored granule at each 



FIG. 59. Plague bacillus. Involution forms from culture on 3 per cent, salt agar. 

 X 1000. (Kolle and Hetsch.) 



end of the rod bipolar staining. Pleiomorphic forms are usually 

 stained faintly or scarcely at all by this method. 



Isolation and Culture. The plague bacillus grows readily on ordinary 

 media and pure cultures are usually readily obtained from the 

 aspirated contents of unopened buboes or other lesions, and frequently 

 from the blood stream in septicemic cases. Colonies on agar after 

 twenty-four hours' incubation are small, somewhat irregular in out- 

 line, translucent, and not distinctive. Similar growths appear upon 

 gelatin after two to three days' incubation. The medium is not 

 liquefied. The bacilli develop with considerable luxuriance in broth 

 forming a granular sediment in the bottom of the tube and frequently 

 adhering to the sides. The addition of a drop of neutral oil as cocoa - 



1 Ztschr. f. Hyg., 1896, xxi, 165. 2 Centralbl. f. Bakt., 1899, xxvi, 362. 



3 Kossel and Overbeck, Arb. a. d. kais. Gesamte, 1901, xviii, 117. 



