THE BACILLUS OF BUBONIC PLAQUE. 313 



Yersin states that it does not retain the color when 

 treated by the method of Gram ; while Kitasato says 

 that it at one time stains by this method and at another 

 it becomes decolorized. Aoyama observed that those 

 bacilli within the suppurating glands were decolorized, 

 while those in the blood retained the stain when treated 

 by Gram's method. 



Since there is often a mixed infection in these cases, 

 it appears likely that the above discrepancy may be 

 attributed to individual peculiarities of different species 

 of bacteria that were nnder examination, an opinion 

 that is borne out by more recent studies, from which it 

 has been decided that the genuine plague or pest-bacillus 

 does not stain by Gram's method. 



It may be cultivated upon ordinary nutrient media, 

 although preference is given by some to a neutral or 

 slightly alkaline 2 per cent, peptone solution containing 

 from 1 to 2 per cent, of gelatin. 



The most favorable temperature for its growth is 

 between 36° and 39° C. Its colonies on glycerin-agar- 

 agar and on coagulated blood-serum are described as 

 iridescent, transparent, and whitish. On gelatin at 

 18°-20° C. it develops as small, sharply defined, white 

 colonies without liquefaction of the medium. In stab- 

 cultures it develops both on the surface and along the 

 track of the needle. Its growth is slow. It does not 

 cause a diffuse clouding of bouillon, but grows rather 

 as irregular, flocculent clumps that adhere to the sides 

 or sink to the bottom of the vessel, leaving the fluid 

 clear. It shows but limited growth on potato. It does 

 not ferment glucose with production of gas, nor does it 

 form indol. It coagulates milk. 



This organism is killed by drying at ordinary room- 



