442 THE HEMORRHAGIC SEPTICEMIA GROUP 
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.^ The center of the cell is practically uncolored and 
the stainable substance is seen as a deeply colored granule at each end 
of the rod — bipolar staining. Pleomorphic 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 aspi- 
rated 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 
Fig. 04. — Plague bacillus. Involution forms from culture on .3 per cent salt agar. 
X 1000. (KoUe and Hetsch.) 
adhering to the sides. The addition of a drop of neutral oil— as cocoa- 
nut oil — provided the culture is maintained free from all vibration, 
causes a characteristic "stalactite" growth; the organisms grow 
down from the oil droplets as filaments (which have been likened to 
stalactites) until they even reach the bottom of the tube. Chains of 
bacilli are most characteristically developed in this medium. The 
addition of 2 to 3 per cent common salt to broth or agar stimulates 
the formation of very irregular involution forms. Milk is not coagu- 
lated, but a slight permanent acidity gradually develops. Growth on 
coagulated blood serum or ascitic agar, although somewhat more 
luxuriant than on ordinary laboratory media, is not characteristic. 
B. pestis is an aerobic organism; it fails to develop with its customary 
vigor in the absence of oxygen. Unlike a majority of pathogenic 
bacteria, the optimum temperature of growth is about 30° C; growth 
' Kossel and Overbeck: Arb. a. d. kais. Gesamte, 1902, 18, 114. 
