548 Plague 



turbed, they fall like snow and are deposited at the bottom, leaving 

 the liquid clear. 



Colonies. — Upon gelatin plates at 22°C. the colonies may be 

 observed in twenty-four hours by the naked eye. They are pure 

 white or yellowish white, spheric when deep in the gelatin, flat when 

 upon the surface, and are about the size of a pin's head. The 

 gelatin is not liquefied. Upon microscopic examination the borders 

 of the colonies are found to be sharply defined. The contents be- 

 come more granular as the age increases. The superficial colonies 

 are occasionally surrounded by a fine, semi-transparent zone. 



Klein* says that the colonies develop quite readily upon gelatin 

 made from beef bouillon (not infusion), appearing in twenty-four 

 hours, at 2o°C., as small, gray, irregularly rounded dots. Magnifica- 

 tion shows the colonies to be serrated at the edges and made up of 



Fig. 229. — Stalactite growth of bacillus pestis in bouillon. (Reproduced 

 from Simpson's "A Treatise on Plague," 1905, by kind permission of the Cam- 

 bridge University Press.) 



short, oval, sometimes double bacilli. Some colonies contrast 

 markedly with their neighbors in that they are large, round, or oval, 

 and consist of longer or shorter, straight or looped threads of bacilli. 

 The appearance was much like that of the young colonies of Proteus 

 vulgaris. At first these were regarded as contaminations, but later 

 their occurrence was regarded as characteristic of the plague bacillus. 

 The peculiarities of these colonies cannot be recognized after forty- 

 eight hours. 



Gelatin Punctures. — In gelatin puncture cultures the develop- 

 ment is scant. The medium is not liquefied; the growth takes place 

 in the form of a fine duct, little points being seen on the surface 

 and in the line of puncture. Sometimes fine filaments project into 

 the gelatin from the central puncture. 

 Abel found the best culture-medium to be 2 per cent, alkaline 

 *"Centralbl. f. Bakt. u. Parasitenk.," July 10, 1897, xxi, Nos. 24 and 25. 



