Cultivation 573 



the age increases. The superficial colonies are occasionally 

 surrounded by a fine, semi-transparent zone. 



Gelatin Punctures. In gelatin puncture cultures the 

 development 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. Some- 

 times fine filaments project into the gelatin from the central 

 puncture. 



Agar-agar. Upon agar-agar the bacilli grow freely, but 

 slowly, the colonies being whitish in color, with a bluish tint 

 by reflected light, and first appearing to the naked eye when 

 cultivated from the blood of an infected animal after about 

 thirty- six hours' incubation at 37 C. Under the micro- 

 scope they appear moist, with rounded, uneven edges. 

 The small colonies. are said to resemble tufts of glass-wool. 

 Microscopic examination of the agar-agar culture shows the 

 presence of chains resembling streptococci. 



Upon glycerin agar the development of the colonies is 

 slower, though in the end the colonies attain a larger size 

 than those grown upon plain agar. 



Klein * says that the colonies develop quite readily upon 

 gelatin made from beef bouillon (not infusion), appearing 

 in twenty-four hours, at 20 C., as small, gray, irregularly 

 rounded dots. Magnification shows the colonies to be 

 serrated at the edges and made up of short, oval, some- 

 times 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 he was led to believe that their 

 occurrence was characteristic of the plague bacillus. The 

 peculiarities of these colonies cannot be recognized after 

 forty-eight hours. 



Hankin and Leumannf recommend for the differential 

 diagnosis of the plague bacillus a culture medium prepared 

 by the addition of 2.5-3.5 per cent, of salt to ordinary 

 culture agar-agar. When transplanted from ordinary agar- 

 agar to the salt agar-agar, the involution forms so charac- 



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

 and 25. 



t "Centralbl. f. Bakt. u. Parasitenk.," Oct., 1897, Bd. xxn, Nos. 

 16 and 17, p. 438. 



