BACILLUS PESTIS 



237 



FIG. 130 



has spore formation been observed. Even though bodies somewhat 

 resembling spores are occasionally seen in the bacilli they are not 

 genuine spores, because such bacilli are not 

 more resistant to heat, antiseptics, etc., than 

 the other pest bacilli. 



Cultural Properties. The plague bacillus 

 grows on all ordinary laboratory culture 

 media, best on such as are faintly alkaline. 

 Even a minor degree of acidity as well as 

 a higher degree of alkalinity prevents devel- 

 opment. It develops at temperatures ranging 

 from 5 to 38 C., and in artificial media best 

 at 25 to 30 C. It is almost strictly, though 

 perhaps not absolutely, obligate aerobic. As 

 a rule, it develops on artificial media only in 

 the presence of free oxygen; some observers, 

 however, have occasionally seen a weak 

 growth in its absence. When a favorable 

 solid culture medium (agar or gelatin, slightly 

 alkaline) is inoculated from the organs (bubo, 

 spleen, etc.) of a plague case the development 

 of the bacilli is at first generally quite slow, 

 and frequently very little can be seen with the 

 naked eye in the first twenty-four hours. 

 After this time a typical picture may appear 

 in a considerable number of cases, and it is 

 always present after forty-eight hours. The 



FIG. 131 



Culture of plague bacillus 

 on agar, five days' growth, 

 fixed with formalin vapor. 

 (Author's preparation.) 



Colony of plague, gelatin plate culture, forty-eight 

 hours old, showing dark elevated centre and trans- 

 parent homogeneous marginal zone. (Author's 

 preparation.) 



surface of the agar or gelatin shows small, delicate, round, moist, dew- 

 drop-like colonies. They are light gray in reflected light and grayish 

 white in transmitted light. If these colonies are inspected with a 

 hand lens or with a low power of the compound microscope they 

 show an elevated, finely granular, rounded centre and a perfectly 

 transparent, very thin, flat marginal zone. The colonies, on the whole, 



