16 



chracteristic in the capsule^ so its exhibition will not assist in the 

 microscopic diagnosis of plague. The bacillus is not motile, does not 

 possess any flagellse, nor has spore formation been observed. Even 

 if one occasionally sees in the bacilli bodies some^yhat like spores, 

 they are not genuine spores, because such bacilli are not more 

 resistant to heat, antiseptics, etc., than the other pest bacilli. 



CULTIVATION OF THE BACILLUS. 



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 its 

 development. It develops at temperatures ranging from 5° to 

 38° C, and in our artificial media best at 25° to 30° C. It is 

 quite strictly, though perhaps not absolutely, obligate aerobic. 

 As a rule it develops on artificial media only in the presence of 

 free oxygen. However, some observers have occasionally seen a 

 weak growth even in the absence of the latter. When a favorable 

 solid culture medium (agar or gelatin, slightly alkaline) is inoculated 

 from the organs (bubo, spleen, etc.) of a plague case, the develop- 

 ment of the plague bacilli is at first generally quite slow, and 

 frequently very little can be seen with the naked eye within the 

 first twenty-four hours. On the other hand, in a considerable 

 number of cases a tjqjical picture may appear after this time and it 

 is always present after forty-eight hours. The surface of the agar or 

 gelatin shows small, delicate, round, moist, dewdrop-like colonies. 

 They are light gray in refiected light and grayish-white in transmit- 

 ted. If these colonies are inspected with a hand lens or with a lower 

 power of the compound microscope they show an elevated, finely 

 granular, rounded center and a perfectly transparent, very thin, 

 flat marginal zone. The colonies on the whole are circular, but the 

 transparent marginal zone early shows a somewhat irregular 

 boundary line. Colonies of this type are not formed by any other 

 micro-organism, except occasionally by the influenza bacillus. How- 

 ever, in the case of the plague bacillus they develop most characteris- 

 tically on ordinary gelatin plates kept at 20° to 24° C, under 

 which conditions the influenza bacillus would not show any growth 

 at all. While these typical young colonies are seen both on agar and 

 on gelatin, we think that they are best shown on the latter, hence 

 the use of this medium is to be greatly recommended in the bac- 

 teriologic diagnosis of plague. Gelatin plates can, of course, be 



