ii CHAEACTEKS OF THE B. PESTIS 25 



human cases of plague ; the second type (type 2) — rat 

 type — is the one which is less virulent, and which is 

 characteristic for the rat, that is when it has been pro- 

 pagated through rats. These two types differ in the 

 nature and aspect of their colonies during the early period 

 of their growth on the surface of nutrient gelatine ; type 

 2 forming colonies distinctly more translucent and less 

 granular than those of type 1, and at the same time those 

 of the former are less angular, more rounded, than those 

 of the latter or type 1. It will be understood that these 

 differences, although distinct and easily ascertained in the 

 early phases — two to five days at 20° to 21° C, — become 

 lost as development proceeds ; after ten days or more the 

 angular thinned margin, the granular opaque raised centre, 

 appear the same in both sets. 



Microscopic specimens of the two types of colonies 

 during the earlier phases show also a marked distinction, 

 inasmuch as those of type 1 are distinctly more cylindrical 

 than those of type 2 (see Figs. 38 and 39). 



In stab culture of B. pestis, both in gelatine and in 

 agar, the stab becomes marked as a series of opaque 

 granules, and after some progress has been made each 

 granule shows a filmy projection at one side or the other 

 (Fig. 33), at the same time the upper or free end of the 

 stab being marked by a greyish- white filmy expansion of 

 growth, which in cultures (agar) of some standing (two 

 to three weeks) has the character of a rounded shield, 

 showing distinct concentric and radial markings as shown 

 in Fig. 30. This surface growth possesses the viscid 

 nature of plague growth in a marked manner. 



Streak cultures of B. pestis on agar show a filmy, grey, 

 translucent growth, which, as mentioned on a former page, 



