26 ORIENTAL PLAGUE chap. 



is of a characteristic viscid character. Streak cultures on 

 serum and other agar compounds are of the same character. 

 Later on the growth becomes thicker, less translucent, in 

 reflected light of a slightly brownish tint, with numerous 

 raised droplike round thickenings. 



On gelatine surface streak culture the streak becomes 

 marked as an at first grey, then whitish, dry band, 

 gradually thickening, and becoming more opaque and 

 granular ; the centre of the streak is thicker than the 

 margin, which latter is crenate, or, more correctly 

 speaking, knobbed and with fine projections. The gelatine 

 is not liquefied at any time. A gelatine streak culture of 

 B. pestis, as also a gelatine surface culture containing 

 isolated colonies of B. pestis, is, after several weeks' 

 incubation, extremely characteristic : whitish, dry, thick, 

 granular, opaque, thicker in centre than at margin ; in 

 streak, knobbed margin ; in isolated colonies with filmy 

 irregular margin and conically raised centre. 



In milk B. pestis grows well at 20° to 37° C. without 

 causing any change of the milk either in aspect or its fluid 

 character. In litmus milk a slow and gradual change of 

 the at first blue colour into less blue, then violet, and 

 ultimately slight red colour takes place, thus showing 

 that the B. pestis is a slow acid producer. This can be 

 proved also in this way, that if of a culture of B. pestis in 

 alkaline glucose broth, incubated at 37° C. for two or 

 three days, a few drops be added to a few cc. of a watery 

 solution of litmus, this at once turns distinctly red. 



B. pestis grows on steamed potato but feebly, forming 

 thereon a transparent film ; the growth is therefore 

 invisible to the unaided eye, and only by taking a 

 particle of the inoculated surface and examining it in a 



