BACILLLUS MESEJJTERICUS 117 



temperature (of about 37° C.) a reddish-yellow or rose-red 

 colour. The individuals of all the varieties included under 

 the name of potato bacillus adhere together and form an ex- 

 tensive wrinkled membrane, which can easily be detached 

 from the slice of potato. The Bacillus mesentericiis vulgatus 

 has the property of curdling milk, as rennet does, and render- 

 ing it stringy, the substance to which it owes its viscidity 

 being probably metamorphosed cellulose. It displays upon 

 the whole the same behaviour towards gelatine and agar that 

 the two other potato bacilli do, but whereas the cultures of 

 Bacillus mesentericiis fuscus have a yellowish colour, and 

 those of the ruber variety a reddish, the membrane on the 



Peripheral radiating 



~ Liquefied part 

 Fig. 41. — Islet op Bacillus mesentericus vuL&A'njs on a Gelatine Plate. 



potato shows no pigmentation at all in the case of Bacillus 

 mesentericus vulgatus. 



The potato bacillus develops with particular readiness 

 on pieces pf potato which are not completely sterilised, often 

 destroying the cultures of other micro-organisms. 



Bacillus liodermos. — Pliigge found very widely distributed 

 in the atmosphere, and often as a guest upon our nutrient 

 materials, short, exceedingly motile rods, the growth of 

 which on gelatine causes it to liquefy with great rapidity, a 

 white pellicle floating on the surface. In thrust-cultures 

 dirty grey flakes swim about in the fluid mass. A smooth, 

 glossy coat resembling thin mucilage develops on potato, 

 changing as the spores form into a thick and much- 

 wrinkled membrane. The mucilaginous mass is soluble in 



