238 BACILLI OF THE HEMORRHAGIC SEPTICEMIA GROUP 



are circular, but the transparent marginal zone early shows a some- 

 what irregular boundary line. If a young plague culture is touched 

 with a platinum loop it is found to be viscous and sticky. It is, 

 however, easily removed from the surface on which it grows. 



The plague bacillus, as already stated, has a marked tendency to 

 develop involution forms early in its growth. As first shown by 

 Hankin, this tendency is most pronounced in cultures on a 3 to 4 per 

 cent, salt agar, one of the most valuable media for the bacteriologic 

 diagnosis of plague. It is prepared and standardized like an ordinary 

 agar, except that it contains, instead of J per cent., 3 to 4 per cent, of 

 common salt. Generally the greater number, or all, of the organisms 

 from such a growth present themselves as large spherical bodies, 

 looking very much like yeast cells; later, large, swollen club- or dumb- 

 bell-shaped or irregular forms make their appearance. The most 

 typical and most constant form grown on a 3.5 to 4 per cent, salt 

 agar, after twenty-four to forty-eight hours, is the yeast-like, large, 

 spherical plague organism. No other microorganism forms this 

 type so promptly and regularly on salt agar that it might be con- 

 founded with the plague bacillus. Hence, it is advisable at the autopsy 

 in a suspected case of plague to inoculate besides gelatin plates also 

 ordinary agar tubes, bouillon flasks, and salt-agar tubes or plates. 

 In bouillon flasks the bacilli, at temperatures between 30 to 35 C., 

 show a finely flocculent whitish, slowly increasing sediment after 

 twenty-four hours. During the next twenty-four hours the flocculi 

 extend upward from the bottom along the walls. A fine whitish ring 

 of growth then forms on the surface, and in course of time covers it. 

 If the flask is kept motionless and undisturbed, bands and strands of 

 bacilli finally grow downward from the surface membrane. The 

 contents of the flask now present an appearance somewhat resembling 

 stalactite and stalagmite formations. 



The plague bacillus does not liquefy gelatin or blood serum; does 

 not ferment dextrose, levulose, lactose, or mannite; and grows spar- 

 ingly on potato and in milk. It does not coagulate the latter. 



Resistance. The resistance of the plague bacillus is not great; it 

 is about the same as that shown by other members of the group. At 

 70 C. the bacilli are safely killed within one hour; sunlight and drying 

 destroy them within twenty-four hours; 1 per cent, lysol solution in 

 ten to fifteen minutes; 1 per cent, caustic lime in ten minutes; 1 per 

 cent, corrosive sublimate solution in half a minute; 1 per cent, hydro- 

 chloric acid solution in one minute; J per cent, sulphuric acid in five 

 to ten minutes. 



Vaccine and Serumtherapy. Killed cultures of the plague bacillus 

 have been prepared and used as vaccines by Yersin, Calmette, Borrell, 

 Haffkine, and others. Haffkine's has been used extensively in India. 

 Attenuated but live cultures have been used by Kolle and Otto and 

 Strong. An antiplague serum has been prepared by a number of 

 investigators by the inoculation of horses. Vaccination against plague 



