Vital Resistance 549 



peptone solution containing i or 2 per cent, of gelatin, as recom- 

 mended by Yersin and Wilson. 



Agar-agar. — Upon agar-agar the baciUi grow freely, but slowly, 

 the colonies being whitish in color, with a bluish tint by reflected 

 light, and first appearing to the naked eye when cultivated from the 

 blood of an infected animal after about thirty-six hours' incubation 

 at 37°C. Under the microscope they appear moist, with rounded 

 uneven edges. The small colonies are said to resemble tufts of 

 glass-wool. Microscopic examination of the agar-agar culture 

 shows the presence of chains resembling streptococci. 



Upon glycerin-agar the development of the colonies is slower, 

 though in the end the colonies attain a larger size than those grown 

 upon plain agar. 



Hankin andLeumann* recommended, for the differential diagnosis 

 of the plague bacillus, a culture-medium prepared by the addition 

 of 2.5 to 3.5 per cent, of salt to ordinary culture agar-agar. When 

 transplanted from ordinary agar-agar to the salt agar-agar, the in- 

 volution forms so characteristic of the bacillus occur with ex- 

 ceptional rapidity. In bouillon containing this high percentage of 

 salt the stalactite formation is beautiful and characteristic. 



Blood-serum. — Upon blood-serum, growth, at the temperature 

 of the incubator, is luxuriant and forms a moist layer, of yellowish- 

 gray color, unaccompanied by liquefaction of the serum. 



Potato. — Upon potato no growth occurs at ordinary temperatures. 

 When the potato is stood in the incubator for a few days a scanty, 

 dry, whitish layer develops. 



Vital Resistance. — Kitasato found that the plague bacillus did 

 not seem able to withstand desiccation longer than four days; but 

 Rappaportf found that they remained alive when kept dry upon 

 woolen threads at 2o°C. for twenty-three days, and Yersin found 

 that although it could be secured from the soil beneath an infected 

 house at a depth of 4 to 5 cm., the virulence of such bacilli was 

 lost. 



Kitasato found that the bacillus was killed by two hours' ex- 

 posure to 0.5 per cent, carbolic acid, and also by exposure to a 

 temperature of 8o°C. for five minutes. Ogata found the bacillus 

 instantly killed by 5 per cent, carbolic acid, and in fifteen minutes 

 by 0.5 per cent, carbolic acid. In o.i per cent, sublimate solution 

 it is killed in five minutes. 



According to Wyman, the bacillus is killed by exposure to 5S°C. 



for ten minutes. The German Plague Commission found that the 



bacilli were killed by exposure to direct sunlight for three or four 



hours; and BowhillJ found that they are killed by drying at ordinary 



room temperatures in about four days. 



* "Centralbl. f. Bakt. u. Parasitenk.," Oct., 1897, Bd. xxii, Nos. 16 and 17, p. 

 438. 

 t Quoted by Wyman. 

 I " Manual of Bacteriological Technique and Special Bacteriology," 1899, p. 197. 



