484 INFECTION AND IMMUNITY. 



100 C. kills them at once, and 80 C. in from five 

 to ten minutes (moist heat) . They are very resis- 

 tant to cold, remaining alive at a temperature of 

 20 C. for several weeks, even when repeatedly 

 thawed out during this time, and they even prolif- 

 erate slowly at from 4 to 7 C. 



virulence Cultures of the plague bacillus retain their viru- 

 ad Toxins. } ence over a j on g p er i 0( j w hen kept in a cool dark 



place and when not allowed to dry. However, 

 they often loose in virulence unaccountably. The 

 nature of the toxic substance is as yet obscure. A 

 concentrated soluble toxin has never been obtained 

 in cultures. Filtrates of young cultures show lit- 

 tle or no toxicity, whereas in older cultures the 

 fluid becomes more or less toxic (liberation of en- 

 dotoxin?). Lustig and Galeotti extract cultures 

 with 0.75 to 1 per cent, potassium hydroxid, from 

 which they precipitate a toxic substance with ace- 

 tic or hydrochloric acid. Markl found the cell 

 bodies to be very toxic after eight weeks' growth at 

 room temperature, provided the organisms were 

 killed by chloroform rather than by heat; killing 

 by heat destroys the toxic substance largely. He 

 believes some metabolic product of the organism 

 is the chief toxic constituent, claiming at the same 

 time the presence of a certain amount of soluble 

 toxin. 



virulence The plague bacillus is exceedingly virulent for 

 for Animals. r& ^ S q u j rre i g ^ guinea-pigs and monkeys ; somewhat 

 less virulent for mice and adult rabbits ; other ani- 

 mals, cats, dogs, swine, cows, horses, sheep, goats, 

 may be infected artificially, although they com- 

 monly recover even after large doses. Guinea-pigs 

 and rats may be infected by subcutaneous, intraperi- 

 toneal and intr a vascular injections, by the feeding 



