2 STUDIES IN IMMUNITY. 



properties; on the contrary, their virulence is to a considerable 

 extent increased. With an organism modified in this manner, 

 Metchnikoff has succeeded in killing a guinea-pig in from G to 7 

 hours, whereas the ordinary culture takes about 20 hours. 



What is the cause of this marked increase in pathogenicity? By 

 what modification has the virus acquired this powerful activity? 

 This point we have attempted to determine. 



Let us immunize a guinea-pig of 550 grams weight by injecting 

 3.5 c.c. of a sterilized culture of V. Metchnikovi, followed 8 days 

 later by 0.5 r.r. of a living culture of the organism. Ten days later, 

 when the animal has returned to a normal condition, we introduce 

 1 c.c. of a vigorous culture under the skin of the belly. Twenty 

 hours later a small amount of the exudate which has been formed 

 is withdrawn and grown in veal bouillon containing peptone. 



When this culture has grown out, a cubic centimeter of it is taken 

 and injected subcutaneously in another similarly vaccinated guinea- 

 pig. Twenty-seven hours after inoculation, a drop of the exudate 

 is withdrawn as from the first animal and likewise grown in bouillon. 



This second guinea-pig, although previously in good condition, is 

 much sicker than the first vaccinated animal. There is a large area 

 of necrosis about the point of inoculation; the eyes are half closed, 

 the animal inert, and prostration manifest. This is in harmony 

 w T ith the experiments of Metchnikoff, who demonstrated that the 

 vibrio that had remained under the skin of a vaccinated guinea-pig 

 for 20 hours became more pathogenic. 



We have now three different cultures of the vibrio Metchnikovi : a 

 culture of the organism modified by a single passage through an 

 immunized animal; a culture of the same organism modified by two 

 passages; and a culture of the vibrio grown on agar in successive 

 generations during several months. We may obtain a fourth cul- 

 ture by passing the vibrio through a non-vaccinated guinea-pig. 

 This last constitutes a culture of normal vibrios whose virulence has 

 undergone no modification through prolonged growth on artificial 

 media. 



The increase in virulence of cultures which we call "modified," 

 or very virulent, may be due, according to our present ideas, to one 

 of two causes: either their chemiotactic power toward leucocytes 

 has diminished, which would allow them to escape more readily 



