PROBLEM OF VIRULENCE 293 



tosis. These substances (referred to by Kruse as "Lysins") were named 

 by Bail, "Aggressins." The production of aggressins by pathogenic 

 germs is probably absent in test-tube cultures, or, at any rate, is greatly 

 depressed under such conditions, but is called forth in the animal body 

 by the influences encountered after inoculation. 



These aggressins can be found, according to Bail, in the exudates 

 about the site of inoculation in fatal infections. He obtained them, 

 separate from the bacteria, by the centrifugation and subsequent de- 

 canting of edema fluid, and pleural and peritoneal exudates. 



Two experimental observations are brought by Bail in support of 

 the truth of his contentions. In the first place, he was able to show 

 that fatal infection could be produced in animals by the injection of 

 sublethal doses of bacteria, when these were administered with a small 

 quantity of "aggressin." He inferred from this that the injected ag- 

 gressin had paralyzed the onslaught of phagocytic and other protective 

 agencies, and had thus made it possible for the bacteria to proliferate. 



The second experimental support of Bail's theory consists in the 

 successful immunization of animals with aggressin. Animals were 

 treated with aggressive exudates, from which all bacteria had been re- 

 moved by centrifugalization and which had been rendered sterile by 

 three hours' heating to 60 C. and addition of 0.5% phenol. Animals 

 so treated were not only immune themselves, but contained a substance 

 in their serum which permitted the passive immunization of other un- 

 treated animals. Bail explained this by assuming the production of 

 anti-aggressins in the treated subjects. His experiments and those of 

 his pupils were conducted with the typhoid and dysentery bacilli, the 

 bacilli of chicken cholera and of plague, the cholera spirillum, and va- 

 rious micrococci. According to whether a microorganism is capable of 

 producing an aggressin and consequently of invading the animal body, 

 he divides bacteria into "pure parasites," "half parasites," and "sapro- 

 phytes." 



The theory of Bail has been attacked chiefly by Wassermann and 

 Citron, 1 Wolff, 2 and Sauerbeck. 3 The criticism which these investigators 

 make of Bail's views has succeeded in placing the "aggressin" theory 

 in doubt. It is claimed by them that much of the "aggressive" char- 

 acter of Bail's exudates is due to their containing liberated bacterial 

 poisons (endotoxins) . This they have maintained both because the 



1 Wassermann and Citron, Deut. med. Woch., xxviii, 1905. 



2 Wolff, Cent, f . Bakt., I, xxxviii, 1906. 3 Sauerbeck, Zeit. f . Hyg., Ivi, 1907, 



