CHAPTER XIX 



AGGRESSINS 



THE term virulence, as applied to microorganisms, is not 

 readily defined. The reaction of an organism in the body cannot 

 be predicted by its behavior in culture-media. The most virulent, 

 diphtheria bacillus, for example, is not necessarily the one that 

 produces the largest amounts of toxin in culture-media. Con- 

 cerning this mechanism of virulence there have been much dis- 

 cussion and speculation. 



Bail has developed what he has termed an aggressin hypothesis 

 to account for these variations in virulence. He has defined an 

 aggressin as a substance which is secreted by pathogenic organisms, 

 when growing in the animal body, which will neutralize the efforts 

 of the tissues to destroy the organism. The work on this subject 

 has been done principally by Bail and his students, although many 

 substantiating facts have been developed by other investigators. 

 The theory is of practical importance, inasmuch as it is claimed 

 that the injection of bacteria-free aggressins into an animal results 

 in the ultimate development of antiaggressin and of an active 

 immunity. 



Bail divides bacteria into three groups true parasites, half 

 parasites, and saprophytes. The true parasites include such 

 forms as the anthrax bacillus, in which the injection of a very 

 small number results fatally through a rapid multiplication of the 

 organisms. The half parasites are those having a lower degree 

 of virulence such that relatively large injections must be made to 

 produce infection. The third group includes those totally devoid 

 of pathogenic properties. The difference between these forms is a 

 difference in the ability to produce aggressin. 



Aggressin production is demonstrated by Bail as follows: 

 A guinea-pig is injected intraperitoneally with many times a fatal 

 dose of culture of Bacillus typhosus or Spirillum cholerce. The 



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