36 OFFENSIVE FORCES OF THE INVADING MICROORGANISM 



organism in serum-containing media and to examine microscopically 

 for capsules. In the non-capsule formers, on the other hand, micro- 

 scopic examination is insufficient to determine whether an organism 

 under consideration is virulent or not; in that case the animal 

 experiment alone will decide the question. 



Factors Determining Capsule Formation. Of the factors which 

 are operative in determining the formation of capsules very little is 

 as yet known. One could imagine, of course, that as the result of 

 favorable changes in nutrition, certain biological changes would 

 result of which the hypertrophy of the ectoplasm is one of the con- 

 sequences; in other words, that capsule formation is an index of a 

 condition of particularly active nutrition. There are certain facts, 

 however, which suggest that this explanation is not correct, for we 

 find that capsule formation may be evoked by agents which have no 

 nutrient properties whatever. Danysz thus found that the anthrax 

 bacillus when grown in arsenical media of increasing concentration 

 forms enormous mucinous capsules which protect the organism 

 against the bactericidal action of the chemical in question. 



Organ Virulence. After the virulence of an organism has been 

 artificially raised, one would imagine that this increase would mani- 

 fest itself not only in animals of the same species through which it 

 has been passed, but in others as well. This, however, is not the 

 case, and here as elsewhere in immunological work one meets with 

 remarkable examples of specificity for which no explanation can as 

 yet be given. If, for example, the virulence of the chicken cholera 

 bacillus is increased by passage through the chicken, the increase 

 affects this animal but remains unchanged for the guinea-pig. Simi- 

 larly a certain selective affinity develops for certain organs if the 

 increase in virulence has been brought about through the specific 

 intervention of those organs, and then shows itself irrespective of the 

 manner in which infection is produced. When rats, for example, 

 have been serially infected through the respiratory tract with the 

 lung juice of animals dead with the plague, the virulence of the 

 organisms is not only increased, but plague pneumonia invariably 

 develops on infecting other rats even by the subcutaneous or intra- 

 peritoneal route. 



Virulence of this order which is specifically directed against cer- 

 tain organs is spoken of as organ virulence and is evidently destined 

 to play an important role in the future study of infection. Capsule 



