170 JOHNE^S DISEASE 



otherwise Johne's bacillus would grow on a broth pre- 

 pared from such tissues, or on pieces of fresh gland 

 removed aseptically, and this we know is not the case. 

 What is far more probable is that the necessary food 

 exists in very small quantity, but is constantly re- 

 plenished as it is used by the bacillus. As we have 

 pointed out (Chapter VI.), we have evidence that the 

 substance contained in the body of B, phlei, which 

 stimulates the growth of Johne's bacillus in artificial 

 cultures, is an organic acid ; and it is probable that it 

 is some such acid — possibly in combination — existing 

 in the intestinal wall, which stimulates the growth of 

 the bacillus in this situation. Such an acid might be a 

 product of metabolism of the neighbouring glands, or 

 a product of digestion passing through the intestinal 

 walls, or, indeed, it might be produced from such sub- 

 stances as grass by the action of certain intestinal 

 bacteria, and be absorbed by the intestine, thus reach- 

 ing the vicinity of the baciUi. However this may be, 

 the facts remain that the disease is confined to the 

 intestine and mesenteric glands of certain ruminants, 

 and that a large dose of infective material is necessary 

 to reproduce the disease experimentally. From this it 

 seems probable that under good hygienic conditions a 

 healthy bovine has but little chance of infection if a 

 single dose of infective material is taken with the food. 

 Repeated ingestion of infected food, especially by 

 animals poor in condition, is probably essential for 

 infection in the majority of cases. 



Considering now the case of rodents and birds, we 

 have already seen that these do not naturally contract 

 the disease, and that very large doses, and in some 

 cases repeated doses, are necessary to produce patho- 

 logical changes ; but even in this case the bacilli 

 rapidly die, and cannot be recovered in pure cultures. 



