INFECTIONS OF THE DIGESTIVE TRACT 5 



cellulose eaten by herbivora (in which the digestive 

 function of the bacteria chiefly comes into question) is 

 provided with intracellular enzymes capable of decom- 

 posing cellulose. 



The real significance of the normal intestinal flora 

 probably lies not in any immediate relation to processes 

 of digestion, but in a wholly different direction. 



It is impossible to avoid the entrance of bacteria into 

 the digestive tract. As will be seen when we come to 

 consider the normal flora of the alimentary tract, the 

 obligate bacteria (e.g. B. lactis aerogenes, B. coli, B. 

 bifidus) have adapted themselves to the secretions of 

 this part of the body and ordinarily hold their own 

 against new-comers. By virtue of their adaptation 

 they are not ordinarily harmful to their host, but on 

 the contrary, they are under some circumstances cap- 

 able of doing a service by giving rise to conditions that 

 discourage the growth of many harmless and harmful 

 species which the human animal cannot readily exclude 

 from his digestive tract. I believe the chief significance 

 of the obligate intestinal bacteria lies in their potential 

 capacity for thus checking the development of other 

 types of organisms capable of doing injury. 



Under ordinary conditions of life, in temperate cli- 

 mates, and still more so in hot ones, a human being is 

 liable to take into the alimentary tract, with food and 

 drink, microorganisms which are capable of doing injury 

 if they find opportunity to multiply in the digestive 

 tract. Water, milk, cheese, oysters, game, preserved 

 and fresh meats, etc., are liable to contain injurious 



