580 GASTRO-INTESTINAL BACTERIOLOGY 



bacteria that appear in the intestinal flora of a normal person are 

 fairly constant in their occurrence, but there may be well-marked 

 seasonal and even annual variations in the relative proportions of the 

 individual groups of organisms which comprise this flora. This sug- 

 gests that the normal bacterial flora is acclimatized to the intestinal 

 environmental conditions of temperature, reaction and composition 

 of food, and of intestinal secretions at different levels. It also indi- 

 cates that the activities of the organisms which comprise the normal 

 intestinal flora are not in active opposition to those of the host. 1 



Adventitious bacteria, frequently in considerable numbers, 

 undoubtedly reach the intestinal tract from time to time. The fate 

 of these organisms depends upon a number of factors, some of which 

 are little understood. If their activities are greatly at variance with 

 those of the normal types they usually fail to gain a foothold; either 

 they are unable to develop in competition with the well-acclimatized 

 normal flora, or they cannot accommodate themselves to the physio- 

 logical and chemical conditions which prevail there. If, on the con- 

 trary, these organisms can adapt themselves readily to the prevailing 

 conditions at some level of the alimentary canal they may continue 

 to develop either in association with preexisting types, or gradually 

 replace the latter. 2 It is doubtless through this process that the sea- 

 sonal prevalence of some types of intestinal bacteria has its origin. 

 It is not unlikely, furthermore, that the occasional unusual type of 

 organism characteristic for an individual or a group of individuals 

 gains entrance to and develops in the intestinal tract in this manner. 



The nature of the process whereby progressively pathogenic bacteria 

 (usually of exogenous origin) replace or modify the normal intestinal 

 flora is as yet little understood. There is evidence in favor of the view 

 that exogenous bacteria which invade the body through the intestinal 



1 The general phenomena governing the parasitism of bacteria in the alimentary canal 

 are not unlike those leading to bacterial parasitism upon the skin, the conjunctiva, 

 or other surfaces of the body which are in communication with the exterior. One 

 important phase of intestinal parasitism is not manifested in other parts of the body, 

 however. The bacteria of the intestinal flora change along rather definite lines from 

 infancy to adult life, as the diet of the host changes from the monotonous pabulum of 

 infancy to the varied regimen of the adult. The organisms parasitic upon the skin and 

 other surfaces of the body do not exhibit this change in type, and it is reasonable to 

 attribute the relative stability of the skin flora to the relative constancy of environ- 

 mental conditions there, while the succession of types of intestinal bacteria from infancy 

 to adult life is rather definitely associated with corresponding changes in the diet of the 

 host. 



2 Undoubtedly repeated inoculation of the alimentary canal with adventitious strains 

 of bacteria plays an important part in determining their acclimatization in the intestines; 

 possibly a simultaneous absence of the preexisting intestinal types in the environment, 

 leading to a reduction or even absence of these normal inhabitants in the food of the 

 host may materially affect the outcome of the "replacement" process. 



