THE GASTROINTESTINAL FLORA OF NORMAL INFANTS 593 



conditions, extend their normal habitat, and crowd out some of the 

 existing organisms, eventually leading to abnormal reactions in the 

 alimentary canal which may be detrimental to the host. 



There are many intestinal disturbances of unknown causation, pre- 

 sumably unrelated to bacterial activity, which naturally are not of 

 interest in this connection. There is a second group of conditions in 

 which bacteria may conceivably play a secondary part; in some 

 instances abnormal physiological conditions in the alimentary canal 

 may be justly regarded as the antecedent factors. The boundaries 

 of these two groups are poorly circumscribed and they merge through 

 imperceptible or poorly" defined limits into a third group of cases in 

 which the activities of endogenous or exogenous bacteria in the alimen- 

 tary canal may be the causative factor in morbid processes of the 

 gastro-intestinal tract. 



For convenience of discussion this last group may be divided into 

 three types: (a) Those cases in which products resulting from the 

 action of bacteria upon proteins or their derivatives appear to be the 

 prominent factors in the production of the morbid process; (6) those 

 cases in which products resulting from the fermentation of carbohy- 

 drates by the action of bacteria are the prominent substances concerned 

 in the morbid process. A third group, practically unstudied at the 

 present time, would include those cases in which symbiotic activities 

 of proteolytic and fermentative bacteria would result in the production 

 of substances derived both from proteins and from carbohydrates. 1 



The action of bacteria on fats is little understood at present and 

 no statement can be made covering this type of abnormality. It is 

 expressly understood that products of the nature of endotoxins result- 

 ing from the dissolution of bacteria are not considered in this connec- 

 tion, which relates exclusively to a discussion of the activities of living 

 organisms. 



The symptomatology induced from the products arising from 

 the decomposition of proteins or protein derivatives by the action 

 of bacteria in the intestinal tract depends largely upon the organism 

 or organisms concerned; it varies from the somewhat insidious, slowly 

 progressing, so-called auto-intoxication, in which a marked increase 



1 Thus, in occasional severe diarrheas of children strains of Bacillus coli and Bacillus 

 mesentericus are occasionally isolated, which grow symbiotically in milk, causing a deep- 

 seated change both in the protein and carbohydrate content of the medium. The result 

 of their mutual development is much greater than the sum of their separate activities. 

 Ordinary strains of these organisms frequently do not exhibit this symbiotism. It is by 

 no means improbable that similar symbiotic activity in the intestines, if unrestrained, 

 may lead to conditions incompatible with the well-being of the host. 

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