G ASTRO-INTESTINAL FLORA OF NORMAL INFANTS 073 
finally, that proteolytic organisms are most abundant in the large 
intestines, where carbohydrate in significant amounts is practically 
absent, but where the protein concentration is still considerable. 
It must be remembered that the perpetuation of a dominantly fer- 
mentative intestinal flora depends upon the practically continuous 
presence of carbohydrate throughout the alimentary canal. The 
amount of carbohydrate required to attain and maintain this carbo- 
hydrate excess varies materially in different individuals.^ Practically 
all the bacteria found in the large intestine of normal adults exliibit 
a preferential action upon glucose (a product of the hydrolysis of 
starches and many bioses as well), but they are, for the most part, 
unable to utilize lactose. 
There are, therefore, two important factors to consider in dis- 
cussing the influence of diet upon the intestinal flora: The substitu- 
tion of types of organisms, which frequently follows a monotonous 
diet; and a change in the metabolism of existing types of intestinal 
bacteria when dietary conditions are such that the intestinal medium 
at one or another level fluctuates in its content of utilizable carbo- 
hydrate and other nutrient substances. ^ 
From time to time modifications or changes in the types of bacteria 
in the intestinal flora and of their activities takes place. The nature 
and extent of these modifications and their effects upon the host vary 
very much, not onl\- qualitatively, but quantitatively as well. An 
invasion of the intestinal tract by exogenous bacteria, as the dysen- 
tery bacillus or the cholera vibrio, may lead to a more or less pro- 
nounced replacement of some of the normal intestinal types by these 
alien organisms, and to the production of disease. Normal intestinal 
organisms or types indistinguishable from them by ordinary methods 
of study also may multiply with abnormal luxuriance through luiusual 
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 boimdaries 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 actiA'ities of endogenous or exogenous bacteria in the aliment- 
ary canal may be the causative factor in morbid ])i-()cesses of the 
gastro-intestinal tract. 
1 See Torrcy: Jour. Infec. Dis., 1909, 4, 538. 
2 For a brief general discussion of the influence of nutritional factors ui)on bacterial 
metabolism, see Section on Bacterial Metabolism. 
3 Kendall: ' Jour. Am. Med. Assn., 1926, 86, 737. 
43 
