592 GASTRO-INTESTINAL BACTERIOLOGY 



such, are not readily fermented by most bacteria. Again, sugars, if 

 they are present, are largely absorbed from the higher levels of the 

 small intestine, leaving residual unhydrolyzed starches and protein 

 in relatively great concentration in the lower levels of the large intes- 

 tine. It is not surprising, under these conditions, to find that the 

 more obligate fermentative bacteria the Cocci are prominent at 

 the higher levels, as is the case normally in infants; that facultative 

 bacteria, as Bacillus coli, are common in a transitional zone between 

 a medium containing moderate amounts of utilizable carbohydrate 

 and one in which the utilizable carbohydrate is frequently absent, 1 

 and 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. 

 Practically all the bacteria found in the large intestine of normal 

 adults exhibit a preferential action upon dextrose (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 substitution 

 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 carbohydrate 

 and other nutrient substances. 2 



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 only 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 unusual 



1 Bacillus coli and various closely related bacilli are among the most labile of intes- 

 tinal bacteria in adapting their metabolism to the composition of the intestinal contents. 

 In a medium containing both utilizable carbohydrate and utilizable protein these 

 organisms act principally upon the carbohydrate, forming lactic and smaller amounts 

 of other acids. In a protein medium the products of metabolism are indol, phenols, 

 and other products of proteolysis. 



2 For a brief general discussion of the influence of nutritional factors upon bacterial 

 metabolism, see Section on Bacterial Metabolism. 



