588 GASTRO-INTESTINAL BACTERIOLOGY 



and aerobic spore-forming bacteria of the mesentericus group are 

 the most readily recognized. The important feature of the intestinal 

 flora at the lower levels of the intestinal tract of adolescents, and 

 more especially of adults, is the presence of facultative fermentative 

 bacteria which appear to thrive equally well when the intestinal con- 

 tents at this level contain protein and carbohydrate as when the 

 carbohydrate is absent. Members of the colon-proteus group, par- 

 ticularly the former, various aerobic liquefying bacilli both spore- 

 forming, and non-spore-forming and, to a limited extent, anaerobic 

 bacteria as well are characteristic of the bacterial flora of the large 

 intestines of adults. This is in striking contrast to the distinctive 

 monotonous fermentative flora of the normal nursling, whose diet 

 contains a sufficient amount of carbohydrate (lactose) to bathe the 

 entire alimentary canal. It contrasts also, to a somewhat lesser degree, 

 with the lower intestinal flora of young children on a cow's milk diet, 

 where the proportion of carbohydrate to that of protein, although 

 decidedly less than that of the nursling, is usually still sufficient to 

 restrain an excessive development of proteolytic bacteria. 



It will be seen that the carbohydrate of the infant diet is lactose, 

 which is utilizable as such by the dominant bacteria of the infantile 

 intestinal and fecal flora. A not inconsiderable portion of the carbo- 

 hydrate of the adult, on the contrary, is starch, which is not readily 

 utilizable as such by a great majority of the intestinal or fecal bacteria; 

 it is very probable that a very considerable proportion of the assimil- 

 able products of hydrolysis of the starch are absorbed rapidly from the 

 intestinal contents and therefore there is normally but little utilizable 

 sugar available for the intestinal flora of adults. This is especially 

 the case in the lower levels of the intestinal tract, where the stasis of 

 the intestinal contents results in a differential accumulation of the 

 more slowly hydrolyzed and absorbed protein. It would appear from 

 these considerations that the relative absence of utilizable carbo- 

 hydrate in the large intestine of adults would naturally be associated 

 with a diminution of the obligate fermentative or carbohydrophilic 

 organisms, and available evidence indicates that such is the case. 



Significance of Intestinal Bacteria. The striking differences in 

 morphology, chemistry and in cultural characters between the intes- 

 tinal floras characteristic respectively of nurslings, artificially fed 

 infants and adults suggest at once that nutritional stimuli may be an 

 important factor in determining the dominance of types of bacteria. 

 An intestinal flora does not appear to be essential for the well-being 



