704 ARTHUR ISAAC KEJSTDALL 



residuum, bacteria which are accommodative to alternations in metabolism 

 are confidently to be looked for. Such happens in the adult alimentary 

 canal, and facultative bacteria, as Bacillus coli, which can accommodate 

 their metabolism to protein or carbohydrate energy, become the dominant 

 organisms. 



The nature and extent of bacterial acclimatization in the intestinal 

 tract is not a matter of indifference to the host; the character of the 

 normal resident flora is of equal or greater importance. 



It is conservatively estimated that a normal, healthy adult, enjoying 

 an average mixed diet, excretes daily in the feces from one hundred to 

 thirty hundred billion of bacteria (Schmidt and Strasburger, McNeil, 

 Latzer and Kerr, and Cammidge). The dried weight of this bacterial 

 mass would exceed five grams, and the nitrogen in it alone would weigh 

 nearly seven-tenths of a gram. It is apparent that the ingested food 

 does not contain this prodigious number of bacteria, and, furthermore, 

 the kinds of organisms isolatable from the excreta do not coincide in 

 type or proportion with those of the regimen. Indeed, many of the latter 

 do not appear to endure intestinal conditions and the bacterial antagonisms 

 therein. It must be conceded, therefore, that the alimentary canal is a 

 singularly efficient incubator and culture medium from the bacterial point 

 of view; an environment in which bacterial growth along rather definite 

 lines exceeds in intensity and selectiveness that of any known natural 

 process. 



The range of reaction and the composition of nutritive substances 

 at different levels are such that theoretically a great variety of organisms, 

 capable of growing at body temperature, might find conditions favorable 

 for their development. Notwithstanding the nutritive possibilities 

 throughout the alimentary canal, from starches to glucose and fermenta- 

 tion acids, from practically unaltered protein to amino acids and extrac- 

 tives, and from fats to fatty acids and glycerin, the number of types 

 of bacteria which occur normally and in significant numbers in this in- 

 cubator-culture medium is surprisingly small. They are also fairly well 

 known. 25 



The underlying principles of normal intestinal bacteriology, in the 

 light of available information, may be summarized from the clinical view- 

 point as follows: 



1. The constant temperature, variety of food, and range of reac- 

 tion in the alimentary canal create conditions favorable to bacterial 

 growth. 



2. The bacterial response to these conditions is enormous, viewed 



25 A distinction is made between the resident bacterial types which persist under 

 normal dietary conditions for considerable periods of time, and those transient forms 

 which successfully run the intestinal gauntlet, and which may be encountered in any 

 massive bacterial' process. Exogenous pathogenic bacteria, which will be discussed 

 below, are specifically excluded from the present discussion. 



