PARASITIC AND PATHOGENIC BACTERIA 103 



Intestines. The abundant intestinal contents, which vary some- 

 what in composition and reaction at different levels, provide conditions 

 which make the intestinal tract a very efficient combined incubator 

 and culture medium. Many kinds of bacteria may theoretically find 

 conditions well adapted to their rapid development there and it is 

 not surprising to find that bacterial proliferation is greater both in 

 nature and extent in the intestinal tract than in any other known 

 medium. It has been conservatively estimated that the average 

 daily fecal excretion of bacteria in a healthy adult on a normal diet 

 is expressed by the truly enormous number, 33 x 10 12 . About 47 per 

 cent, of the nitrogen of the feces is contained in the bodies of these 

 bacteria which, when dried, weigh nearly 0.5 gram. 



The upper level of the intestinal tract, particularly the duodenum, 

 is relatively free from bacteria during interdigestive periods. The 

 duodenal bacterial population increases rapidly when food enters 

 this section of the alimentary canal and decreases when the food 

 passes to lower levels. The numbers of bacteria increase very greatly 

 where stasis of food becomes more marked and in the cecum and 

 large intestines generally there are continually present enormous 

 numbers of bacteria. 1 



The types of bacteria found in the intestinal tract are influenced 

 markedly by the nature of the food of the host and by the ability of 

 the organisms themselves to change their metabolism to meet varia- 

 tions in the composition of this food. Those bacteria which can best 

 meet alternations in diet of the host are the ones which naturally 

 persist. The bacteria contained in the food itself may also play a 

 prominent part in determining the nature of the organisms which 

 are found in the intestinal tract. The colon bacillus is particularly 

 labile in meeting dietary alternations in the intestines and this organism 

 constitutes about 80 per cent, of the bacteria which can be isolated 

 from the feces of the adult. 



At birth the intestinal tract is sterile and the embryonal feces, the 

 meconium, which is passed during the first eighteen hours after birth, 

 is sterile. Following this period of sterility there is a period lasting 

 about three days on the average, in which various adventitious organ- 

 isms are met with in the dejecta. The normal nursling flora begins 

 to appear by the end of the third day, following the ingestion of 

 breast milk. The dominant organism of this nursling flora is ordi- 

 narially an obligate anaerobe, Bacillus bifidus, which is one of the 



1 Kendall, Jour. Med. Research, 1911, xxv, 126-130. 



