CHAPTER XIX 



INTESTINAL BACTERIA. THE COLON-TYPHOID 



GROUP 



DURING life a great variety of organisms find suitable conditions 

 for development in the intestinal tract. At birth the meconium 

 of healthy infants is sterile. Very soon, however, bacteria gain 

 entrance to the alimentary canal through the rectum or by way 

 of the mouth from swallowing saliva or food. As the reaction 

 and amount of oxygen at different levels vary, each species tends 

 to remain in that portion of the tract in which it finds its optimum 

 conditions. Aerobic organisms are most abundant in the mouth, 

 although hidden in the crypts of the tonsils and in folds of mucous 

 membrane anaerobes may flourish. Those organisms which are 

 able to withstand the acidity of the gastric juice and succeed in 

 reaching the duodenum find the alkaline reaction there much 

 more favorable to development; also the diminished amount of 

 oxygen renders conditions particularly suitable for anaerobic or 

 facultative anaerobic growth. The available food supply is 

 probably the most important factor in determining the type of 

 organisms likely to develop. In breast-fed infants, for example, 

 lactose being more abundant than any other ingredient in the 

 milk, fermentative organisms predominate. Later as protein is 

 added to the diet proteolytic bacteria become more numerous 

 and the fermentative type relatively decrease. Many investiga- 

 tors have tried to determine whether the presence of bacteria in 

 the intestinal tract is of physiological benefit to the individual. 

 Successful experiments have shown that at least they are not a 

 necessity. An infection with pathogenic organisms, such as 

 dysentery or cholera, totally changes for a time intestinal condi- 



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