EXPERIMENTS WITH WHITE RATS 61 



There appears to be, then, a definite correlation between the rate of 

 absorption in the alimentary canal of a utilizable carbohydrate and its 

 tendency to effect, when administered per os, a transformation of the 

 intestinal flora from the ordinary complex type to one in which the 

 dominating organism is B. acidophilus. This conclusion is further 

 strengthened by the results of the following observations. 



RELATION OF HYDROGEN ION CONCENTRATION TO THE 

 CHARACTER OF THE INTESTINAL FLORA 



As has been stated in the historical resume of this paper, the possible 

 relation of acidity of the intestinal tract to the prevailing flora has 

 been a subject of much debate. Metchnikoff's theory underlying the 

 alleged implantation of B. bulgaricus rests largely on the assumption 

 that this organism exerts its remedial influence on the host because of 

 the acid production which it causes within the enteric tract, particu- 

 larly the large intestine. 



In the present study the hydrogen ion concentrations of the feces 

 and of the contents of the intestine at different levels were determined 

 partly by the Sorensen indicator method as modified by Henderson and 

 Palmer (1912-1913) and partly with the aid of the colorimetric stand- 

 ards prepared according to the description of Clark and Lubs (1917). 

 In preparing the fecal extracts for the determinations the general plan 

 first suggested by Howe and Hawk (1912) and later employed by 

 Nelson and Williams (1916-1917) was followed, with the exception that 

 500 milligrams of the fresh feces in place of two grams, and that twenty 

 cubic centimeters of practically neutral water instead of fifty cubic 

 centimeters of N/2 sodium sulphate, were employed. 



The filtered fecal extracts were usually colored light yellow to brown, 

 and gave very little if any sediment upon standing. Contrary to the 

 experiences of Howe and Hawk, comparatively sharp readings could be 

 made. Four color indicators were used, namely brom thymol blue, brom 

 cresol purple, methyl red and sodium alizarine sulphonate, the first two 

 being chosen because they offer more distinguishable shades of color 

 within the same range of pH than does para-iiitrophenol which was used 

 by Nelson and Williams (1916-1917). Owing to the wide color range 

 of sodium alizarine sulphonate (4.7 to 8.0), preliminary tests were 

 made with this agent to determine the narrow ranges within which the 

 reactions fell, and thus to facilitate the more exact determinations made 

 with the other indicators. Brom thymol blue and brom cresol purple 

 often serve as excellent checks for each other. 



Altogether 146 hydrogen ion concentration determinations were 

 made. The results of some of the individual tests are given in Table 24. 

 The regular bacteriological examinations and reduction tests with 



