HYDROGEN IONS, ETC., OF BACTERIOLOGICAL MEDIA 557 



A culture of Bad. colt in Bacto bouillon (poorly buffered) plus 0.1 

 per cent glucose reached a hydrogen ion concentration of pH 4,8 in 

 24 hours and remained at this acidity during incubation for 5 days. 

 On the other hand a similar culture in a highly buffered fermented 

 veal infusion bouillon plus 0.1 per cent glucose showed a slight increase 

 in acidity up to 48 hours and then became progressively alkaline reach- 

 ing a hydrogen ion concentration of pH 8,5 in five days. 



It is conceivable that there may be encountered an organism 

 of very low acid tolerance (e.g., pH 6.0 or 6.5) but which may 

 be an active fermenter of various carbohydrates so long as the 

 hydrogen ion concentration is kept down by a well buffered 

 medium. In such a case titration would reveal a considerable 

 amount of acid formed whereas the final hydrogen ion concen- 

 tration would lead one to believe that little or no fermentation 

 had occurred unless the buffer content of the medium was well 

 known. 



To be impressed by the importance of a knowledge of the 

 buffer content of media one needs only to note the frequent 

 references to it in the literature, notably the papers by Kligler 

 (1916), Berman and Rettger (1918), Bronfenbrenner and 

 Schlesinger (1918), H. Jones (1920a) and Wolf (1920). Most of 

 the discussion between the protagonists of titration and those of 

 hydrogen ion determination centers about the question of buffer 

 substances. By the use of a color standard of known hydrogen 

 ion concentration and a comparator block the titrationist need 

 no longer be embarrassed by the variable personal equation in 

 judging a poor end point, but to both the titrationist and the 

 hydrogen ion determinist the presence of variable and unknown 

 amounts of buffer substances in media constitutes a real 

 difficulty. 



A complete analysis of the reaction between buffer content 

 and growing culture would require a detailed knowledge of the 

 metabolism of the particular organism being cultivated, taking 

 into consideration the fact that the buffer content itself may be 

 modified by the culture. Nevertheless an index of the buffer 

 content at the beginning or at any time during the growth of 

 the culture is readily obtained by titrating the medium with 



