BACTERIA OF THE COLON TYPE IN HUMAN INTESTINES 235 



kept clear and it is therefore necessary to emphasize the fact that 

 we are in this paper deahng with the production of acid in carbo- 

 hydrate media only as a cultural test. 



It is now well established that the activity of cultures of B. coli 

 is inhibited within a zone of Jhydrogen ion concentration near 

 Ph 5.0 (Michaelis and Marcora, 1912; Clark, 1915). In any 

 particular carbohydrate medium various cultures of B. coli 

 arrive at a rather definite point and in media of various composi- 

 tion such points are generally not far apart (Clark, 1915). It 

 has been clearly established that the concentration of the hydro- 

 gen ions is of far greater significance than the titratable acidity. 

 But it can hardly be expected that any one definite hydrogen ion 

 concentration would be found to characterize an organism in all 

 media. If the hydrogen ion concentration exerts a constant 

 influence upon the metabolism or growth of a culture the inhibi- 

 tion will become more pronounced as time passes so that a lower 

 hydrogen ion concentration will be attained in a prolonged 

 fermentation than in a rapid one. There are doubtless other 

 substances formed which inhibit activity and the effect of these 

 will be proportional to the time in which they act and to their 

 concentration. There also may be present in a culture enzymatic 

 actions proceeding independently of growth and general me- 

 tabolism. Then too there are those opposing processes of acid 

 production and destruction which the work of Ayers and Rupp 

 (1917) has emphasized. In some cases the reaction observed 

 at any time may be merely the resultant of these opposing 

 processes. 



The last condition is one which doubtless plays an important 

 role in fermentations by the group of high ratio organisms. The 

 tendency of these cultures to undergo a comparatively rapid 

 reversion of reaction makes it difficult except under anaerobic 

 conditions to supply enough sugar and to balance the buffer 

 action of the medium so that the cultures can reach their limiting 

 hydrogen ion concentrations. Apparently their limit is about 

 the same as that of the B. coli or low gas ratio cultures. 



All of these factors which enter into the determination of a 

 reaction observed after any given period of time vary with the 



