236 L. A. ROGERS, W. M. CLARK AND H. A. LUBS 



particular sugar which is being fermented so that an analysis of 

 the situation and the determination of those factors which are of 

 fundamental importance in the fermentation of any given sub- 

 stance require more study than the mere determination of the 

 Ph values attained in media indiscriminately put together. 

 Studies of the fermentation of glucose have been sufficiently 

 extended to enable Clark and Lubs (1915) to devise the so-called 

 methyl red test for the differentiation of the high and low gas 

 ratio groups. This test has been used in the present study and 

 has been found to correlate perfectly with the gas ratios. 



In testing the fermentation of other carbohydrates we have 

 attached no particular significance to the Ph values but have used 

 them merely to indicate whether a distinct acid fermentation has 

 been dominant. It may be said, however, that the Ph values 

 observed with similar organisms are not only fairly constant in 

 media containing any easily fermented carbohydrate but they 

 are obtained with much greater ease and convenience than the 

 titration values formerly used in such tests. 



In certain instances it has been difficult to decide whether the 

 Ph values indicate a fermentation. This is particularly true 

 of the fermentation of glycerol. The Ph of the medium changes 

 very slowly and we suspect that in many instances there may have 

 been a concomitant formation of alkali or destruction of acid 

 which masked whatever acid formation may have occurred. A 

 few examples showing how slow is the change in Ph in glycerol 

 cultures are given in table 2. Results with sucrose, dulcitol and 

 glycerol are assembled in figure 1 . In dulcitol and sucrose broths 

 there are two distinct modes separating the cultures sharply into 

 fermenters and non-fermenters, but with glycerol there is only 

 one distinct mode and that falls between the modes for dulcite 

 and sucrose. It is evident that nearly all of these cultures 

 ferment glycerol to some extent. Those cultures which show 

 little or no change from the original reaction would possibly also 

 be classed as fermenters if they were allowed sufficient time. 



Notwithstanding the difficulty of making a sharp distinction in 

 all cases between fermenters and non-fermenters by means of 

 acidity determinations we prefer this to the gaseous fermentation 



