152 WINSLOW, ROTHBERG AND PARSONS 



The bromcresol purple indicator gives a reasonably sharp 

 color change between pH values of 5.2 and 6.8 but above 6.8 the 

 color change is slight and the adjustment of the final endpoint 

 of the peptone medium was therefore somewhat uncertain. 

 It was always, however, over 6.8. 



In general we found that glucose, lactose, maltose and sucrose 

 were fermented by a large majority of the cultures studied while 

 raffinose, mannitol, dulcitol, salicin and inulin were but rarely 

 attacked. The analysis of the results was materially compli- 

 cated, however, by the presence of three distinct modal points 

 of acidity and by the variation between the results obtained in 

 the two different media studied. 



In figure 1 we have presented the distribution of the pH values 

 recorded after seven days of incubation in the two types of 

 broth and with the four carbohydrates which were most readily 

 attacked. It will be noted that in nearly all the curves there 

 are three more or less distinct modes, one at a pH value between 

 5.0 and 5.3, indicating vigorous fermentation, a second at a pH 

 figure between 6.4 and 6.9, and a third at 7.4. In the succeeding 

 discussion we shall designate these three reactions as acid (+), 

 neutral (±), and alkaline (— ). A neutral reaction indicates no 

 change in the case of the dehydrated medium (which started at a 

 pH of 6.8) while it implies a slight production of acid in the case 

 of the peptone medium which started at 7.4. A neutral reaction 

 in the dehydrated medium and an alkaline reaction in the pep- 

 tone medium alike indicate no change in reaction while an 

 alkaline reaction in the dehydrated medium implies a real 

 decrease in acidity. 



In general, as might be expected, the dehydrated broth with 

 its slightly lower initial alkalinity, showed a higher final acidity 

 than did the peptone broth with its slightly higher initial alka- 

 linity. Out of 180 strains tested in glucose, 116 were ultimately 

 acid in both media, 29 were acid in dehydrated and neutral 

 in peptone broth, 8 were acid in dehydrated and alkaline (indicat- 

 ing no change) in peptone broth, 12 were neutral in dehydrated 

 and alkaline in peptone broth (indicating no change in either 

 case) and 16 were alkaline in both (indicating slight alkali pro- 



