28 MAX LEVINE 



H-f ion concentration is reached. The ratio of the total titratable 

 acid formed to the maximal or limiting H-f-ion concentration is not 

 constant, but varies, within limits, with the amount of buffer materials 

 present in the medium. 



The limiting H-| ion concentration may be an index of (1) the 

 resistance of an organism to acid (H-f- ions), or (2) the point of 

 equilibrium between the decomposing carbohydrate and its end prod- 

 ucts under the influence of an organism. 



If the limiting H-| ion concentration in glucose broth is such as 

 to inhibit further growth of the organism, then the organism will die 

 and the H-| ion concentration will rejpiain constant. This seems to 

 be the course of events with the Voges-Proskauer-negative group. 

 With the Voges-Proskauer-positive organisms the H-] ion concentra- 

 tion rises to a maximum and then decreases, tjhe medium becoming 

 alkaline to methyl red. Under these conditions it is inferred that the 

 maximal H-f^ion concentration is a measure of the point of equili- 

 brium between glucose and its end products under the influence of the 

 organism in question. 



It may be further considered that after the limiting H+- ion con- 

 centration is reached, the organism, if not destroyed, will, if capable, 

 attack the peptones forming alkali. Some of the free acid becomes 

 neutralized and more carbohydrate may be decomposed. The H-J-- ion 

 concentration would remain constant as long as there is any ferment- 

 able carbohydrate present. If this assumption is correct, then an 

 increase of the carbohydrate should retard the reversion from an acid 

 to an alkaline reaction. This is exactly what takes place. In some 

 work now in progress it has been found that Voges-Proskauer-positive 

 strains were alkaline to methyl red after 24 hours' incubation in 0.5% 

 peptone dipotassium-phosphate solution containing 0.5% glucose. In 

 the -same medium with 1% glucose, the reaction was acid after 24 

 hours but alkaline after from 48 to 72 hours. With 2% glucose, the 

 acid reaction persisted until the 4th or 5th day. With 5% glucose 

 there was no reversion to an alkaline reaction even after several weeks. 



THE CORRELATION OF ACID- AND GAS-FORMATION 



Table 12 shows the relation of gas-production to the amount of 

 acid formed from sucrose, raffinose, dulcitol, glycerol, and salicin. 

 The other test substances are not indicated because they were invari- 

 ably fermented with production of gas. Cultures are regarded as gas- 



