382 METABOLISM OF MICROORGANISMS 



compounds appears to be the same as in the oxidative pathway of aerobic 

 organisms : 



Glucose *■ glucose-6-phosphate *• gluconic acid-6-phosphate 



(CO2 



2-ketogluconic acid-6-phosphate 



I ribulose-5-phosphate 



{C2 compound *■ ethyl alcohol or acetic acid 

 triose-3-phosphate *" lactic acid (+ H3PO 4) 



The fermentation of glucose becomes a pentose fermentation after the 

 loss of carbon 1. Tlie latter part of this formulation fits well for the 

 fermentation of pentoses by the same bacteria. A main difference be- 

 tween the two is that ethyl alcohol is a major product in the fermentation 

 of glucose; it is absent from that of pentose. On the other hand, acetic 

 acid is usually a minor product from glucose and a major product from 

 pentoses. The explanation for these differences probably lies in the 

 accumulation of a reduced coenzyme, perhaps DPN • Ho, in the glucose 

 fermentation. The accumulated coenzyme may then be used for the 

 reduction of the C2 intermediate to give ethyl alcohol. In the fermenta- 

 tion of the pentoses there is no such accumulation of reduced coenzyme. 



The discussion just given applies to the aldohexoses (glucose, galactose, 

 and mannose) but does not fit the fermentation of fructose. Fructose 

 gives approximately 1 mole each of carbon dioxide, acetic acid, and lactic 

 acid, no ethyl alcohol, and 2 moles of mannitol. IMannitol is evidently 

 formed by reduction of other molecules of fructose and appears to be 

 paired with acetic acid as a reduction product. The interrelations of the 

 two may be seen from the following equations: 



CgHisOe + H2O -> COo + CH3COOH + CH3CHOHCOOH + 4H 

 2C6H10O6 + 4H -^ 2C6H14O6 

 Fructose Mannitol 



Usually less than 2 moles of mannitol are obtained per 3 moles of fruc- 

 tose fermented. 



The reasons for the differences between fermentation of glucose and 

 fructose will probably appear when labeled fructose can be used and 

 data are obtained regarding the intermediary compounds that are formed. 

 To date, there are no such data. 



The mechanisms of other fermentations have been studied extensively, 

 but space in this chapter does not permit a detailed discussion of the 

 various steps leading from substrate to product. Only a few short state- 

 ments and overall equations will be given. 



Propionic acid appears to be either a reduction product of lactic acid 



