102 PHYSIOLOGY OF BACTERIA 



With some anaerobic and facultative bacteria, the 

 potential is very low. With those organisms producing 

 hydrogen gas, the reduction potential equals or approxi- 

 mates that of the hydrogen electrode at the same pH. 



The reduction potential is quite meaningless unless 

 we also know the pH of the culture. 



V. RATE OF FERMENTATION 



(a) METHODS OF MEASURING THE RATE OF FERMENTATION 



The very rapid rate of multiplication of bacteria makes 

 it difficult to perform experiments with a constant 

 number of acting cells. The respiration of a leaf or 

 the oxygen consumption of a mouse can be measured 

 over a period of several hours without the organism 

 changing considerably in size or weight during the obser- 

 vation. With bacteria, we may have a doubling of the 

 active mass in less than half an hour. It requires 

 special precautions, therefore, if the rate of fermentation, 

 i.e. the amount of products per cell, or per gram of cells, 

 or per gram of cell solids, is to be determined. 



The choice of the unit is rather difficult to make. 

 Physiologists working with larger organisms usually take 

 the weight of the organism or tissue as the basis of 

 measurement. Feeding experiments with animals are 

 computed on the basis of kilogram body weight. Since 

 this includes bones and fat, the proportions of which vary 

 greatly, the basis is not really ideal, but it is more accu- 

 rate than the basis of the single organism. 



With microorganisms, the basis by weight would be 

 possible, but rather difficult to ascertain. It is compara- 

 tively easy with yeast, but much more difiicult with 

 bacteria. The determination of the weight of bacterial 

 cells is possible only if they can be filtered or centrifuged 

 from their medium. It is impossible with cultures in 



