ENERGY SUPPLY OF THE CELL 99 



facultative microorganisms will remove dissolved oxygen 

 from the medium in which they grow, the potential 

 difference between a sterile medium and a culture in the 

 same medium may be due only to the removal of oxygen 

 which brings the natural reducing properties of the 

 medium into play. Thus, Thornton and Hastings 

 (1929) confirm the conclusion of Barthel that the reduc- 

 tion of methylene blue in the so-called reductase test of 

 milk takes place in two stages: (1) the removal of dis- 

 solved oxygen by bacteria, (2) the reduction of the dye by 

 constituents of the milk. 



Only if the potential of a culture drops below that 

 observed in the oxygen-free medium, are we justified in 

 stating that we are dealing with a reduction potential 

 characteristic for that microorganism. 



It seems, from all these experiments, that the oxida- 

 tion-reduction potential in bacterial cultures is compara- 

 tively simple. The cells of bacteria, as such, do not 

 establish the potential. If cells are washed once and 

 suspended in salt solution, they are still able to hold a 

 distinct potential, but after repeated washing, it becomes 

 indefinite. The potential must be produced by some 

 metabolic products of the bacteria. It cannot be 

 doubted that a definite potential is established inside 

 of the cells, but this cannot be measured in the cultures. 



An oxidation-reduction system must contain a reversi- 

 bly oxidizable substance. This substance may act as a 

 catalyst and transfer the oxygen to another substance 

 which is irreversibly oxidized. 



The reversibly oxidizable compound is represented by 

 compounds like glutathione, cysteine, thioglycollic acid, 

 or perhaps by a dye (all oxidation-reduction indicators 

 being reversible) or even by an oxidizing enzyme. 

 Methylene blue is a very active catalyst of cell respiration. 



