96 PHYSIOLOGY OF BACTERIA 



Another example of poising is shown in Fig. 9 which 

 represents potentials in the same milk with two different 

 dyes as measured by Thornton and Hastings (1929b). 

 Methylene blue has only a negligible poising effect 

 as was shown in other experiments. Janus green Bj 

 however, though it stimulated reduction at first, did not 

 let the potential drop as suddenly as is the custom in 

 milk, or in milk with methylene blue ; it came down very 

 slowly, and not to as low an end point as with methylene 

 blue. 



Cohen (1931) added substances of the type of potas- 

 sium ferricyanide or benzoquinone which, after partial 

 reduction by the bacteria, maintained a readily reversible 

 oxidation-reduction system in the medium. 



(e) THE FACTORS INVOLVED IN REDUCTION 



All living protoplasm seems to have the power to 

 effect chemical reductions, and this holds true also with 

 bacterial cells, and consequently with bacterial cultures. 

 With some of them, this power is very pronounced, as 

 with anaerobic bacteria. 



The power to reduce is identical with the power to 

 oxidize, both being accomplished simultaneously by 

 the transferring of hydrogen from one molecule to 

 another. The reduced substance may eventually be 

 the oxygen gas dissolved in the medium. 



Aerobic bacteria will soon exhaust the oxygen dissolved 

 in a medium, and the medium will then remain free from 

 oxygen as long as the metabolism of the bacteria remains 

 active, excepting a very thin surface layer into which 

 the oxygen diffuses from the air (see p. 80). Many 

 anaerobic bacteria can accomplish the same if they are 

 present in sufficient quantity at the start to overcome 

 the harmful effect of the oxygen. 



