98 OXIDATION-REDUCTION POTENTIALS 



and presumably cellular enzymes are still capable of activity even though the synthesis 

 of new cells has slowed down. It is possible that the distribution of enzyme systems in 

 bacterial cells varies with age. In the logarithmic growth phase the syntheses of brand 

 new enzyme systems for the multiplying cells must follow one another so rapidly that 

 it is difficult to conceive or observe a sequence of enzyme distributions, but when the 

 tempo of multiplication has become less hectic it is possible to visualise a succession of 

 enzyme distributions, one type of reaction predominating for a time to be followed by 

 another, perhaps by the unmasking of reaction catalysts previously unimportant on 

 account of their slowness, but assuming priority when the faster reaction catalysts 

 have decayed. In many biologically important systems the slow ripening processes in 

 the autumnal harvesting may be as important as the vernal growth. 



REDUCING CONDITIONS IN BACTERIAL CULTURES 



It is a commonplace that bacterial cultures develop reducing conditions. Dyes 

 such as methylene blue added to cultures are reduced ; samples of milk contaminated 

 with many bacteria rapidly reduce methylene blue (or resazurin) when incubated — 

 this constitutes an official test for contaminated milk. As will be seen in many later 

 curves in this chapter the oxidation-reduction potentials in bacterial cultures begin 

 to fall soon after the fresh sub-culture is made, the lowest }>otential being reached when 

 the metabolic activities of the bacteria are most intense in the logarithmic growth 

 phase. In electrode potentials we have a quantitative ■measure of the reducing inten- 

 sities of systems as indicated in Chapter I. 



What is the reason for the development of reducing conditions in bacterial 

 cultures ? 



Coulter and Isaacs (1929) discuss the mechanism of establishment of reducing 

 oxidations in bacterial cultures {B. typhosus). They suggest that during the phases 

 of active proliferation the organisms exhaust all the free oxygen in the medium 

 " thus permitting the characteristic potential of the culture medium to become 

 manifest." When the phase of active multiplication of the organisms is over and 

 the " rate of dying of the bacteria approaches and exceeds the rate of multiplication, 

 it is suggested that dissolution of bacteria liberates reductive substances," and more 

 negative potentials are observed. 



However not only are bacteria able to remove free oxygen from the medium 

 but they are able to effect other reductive processes outside the cell in accord with 

 their nutritive requirements. Coulter and Isaacs state that " the role of the bacteria 

 is the establishment of the anaerobic state " and that " this point of view is opposed 

 to that which regards the potentials of bacterial cultures as a manifestation of 

 reductive substances liberated by the bacteria." It seems reasonable, however, 

 to assume that neither of these hypotheses provides the underlying reason for the 

 reducing conditions in bacterial cultures, but that the bacteria effect reduction as 

 a natural result of metabolic activities. To obtain the energy necessary for prolifera- 

 tion, food materials in the broth nmst be oxidised, and for this purpose free 

 oxygen, combined oxygen, hydrogen acceptors (or electron acceptors) or any 

 suitable type of oxidising agent will be utilised by the cell. The actual agents 

 used by the cell for the oxidations will depend on the enzymes and catalysts available, 

 and one after another of the oxidising systems will be reduced as the cell calls its 



