108 



OXIDATION-REDUCTION POTENTIALS 



Typical curves relating the electrode potentials of aerobic cultures of hsemolytic 

 streptococci and C. diphtherice (Hewitt, 1930, 2) at various stages of development 

 are given in fig. 20. A small inoculum of hsemolytic streptococci was made into 

 sterile peptone-infusion broth and even after 30 minutes the E^ had commenced to 

 fall, a minimum value ( — 0-16 volt) being reached in 12 hours : this is approxi- 

 mately the duration of the logarithmic phase of growth and is characterised, as can 

 be seen, by intense reducing activities. After this period the death rate of the 



o 



W 



100 120 140 



Time (hours) 



Fig. 20 



Electrode potential : time curves of Infusion broth cultures of hsemolytic streptococci and C. diphtherice 



organisms becomes appreciable and the potential rises. Reducing conditions are 

 not maintained under aerobic conditions when the organisms have ceased actively 

 to proliferate. This increase in potential after the cessation of active multiplication 

 is not, however, a general phenomenon exhibited by all organisms. In the case of 

 C. diphthericB, for example, of which the corresponding curve is given in the same 

 figure the potential falls to — 0*2 volt, rather more slowly than with hsemolytic 

 streptococci, but this level is maintained for some days. 



When cultures are aerated, it is found, as would be anticipated, that the generous 

 supply of oxygen prevents to some extent the establishment of intense reducing 

 conditions. Thus in the case of hsemolytic streptococci (top curves in fig. 20) the 

 potential in ordinary aerobic cultures falls to — 0-16 volt, whilst in specially aerated 

 cultures it does not fall below + 0-1 volt, and a marked effect is seen after the phase 

 of active proliferation. In the aerated culture, the potential rises rapidly and reaches 

 -f 0*45 volt, a highly oxidising level, and peroxide may be detected in the culture. 



In the case of C. diphtheria} the aeration has some effect (fig. 21) in preventing 

 the attainment of reducing conditions, but the effect is negligible compared with 

 that observed in the case of hsemolytic streptococci. The electrode potential rises 



