120 



OXIDATION-REDUCTION POTENTIALS 



With heemolytic streptococci the potential in aerated 1 per cent, glucose cultures 

 falls very little despite the very luxuriant growth observed and no peroxide is 

 detected. No obvious relation was noted between the amount of growth and the 

 fall in potential of bacterial cultures. 



With C. diphtJiericB and staphylococci the organisms are able to utilise inter- 

 mediate fermentation products of glucose {e.g., lactic acid) in the presence of a generous 

 oxygen supply, so that in some cases a vigorously aerated culture may be more 

 reducing than in an otherwise similar ordinary aerobic culture. 



+ 0-4k 



o 



W 



Time (hours) 



Fig. 30 



Glucose broth cultures of B. coli 



The measurement of electrode potentials of bacterial cultures reveals distinct 

 diiferences between the effects of different media and cultural methods on the con- 

 ditions to which the proliferating organisms are subject. The characteristic effects 

 of the presence of serum and glucose on the oxidation-reduction conditions in cultures, 

 the effect of aeration and the differences betw^een peptone-infusion broth and tryptic- 

 digest media have already been mentioned. Even differences in the time since 

 heating a culture medium are reflected in the potential time curve, since it is found 

 that the potential falls more rapidly with some organisms when the medium has been 

 freshly heated, and, particularly in the case of tryptic digest medium, the potential 

 in streptococcal cultures rises more rapidly in the freshly boiled medium, with con- 

 comitant more rapid production of peroxide. These differences may be correlated 

 with the observation of Dubos (1929) that smaller implants of organism are necessary 

 for cultivation in freshly heated medium. 



Different batches of the same type of broth vary considerably, as judged by the 

 potential-time curves of cultures as well as by observations of the luxuriance of 

 growth, amount of toxin production, etc. Particularly marked were the diiferences 

 observed between different batches of tryptic-digest broth, which is not surprising 

 when the difficulties in controlling the processes of digestion are considered. Different 

 samples of serum have varying effects on the potentials of cultures. These variations 



