106 OXIDATION-REDUCTION POTENTIALS 



Bacterial cultures develop reducing conditions not so much because of the liberation 

 of reducing substances as on account of the exertion of reducing activities, although, 

 of course, the potential cannot fall unless a reducing substance is present. 



At present our knowledge of the systems present in bacterial cultures is 

 incomplete, but the data accumulating from electrometric studies are giving us some 

 insight into the differences between different culture media and different organisms. 

 The conditions governing the potentials established in bacterial cultures must be 

 complicated and amongst the important factors are the following : — 



(i) Kind of organism — Class, strain, etc. 



(ii) Condition of organism- — Previous history, colony form, etc. 

 (iii) Growth phase of the culture. 



(iv) Nature of constituents and their quantities in culture medium, 

 (v) Conditions of cultivation. 



It is not proposed to attempt a full description of the many publications dealing 

 with the reduction of dyes in bacteriological cultures. That many dyes are reduced 

 in bacterial cultures is well known, and the ease and rapidity of reduction varies with 

 the potential range over which the dyes are reduced. The lower the electrode- 

 potentials at which the dyes are reduced, the slower, in general, is the reduction of 

 the dyes by cultures. Differences in ease of reduction of methylene blue have been 

 used for the differentiation of organisms (Avery, 1922). Applications of dye reduction 

 phenomena are found in testing milk (Schardinger, Thornton and Hastings, Wilson). 

 and the putrescibility of sewage (Spitta and Weldert, Wooldridge and Standfast), 

 detecting traces of oxygen in anaerobic apparatus (Hall, 1921), etc., and the study 

 of enzvmes in Thunberg tubes. 



Before dyes can be used for the study of most of the wider problems of bac- 

 teriology it is essential that the fundamental facts be established and the initial 

 difficulties removed by direct potentiometric studies of reducing conditions in 

 bacterial cultures. At the present stage the disadvantages attaching to the use of 

 dyes are almost insuperable, and the discrepancies between the results obtained by 

 colorimetric methods and those by electrometric measurement are considerable. 



Although the measurements and interpretations of electrode potentials in 

 bacterial cultures are still somewhat empirical, the im23ortance and interest of the 

 subject warrant, in the author's opinion, the space devoted to their consideration. 



ELECTRODE POTENTIALS OBSERVED IN BACTERIAL CULTURES 



Potter (1911) first noted that the electrode potential of inoculated culture 

 medium was lower {i.e., more reducing) than the sterile culture medium. The develop- 

 ment of reducing conditions was observed by following the fall in potential first 

 by Gillespie (1920). This has been confirmed by Cannan, Cohen and Clark (1926) 

 and many others. A systematic and comprehensive study of electrode potentials 

 of bacterial cultures should assist considerably in the understanding of many 

 bacteriological problems. 



Perhaps the greatest difficulty in dealing with biological systems generally and 

 with bacterial cultures in particular is that we have not rigidly defined static systems 



