BACTERIAL CULTURES 107 



of fixed characteristics but constantly changing systems for ever in a state of flux. 

 This proves a source of great difficulty to many students of the problems presented. 

 In a bacterial culture, not only is the oxidation-reduction potential changing (as we 

 shall show) but also the hydrogen-ion concentration, the chemical composition, the 

 number and condition of the organisms and so on. To escape from these difficulties, 

 investigators have tried to fix the hydrogen-ion concentration by means of buffer 

 salts, to determine the chemical composition by use of synthetic culture media, and 

 to stabilise the numbers of organisms by the use of thick suspensions of washed 

 bacteria. These attempts are, however, doomed at the outset. From its very 

 nature a bacterial culture is a constantly changing system, and if any factor is held 

 artificially at a fixed level the culture is no longer the natural system with which the 

 bacteriologist normally has to deal. Attempts to make artificial corrections for 

 various changes, e.g., to make a correction, arbitrarily fixed for pH changes, are also 

 useless. Knight (1930) has shown that, in the batch of culture broth used by him, 

 the curve connecting electrode potential with pH varied between limiting " slopes " 

 of 0-03 and 0-12 (a 300 per cent, variation) in the " physiological " range. 



In synthetic media cultures of bacteria Ward (1938) finds the oxidation-reduction 

 potentials more difficult to measure than in broth cultures, probably owing to the 

 presence of poising and indicator systems in ordinary broth media and their absence 

 from synthetic mixtures. 



Another method of " standardising " conditions, namely, the attempt to fix the 

 pH of culture media by buffering the broth, is full of pitfalls. A common method is 

 to use phosphates for the purpose. Apart from the relative inefficiency of phos- 

 phates for buffering in the usual bacteriological range of pH, there is the fact that 

 phosphates have other effects on culture media. When autoclaved with phosphates, 

 broth darkens and valuable constituents are destroyed, and, on the other hand, 

 phosphates have an accelerating effect on carbohydrate metabolism (Hewitt, 1932). 

 Thus the attempt to simplify the problem by pH fixation results in the introduction, 

 of further complications. 



Various attempts have been made to imitate the effects produced in bacterial 

 cultures by bubbling nitrogen through culture media. Boyd and Reed (1931) have 

 shown, however, that the potentials registered by inert electrodes immersed in culture 

 media through which various gases are bubbled are the same as the potentials 

 obtained under similar conditions with electrodes immersed in salt solutions. This 

 observation, taken together with the obvious fact that the function of proliferating 

 bacteria is so manifestly different from that of an inert gas, would suggest that little 

 of significance can be expected from continued studies of the effects of bubbling 

 nitrogen through culture media. 



The bacteriologist frequently has to deal with pathogenic organisms requiring 

 a rich and complex medium for typical proliferation to occur. The investigator 

 cannot afford to ignore such systems if he wishes to gain an insight into some of the 

 major problems of bacteriology and infection. 



In the author's experiments and in an increasing number of those of others 

 the conditions met with in ordinary bacteriological technique have been retained,, 

 as far as possible, in the study of electrode potentials. 



