BACTERIAL REDUCTIONS 



105 



and hence obtain the energy necessary for life and proliferation. Definite electrode 

 potentials are established in cultures, but we are not fully aware what substances 

 are responsible for these potentials in many cases, nor what reversible oxidation- 

 reduction systems are actually being measured. Nevertheless, valuable conclusions 

 may be drawn from such results. The effect of some of these systems, present in 

 minute quantities, may be merely to indicate the potential of the system — the 

 quantity present may be too small to affect the oxidation-reduction conditions of 



Time 



Time 



Time 



Fig. 19 

 El, : time curves of hypothetical cultures (continuous lines). (Broken line represents Ei, : percentage 



oxidation curve of poising system present.) 



the culture. Other systems will have a poising effect and tend to obstruct oxidation 

 or reduction processes since they themselves have to be oxidised or reduced before 

 the level of electrode potential can be altered appreciably. It will be seen, therefore, 

 that the quantity present of such systems, as well as their oxidation-reduction level 

 of intensity will affect the drift in potential. Thus, in the curves above (fig. 19), 

 the continuous lines represent the fall in electrode potential of hypothetical bacterial 

 cultures during growth and the dotted lines represent the ordinary curve of an 

 oxidation-reduction system, showing the potentials corresponding to different 

 percentages of reduction. In I, the amount of the oxidation-reduction system is 

 so small that it does not affect the potential-time curve of the culture ; in II, the 

 amount is greater, but the culture is easily able to reduce it, so a small kink on the 

 curve is seen ; in III, the capacity of the system is so large that the culture is 

 imable fully to reduce it, so that the potential is poised and prevented from falling 

 to the usual low level. The same effect might be obtained by keeping the oxidation- 

 reduction system constant and using organisms of different reducing powers, I 

 being the most intense reducer and III the least. 



If the broken line in the figure is taken to represent an oxidation-reduction 

 indicator dye, then in system I it would be rapidly and completely reduced, in II 

 slowly reduced and in III it would remain in the oxidised form. 



On the views presented bacterial cultures are to be regarded as dynamic, con- 

 stantly altering systems and not static, fixed systems of unchanging properties. 



