452 Physiology 



varies with pH. For the methylene blue system, Eq at pH 7 is 11 mv; for 

 the cytochrome a system, 290 mv at pH 7.4; for lactate/pyruvate, —180 mv 

 at pH 7. If two systems with different E^' values are mixed together, a 

 reaction, which may be considered the transfer of electrons from one sys- 

 tem to the other, continues until equilibrium is reached. The higher 

 potential is lowered and the lower potential raised to a common level; or 

 reduction of the first system (gain of electrons) and oxidation of the 

 second (loss of electrons) take place. 



The potential of the culture medium undoubtedly influences the 

 growth of inicroorganisms. For bacteria, it is possible to lower the po- 

 tential of a liquid medium with a suitable reducing agent so as to inhibit 

 growth of aerobes and permit growth of anaerobes. In the case of Chilo- 

 monas Paramecium, appropriate additions of the sulfhydryl radical (— SH) 

 both lower the oxidation-reduction potential of the medium and stimu- 

 late growth (238). Influence of the potential on Entamoeba histolytica 

 apparently varies with the type of medium. According to one worker, 

 growth of E. histolytica decreases from a maximum, at a potential below 

 — 300 mv, to almost none at —200 mv; unencysted amoebae die after an 

 hour or more at —50 mv (56). Jacobs (234), on the other hand, found that 

 the potential of the medium was about —25 mv while E. histolytica was 

 growing most rapidly in cultures containing "organism t." In contrast to 

 E. histolytica, Trypanosoma criizi and several species of Leishmania grow 

 best in cultures at a potential of about 330 mv (57). Growth of micro- 

 organisms themselves also may modify the potential. For instance, a drop 

 of about 290 mv has been traced in cidtures of Chilomonas Paramecium 

 (241). It is uncertain just how extensively the potential of the medium 

 influences internal oxidations and reductions, although the "internal 

 potential" of Amoeba proteus, as measured by injected indicators, may 

 be changed from about —70 mv (pH 7) under aerobic conditions to —143 

 mv in anaerobiosis (55, 70). 



Oxygen consumption^** 



Measurements of oxygen consumption make it possible to trace 

 effects of environmental factors on metabolic rates, to investigate the 

 utilization of particular substrates, and to correlate stages in the life-cycle 

 with metabolic activity. Such measurements are necessary in the study of 

 oxidative mechanisms by the use of poisons or stimulants, and may indi- 

 cate the relative importance of particular systems, such as the cytochrome 

 system, in the metabolism of a particular species. Manometric techniques 

 can be used also in the estimation of specific enzyme systems and meta- 

 bolites. Comparative data on oxygen consumption of different species 



" A monograph by Umbreit and his associates (560) supplies a comprehensive survey 

 of manometric techniques and their various applications. The earlier literature on 

 Protozoa has been reviewed by Jahn (246). 



