86 



PHYSIOLOGY OF BACTERIA 



differences, however, as in Figs. 3 and 8, the difference 

 becomes larger as the Eh potential drops. This accounts 

 for the two different types of curves representing reduc- 

 tion potentials. 



Cannan, Cohen and Clark (Clark, Cohen and assoc. 1928, X) 

 obtained the curves shown in Fig. 4 representing the reduction 

 potentials of a culture of Bad. coli and of Bad. vulgare, on the stand- 

 ard or Eh scale. The potential of the colon culture drops considerably 



Fig. 4. 



-Reduction potential of Bad. coli (A); hydrogen potential of Bad. 

 coli (B), and reduction potential of Bad. vulgare (C). 



lower than that of the proteus culture. The time is plotted logarith- 

 mically. The third curve, B, is the potential of a hydrogen electrode 

 in the colon culture. After ten hours (log 10 = 1.0), the reduction 

 potential of the colon culture drops sHghtly below that of the hydro- 

 gen electrode. This is in agreement with Gillespie's data on p. 84. 

 Hydrogen gas can be formed only if these two potentials are the 

 same. 



Several interesting potential-time curves are also given by Thorn- 

 ton and Hastings (1929a and b). Raw milk showed a potential 

 between -1-0.200 and -1-0.300 volts when fresh, and was finally 

 reduced to about —0.200 volts, probably by the action of streptococci 

 (see Fig. 9). 



The change of potential with time, with the medium, with aeration 

 and with different species has been studied in some detail by Hewitt 

 (1930a, b, c, d). Streptococci and Pneumococci produce peroxide; 

 with the latter, aeration brings the potential above that of the sterile 



