112 



OXIDATION-REDUCTION POTENTIALS 



cocci the potential falls very much less than in stationary aerobic cultures. Under 

 the conditions employed the fall in aerated cultures of these two organisms is only 

 some 45 per cent, and 5 per cent, respectively of the fall in aerobic cultures, whilst, 

 with staphylococci and C. diphtherice, the potential fall in aerated cultures is about 

 90 per cent, of the fall in aerobic cultures. After the phase of active proliferation 

 the potential in streptococcal and pneumococcal cultures rises rapidly to the high 

 oxidising level corresponding to peroxide formation, whilst in staphylococcal and 

 diphtheria bacillus cultures the potential rises only slowly and never reaches a high 

 level. 



The high level of electrode potential reached by streptococci and pneumococci 

 corresponds to the formation of hydrogen peroxide, which can be detected in the 

 cultures by reactions such as the benzidine-peroxidase test. [An acetic acid solution 

 of benzidine is added to the liquid or solid medium and a sliver of fresh potato is 

 added. If peroxide is present, the surface of the potato assumes a blue tint of 

 intensity proportional to the amoimt of peroxide present (Avery and Morgan, 1924).] 



+ •5 



+ •4 



+ •3 - 



+ •1 



■■°-°«oo,. 



^ 



I 



Plain broth 



/ 



\i... H2O2 broth / 



J t 1— J ! I I I I. 



4 8 12 16 20 24 28 32 36 40 44 48 

 Time (hours) 

 Fig. 24 

 Tryptic digest broth cultures of hsemolj'tic streptococci 



The potentials observed in cultures in which the presence of peroxide has been 

 detected varies from Ei,+0-42 to-f 0-51 volt according to the amount present. 

 It is worthy of note that broth to which, approximately, 0-006 per cent, of hydrogen 

 peroxide (A.E.) had been added had E^-f 0-48 v. and 0-02 per cent, hydrogen 

 peroxide in saline had £^+0-52 v. Bacterial peroxide is identical therefore, at 

 least in its electrode behaviour and chemical reactions, with hydrogen peroxide 

 (Hewitt, 1930, 1), but reproducible and stable electrode potentials are not observed 

 with peroxide and traces of peroxide disappear from cultures after reaction with 

 constituents of the medium. A typical wave is exerted by peroxides on the polaro- 

 graphic curve. (Chapter VIII.) 



It is evident that the potential-time curves of peroxide-forming bacteria are 

 quite different from those of the non-peroxide producers, but the question whether 

 the differences can be explained solely by the catalase content of the latter organisms 

 is not yet solved. Other enzymes cannot be without significance. 



