126 OXIDATION-REDUCTION POTENTIALS 



B. fluorescens and Proteus vulgaris. — In cultures of B. fluorescens the potential 

 reached a low level, but with Proteus vulgaris the potential fell slowly and did not 

 attain a low value. (Cannan, Cohen and Clark, 1926.) 



Denitrifying bacteria. — Elema (1932) found that the potentials of cultures of 

 Pseudomonas pyocyanea fell rather slowly to a moderately low level. In a synthetic 

 medium composed of potassium nitrate, potassium phosphate and ethyl alcohol the 

 potentials of cultures of B. vulpinum fell to the level —0-116 v. in 7 hours. Similar 

 results were obtained with M. denitrijicans. 



Elema, Kluyver and van Dalfsen (1934) have followed the electrode potentials 

 reached in anaerobic cultures of M. denitrijicans. The culture medium contained 

 phosphate, alcohol and nitrite, and as long as nitrite was present there was a poising 

 effect on the potentials but as soon as all the nitrite had been converted to nitrogen 

 the potential fell to a low level. Addition of nitrite now caused a rise in potential. 

 HCN retarded the disappearance of the nitrite and the corresponding fall in potential. 

 Zobell (1935) investigated the optimum E^ for marine nitrifying organisms. 



Brux^ella. — Tuttle and Huddleson (1934) find that in liver infusion broth the 

 electrode potential of cultures of Brucella fell to a level of E,, between -|-0-09 and 

 +0*15 v. in 8 days ; the Br. suis potential reached a slightly lower level than that of 

 Br. abortus, or Br. melitensis. In the presence of the oxidation-reduction indicator 

 dye thionine Br. abortus failed to reduce the potential of the medium. The potential 

 did not fall in Br. suis cultures containing basic fuchsine. In the case of Br. meli- 

 tensis cultures neither thionine nor basic fuchsine interfered with the fall in potential. 

 In describing their experiments the authors emphasise a point which is evident to 

 practical bacteriologists but frequently overlooked in theoretical studies. " It is 

 ob\aous that in making a comparative study of this nature on closely related aerobic 

 bacteria, the growth conditions that are maintained should closely approximate 

 those normally used in growing the organism." 



Ijactobacillus Casei. — Oxidation-reduction potentials in growing cultures of 

 Lactobacilhis Casei have been followed by Wilhams and Fieger (1947) with results 

 strikingly similar to those obtained with hsemolytic streptococci, (Hewitt, 1930) 

 The original fall in potential was followed by a rise in older cultures, presumably due 

 to peroxide formation. The stimulation of growth and metabolic activity by certain 

 non-ionic synthetic detergents containing oleates, and by biotin was reflected in the 

 electrode potential-time curves. 



SULPHATE-REDUCING BACTERIA 



It is not possible to survey the history of the investigation of sulphate-reducing 

 bacteria during the past fifty years, but their ubiquitous distribution, their importance 

 in the terrestrial and marine sulphur cycle, their nuisance value and the theoretical 

 problems raised by their metabolic problems are of too great importance to be ignored. 



They reduce sulphates to hydrogen sulphide which contributes to the odours of 

 putrefaction and has resulted in contamination of our coal and oil supplies. The 

 sulphate reducers are responsible for the anaerobic corrosion of iron pipes in the 

 ground. Plant needs for sulphur are comparatively small, but are very important. 



The autotrophic mechanism of the sulphate reducing strains is not yet clarified. 

 Hydrogen is oxidised presumably by an hydrogenase system and energy is thus made 



