78 



OXIDATION-REDUCTION POTENTIALS 



conditions, as would be expected. Kesults pointing in the same direction are obtained 

 by both potential measurements and dye methods but effects are complicated in the 

 latter case by the poising action and capacity factor of the dyes. 



Methylene blue is a thiazine dye of the following formula : — 



Methylene Blue 



(C "4Ny^\/S^^^s^N(CH^^ 



N(CH^ 



H 

 leuco-Methylene Blue 



Tetrathionate has been added to culture media to assist in the isolation of 

 Salmonella. A number of cohform organisms possess an enzyme tetrathionase 

 catalysing the interconversion of thiosulphate and tetrathionate in the presence of 

 hydrogen donators or acceptors. 



H2S4O6 + 2H ^ 2H2S2O3 



Jebb (1949) has found that when a coliform organism with tetrathionase activity 

 was added to reduced nile blue in a Thunberg tube the nile blue regained its colour 

 due to transference of hydrogen to the tetrathionate. Nile blue is one of the oxazine 

 dyes which are similar in constitution to thiazine dyes like methylene blue, but in 

 which the sulphur atom in the ring is replaced by oxygen. Nile blue is reduced at a 

 lower oxidation-reduction potential than methylene blue. 



A variation of the dye decolourisation technique has been used by Kun and 

 Abood (1949). Triphenyltetrazonium chloride is reducible to a red insoluble dye, so 

 that when it is added to a dehydrogenase system the red precipitate can be separated, 

 dissolved in an organic solvent and determined colorimetrically and photometric 

 determinations directly on the reduced system, in the Thunberg tube as in the 

 methylene blue technique, are unnecessary for quantitative results. 



Some experiments on passing nitrogen through suspensions of washed bacteria 

 have been described by Yudkin (1935). He found, as would be expected, slow 

 drifts of potential and the greater the damage to the cells by storage the more definite 

 were the potentials observed. Moribund bacteria in the absence of an appropriate 

 culture medium cannot estabhsh an electrode potential, but a certain amount of 

 lysis of dead bacteria occurs during storage. The autolysed cells liberate sufficient 

 nutrient material for the cells to develop, and manifest some of their normal metabolic 

 activities. The mechanism of establishment of reducing conditions in growing 

 bacterial cultures is discussed in the next chapter. 



Reiss and Vellinger report experiments on the effect of the electrode potential 

 -of the medium on the development of fertilised eggs of the sea-urchin {Paracentrotus 



