140 



OXIDATION-REDUCTION POTENTIALS 



BACTERICIDAL DYES 



That dilute solutions of some dyes stain bacteria suggested that they might be 

 effective disinfectants and this was found to be so over fifty years ago. Some dyes 

 inhibit bacterial growth in dilutions of one in a million. Not only have dyes been 

 used as chemotherapeutic agents, with results generally only moderately successful, 

 but study of the constitution of the most active dyes has led to the development of« 

 more successful non-coloured compounds. 



Methylene blue and other effective dyes are in many cases reversible oxidation- 

 reduction potential indicators. Dubos developed the hypothesis that such dyes may 

 be bacteriostatic because they poise the potential at a level at which organisms cannot 

 proliferate. The diffusion of dyes into the bacterial cell followed by oxidation of an 

 essential metabolite was suggested by Fildes. Such oxidation might well bring to an 

 end the proper functioning of an enzyme-catalysed metabolic cycle and thus stop 

 growth. Attempts have been made to correlate the potentials at which the dyes are 

 reduced with their inhibitory effect on bacteria. In addition some dyes interfere 

 with the chemotherapeutic activities of arsenicals and organic antimony compounds 

 (Jancso and Jancso, 1936). 



In table 26 are given the oxidation-reduction potentials at which a number of 

 dyes are 50 per cent, in the reduced state, together with their bacteriostatic effect 

 against Staphylococcus aureus and their interference with the chemotherapeutic 

 activity of 3-amino-4-hydroxy-phenyl-arsenoxide and stibophen (Page and 

 Eobinson, 1943). 



TABLE 26 



Oxidatign-Redtjction Potentials and Bacteriostasis by Dyes 



It will be seen at once that bacteriostatic activity cannot be correlated with 

 ■oxidation -reduction potential at which the dye is reduced. In view of the very differ- 

 ent chemical constitutions of the dyes it is hardly surprising that the potential should 



