BACTERICIDAL AGENTS 141 



be the only factor in bacteriostasis. There is, however, some evidence that inter- 

 ference with the chemotherapeutic activity of organic arsenicals and antimonials is 

 effected only by dyes within a certain range of reduction potential (—0-05 v. to 

 +0-11 v.). 



There is a marked difference in sensitivity to dyes between the Gram-positive 

 and the Gram-negative bacteria. Particularly in the case of basic dyes (triphenyl- 

 methanes, such as brilliant-green and crystal violet, thiazines such as methylene 

 blue and acridines such as acriflavine) the Gram-positive bacteria are generally much 

 more susceptible, possibly due in part to the presence of ribosenucleic acid near the 

 cell surface which may combine readily with the dye. 



FURACIN 



Cramer (1947) has made a potentiometric study of the mode of action of the 

 antibacterial agent furacin, which is 2 - (5-nitro) - furaldehyde semicarbozone. 

 In Staphylococcus aureus cultures the usual fall in oxidation-reduction potentials was 

 delayed by the smaller bacteriostatic doses of the antibiotic, and inhibited altogether 

 by the larger bactericidal concentrations. The polarographic method was used to 

 determine the amount of furacin remaining in the culture after different periods of 

 incubation and the concentration of drug was correlated with the bacteriostatic 

 effect as follows : — 



It has been pointed out in a previous chapter that a number of substances found 

 in nature are 1 : 4-naphthaquinones (the anti-hsemorrhagic vitamin K, phthiocol, 

 many pigments and mould products) or anthraquinones (mould pigments). In 

 addition to their other biological importance and effects many of them also inhibit 

 bacterial growth. This has been attributed to an inhibition of enzyme reactions by 

 combination with SH groups, but it would seem that the inhibition is reversed by 

 addition of thiol compounds only in the case of Gram-negative organisms, so some 

 other explanation of the bacteriostatic effects is sought (Colwell and McCall, 1945 • 

 Cavallito and Bailey, 1944 ; Geiger and Conn, 1943). In view of their oxidation- 

 reduction equilibria and the possibility of semi-quinone formation Page and Robinson, 

 (1943) attempted to find a correlation between the oxidation-reduction potentials at 

 which they were reduced and the bacteriostatic effects of a number of quinone 

 derivations. The results are summarised in table 27. 



It will be seen that no correlation can be detected with the potentials in the 

 case of the bacteriostasis of the Gram-negative Esch. coli, but in the case of the Gram- 

 positive Staphylococcus aureus all the actively bacteriostatic quinones have Eq^ values. 



