122 DISINFECTION 



heavy metal were required to destroy the organisms than had been found by Koch. 

 Geppert showed that minute concentrations of heavy metals were sufficient to 

 inhibit growth, but were unable to kill the organisms. Observations by Fildes 

 (1940) have shown that mercury is even less bactericidal than it was thought to be 

 by Geppert. Fildes has brought some evidence to suggest that mercury acts by 

 combining with the — SH groups of the bacterial cell, which are essential for meta- 

 bolism. If the mercury is neutralized by the addition of — SH compounds, like 

 glutathione, cysteine, or thioglycollate, bacteria are able to grow after treatment 

 with strong mercury solutions which pre\'ious workers have regarded as being 

 actively germicidal. It seems possible, therefore, that mercury, and perhaps other 

 of the heavy metals, act by interfering with essential metaboUtes of the cell. In 

 virtue of this property, its bacteriostatic power is high, but its bactericidal effect 

 is comparatively low. In estimating the bactericidal effect in practice, it is neces- 

 sary to remove the excess of mercury from the suspension at the end of the test 

 period by treatment with HjS or ammonium sulphide, and then to cultivate the 

 organisms in a liquid medium containing 1 per cent, thioglycollate in order to 

 provide an adequate concentration of — SH groups. Records of the germicidal 

 effect of mercury solutions not based on the use of this method must be regarded 

 as unreliable. The same criticism appHes to the organic salts of mercury, such as 

 phenyl mercuric nitrate and several proprietary preparations. If thioglycollate is 

 added to the broth used for subculture, none of these substances is able to destroy 

 Staph, aureus or Bact. coli in a 1/1,000 dilution in 10 minutes at room temperature 

 (Hoyt, Fisk and Burde 1942). 



Kronig and Paul (1897), and later Paul and Prall (1907), made the very important 

 discovery that the toxicity of solutions of HgClg depends not on the molecular 

 concentration of the salt but on the concentration of free Hg-ions in the solution. 

 Thus the halogen salts of mercury were found to be active in proportion to their 

 degree of electrolytic dissociation, 



HgCl,>HgBr,>Hgl3. 

 Solutions of salts in which the mercury was combined with a complex anion, and 

 in which the degree of dissociation was poor, such as mercury acetate or cyanide, 

 were found to be much weaker in germicidal power. The behaviour of the salts 

 of the heavy metals is therefore analogous to that of the mineral acids, the toxicity 

 being in proportion to the concentration of free metallic ions and of free H-ions 

 respectively. 



The mode of action of heavy metals themselves, as apart from their salts, is not clear. 

 Their toxicity may be demonstrated either by adding them to distilled water, or by placing 

 them, in the form of a bar or coin, on the surface of an inoculated agar plate. Kling ( 1932) 

 beUeves that the pure metal goes into actual solution. On the other hand the experiments 

 of Hofmann (1929) and PUod and CodveUe (1932), both of whom found that oxygen was 

 necessary for the manifestation of toxicity, suggest that an oxide of the metal is formed 

 which then undergoes ionization. Pure metals, of course, cannot ionize, and their failure, 

 whether in aqueous or colloidal solution, to prove toxic under anaerobic conditions, points 

 strongly to the necessity of preUminary salt formation followed by their ionic dissociation. 



A vast amount of work has been done on the effect of different salts on bacteria. 

 As the salts of mineral acids are electrolytically dissociated, it is clear that their 

 action may be due either to the undissociated molecule, to the anion, to the cation, 

 or to all three in combination. To assess the importance of each of these factors, 



