134 DISINFECTION 



suspended matter, and hence their concentration is increased in the immediate 

 neighbourhood of the bacteria. This action is interfered with by the presence 

 of other suspended organic matter, which serves to adsorb the germicide, and 

 thus lower its efiective concentration around the bacteria. Emulsoids of the 

 cresol group are generally most active when freshly made up in solution ; after 

 a day or two, probably because of an alteration in their colloidal state, their 

 activity diminishes. Some of the cresols can be employed in true solution, but their 

 solubility in water is very low. Para-chlor-meta-cresol, for example, has a solubility 

 of about 1 : 300 ; towards naked bacteria it is approximately ten times as active 

 as phenol (see With ell 1942a). 



According to lOarmann, Shternov, and Gates (1934a, h), the germicidal activity of 

 phenol derivatives is increased by halogen substitution, and is still further intensified by 

 the introduction of aliphatic or aromatic groups into the nucleus of these compounds. 

 Their general formulae are 



OH OH 



R / Nci 



\/ 



CI R 



jjara-Chlorophenol Derivative. oriAo-Chlorophenol Derivative. 



where R is an aliphatic or aromatic group. Some of the compounds tested by these workers, 

 such as pfirra-chlorophenol or or//w-chlorophenol derivatives with n-butyl to n-octyl sub- 

 stituents, proved highly destructive to bacteria, while being comparatively non-toxic to 

 mice on subcutaneous injection. 



The cMoroxylenols, many of which form a clear solution in water, have come 

 into j)rominence of late years, mainly for skin disinfection. They are comparatively 

 non-irritant, but their bactericidal power, on the whole, is considerably less than 

 that of phenol ; unless employed in 30, 50, or even 100 per cent, concentration, 

 they cannot be relied upon to destroy staphylococci on the skin (see Colebrook 

 1941). (For useful information on the use of tar derivatives in practice, see Keport 

 1942, 1944.) 



Dyes. — Though a few desultory observations had been made at various times 

 on the effects of aniline dyes on bacteria. Churchman (1912) was the first to 

 investigate them thoroughly. Working with gentian violet, he found that if 5 

 drops of a saturated aqueous solution of this dye were added to broth cultures 

 of different organisms, the mixtures allowed to remain for an hour, and transplants 

 then made on to agar, the Gram-negative organisms grew satisfactorily, but the 

 Gram-positive organisms failed to develop. A similar selective property could be 

 demonstrated by seeding the fresh imstained organisms on to plates, one-half of 

 which contained plain nutrient agar, and the other half nutrient agar containing 

 a dilution of about 1-100,000 gentian violet. A large number of different bacteria 

 were tested to ascertain if there was a perfect correlation between Gram-positive- 

 ness and inability to grow in media containing gentian violet. This was found not 

 to be the case ; about 90 per cent, of the Gram-positive organisms were killed by 

 gentian violet and failed to grow on media containing it, but the remaining 

 10 per cent., comprising the acid-fast group in particular, were not affected. 

 Similarly, though about 90 per cent, of the Gram-negative organisms were resistant, 

 the remaining 10 per cent, were susceptible. 



