132 



DISINFECTION 



into cationic and anionic detergents, according to whether the location of the long- 

 chain hydrophobic group is in the cationic or anionic portion of the molecule. 

 Cetyltrimethylammonium bromide, for example, is a cationic detergent, sodium 

 cetyl sulphate an anionic detergent. Generally speaking, these substances are good 

 wetting and cleaning agents, are relatively non-irritant to raw surfaces and destroy 

 vegetative bacteria in dilutions varying from 1 : 100 to 1: 16,000 or so. Some are 

 toxic to leucocytes and others 2:)recipitate proteins. Their bactericidal action is 

 often greatly diminished by the presence of organic matter and of phosphohpins ; 

 their penetrating power is usually low ; and some organisms, hke Ps. pyocyanea, 

 may prove very resistant to them. On the whole, the cationic group appears to 

 be more germicidal than the anionic group, and Gram-positive are more affected 

 than Gram-negative bacteria (see Miller and Baker 1940, Baker et al. 1941a, 6, 

 Barnes 1942, Hoyt et al. 1942, Williams et al. 1943, Hand 1944). 



Alcohols and Ethers. — Epstein (1897) found that absolute ethyl alcohol was 

 not a germicide, but that when diluted it became germicidal. Minervini (1898) 

 confirmed this, and showed in addition that alcohol had little or no action on spores. 

 For the destruction of vegetative bacteria the optimal strength depends on the 

 degree of moisture present. A final concentration of 50-70 per cent, appears to 

 be most effective. Thus, an equal amount of absolute alcohol should be added 

 to an aqueous suspension of bacteria, whereas for dry bacteria a solution of alcohol 

 already diluted to 50-60 per cent, should be used. For the disinfection of moist 

 hands 80-96 per cent, alcohol is recommended ; for the disinfection of dry hands 

 70-80 per cent, alcohol is better. Dry vegetative bacteria are destroyed less 

 rapidly than moist — presumably because the penetration of alcohol takes longer 

 (Table 12). 



TABLE 12 



Time taken by Different Strengths of Ethyl Alcohol to destroy Dry and 



Moist Staphylococci. 

 (After Russ 1904.) 



Note. — The dry staphylococci were dried on silk threads ; 



aqueous suspension. 



the moist staphylococci were in 



The presence of protein increases the disinfection time of alcohol, but not to 

 any considerable extent. The addition of a dilute mineral acid or alkali greatly 

 increases its activity, enabling alcohol to kill spores. Thus Coulthard and Sykes 

 (1936) found that a solution of 70 per cent, alcohol containing 1 per cent, sulphuric 

 acid destroyed spores of B. suhtilis in less than 24 hours, and a solution of 70 per 

 cent, alcohol containing 1 per cent, sodium hydroxide in 24-48 hours. Alcohol 

 lowers the germicidal effect of some substances, like the heavy metal salts, phenol 

 and formaldehyde, that are dissolved in it (Koch 1881, Kronig and Paul 1897), 

 but is said to raise the germicidal effect of others, such as iodine. In fact, a strong 



