SOAPS AND SYNTHETIC DETERGENTS 131 



acids, but are resistant to soaps of the unsaturated fatty-acid series (Reichenbach 

 1908, Walker 1924, 1925, 1926, Belin and Ripert 1937). Staphylococci are resistant 

 to all the common soaps in neutral and alkaline solution (Walker 1924, Bayliss 

 1936), but according to Eggerth (1926) they are susceptible at pH 5. The most 

 generally useful soap in practice is sodium laurate, since it acts on pneumococci, 

 streptococci, and typhoid bacilli, though not on staphylococci (Walker 1924). 

 There is a suggestion that the germicidal activity of soaps increases with increase 

 in molecular weight in the saturated fatty-acid series, but decreases in the un- 

 saturated fatty-acid series (Walker 1924, Bayliss 1936). Of commercial soaps, 

 Nichols (1920) found yellow or brown bar soap, such as is used in washing dishes, to 

 be effective in a 1 : 200 concentration in kilUng pneumococci and streptococci 

 (see also Colebrook and Maxted 1933). Soap prepared from coconut oil, such as 

 salt-water soap, is more germicidal than any other soap to the typhoid bacillus 

 (Hamilton 1917, Walker 1925, 1926). If a stiff lather is made on the hands, even 

 Bad. coli is killed within a minute. The germicidal effect of soaps is increased by 

 rise in temperature (Walker 1924). 



The mode of action of soaps in destroying bacteria is far from clear. It is 

 certainly not due entirely to free alkali, since this may be present in much too small 

 an amount to have any deleterious effect at all. 



- Reichenbach (1908), however, thought that alkali might play a part in some soaps. 

 He observed, for example, that with salts of the higher fatty acids the germicidal effect 

 decreased much more slowly on progressive dilution than with salts of the lower fatty 

 acids, and explained this by the greater hydrolysis of the former group with the consequent 

 liberation of free alkali. Eggerth (1926) found that, generally speaking, soaps of the lower 

 members of the fatty-acid series were more active in acid solution, the higher members in 

 alkaline ; the point of transition varied with the test organism. He explained this result 

 in terms of the effect of the pH on the dissociation residue and on the solubility of the 

 soap. Lamar (1911) is of the opinion that the soap acts on the lipoidal moiety of the 

 ceUs, rendering them more permeable to germicidal substances in the solution. This 

 would explain the adjuvant effect of soap on bacterial lysis by serum, or by substances 

 such as aromatic oUs, which are often added to commercial soap, and would presuppose 

 a germicidal effect of the soap itself. 



Summarizing, we may say that soaps show a strongly selective action towards 

 bacteria, most of the pathogenic respiratory organisms being killed more readily 

 by soaps of the unsaturated fatty acids, and most of the pathogenic intestinal 

 organisms more readily by soaps of the saturated fatty acids. In practice, thorough 

 washing of the hands in a stiff lather with a minimum amount of hot water, preferably 

 using yellow bar household soap, cap be relied upon to kill a high proportion of 

 pathogenic organisms on the hands, with the exception of Staphylococcus aureus. 



It is possible, as Noguchi (1907) suggests, that soaps in blood and lymph are 

 responsible for some part of the natural defence mechanism of the body, since, in 

 his experience, mixtures of soap and inactivated serum resembled complement in 

 many respects. Burtenshaw (1942) likewise suggests that soaps and long-chain 

 fatty acids are mainly concerned in the auto-disinfecting action of the skin. (For 

 a study of soap derivatives, see Eggerth 1929a, h, 1931, and for a review of 

 " germicidal " soaps, see Morton and Klauder 1944). 



During recent years a large group of synthetic detergents have been used in 

 industry. Some of these substances are highly bactericidal and are finding a 

 place in surgery for the cleansing of skin and other surfaces. They are classified 



