130 DISINFECTION 



(4) Electrolytes with bivalent cations are generally more toxic than those 

 with univalent cations. Thus Ba" is more powerful than Na'. 



(5) On the whole, the salts of the heavier metals are more toxic than those 

 of the lighter metals ; thus HgClg is more toxic than CaCla- But there is no 

 strict quantitative relation between the atomic weight of a metal and its toxicity. 



(6) On the other hand there is a fairly close relationship between the lyotropic 

 and the toxic effects of a salt. 



(7) The toxic action of salts is less marked in protein solutions than in dis- 

 tilled water. Thus the activity of HgClg is decreased markedly in the presence 

 of blood serum. 



(8) The more favourable the nutrient qualities of the medium in which the 

 bacteria are suspended, the less manifest is the toxic effect of salts, and of 

 germicidal agents in general, upon them. 



(9) Different organisms vary in their susceptibility to the disinfectant action 

 of the same salt. Closely allied organisms respond in much the same way to 

 the same salts. 



(10) There is evidence that the Gram-positive organisms, with a few exceptions, 

 are more susceptible to the disinfectant action of salts than the Gram-negative 

 organisms. 



(11) The addition of a salt to a solution of a germicide — whether itself a salt 

 or not — may increase or decrease the action of the latter. This action may be 

 due partly to the effect on the electrolytic dissociation of the germicide ; partly, 

 in a colloidal solution, to an effect on the dispersion coefficient of the disinfectant ; 

 and partly perhaps to the disinfectant action of the salt itself. 



(12) There is little evidence that salts, except in high concentrations, owe their 

 germicidal action to the osmotic pressure that they exert, since bacteria are strongly 

 resistant to variations of osmotic pressure ; but they may act by dehydrating the 

 proteins of the medium in which they are suspended. 



(13) The action of salts is complex. It may be referred to an oxidation effect, 

 a reduction effect, a molecular effect, or an ionic effect. Other effects, namely, 

 the sensitization of organisms to COg, .and their interference with proteolytic 

 enzymes, have not been considered in this chapter ; for details of these the reader 

 is referred to an article by Rockwell and Ebertz (1924). 



Soaps and Synthetic Detergents. — Several workers have studied the germicidal 

 effect of soaps, with results that have been at times contradictory. Many of the 

 discrepancies can be ascribed to the use of different test organisms, since certain 

 soaps are highly bactericidal to some organisms and comparatively inert to others. 

 For example, pneumococci are very sensitive to the soaps of the unsaturated fatty 

 acids — oleic, linoleic, linolenic — but much less so to soaps of the saturated fatty 

 acids — stearic, palmitic, myristic, lauric. According to Lamar (1911) virulent 

 pneumococci are killed by a 0*5 per cent, solution of sodium oleate in 15-30 minutes. 

 Bayliss (1936) found that to kill pneumococci in 10 minutes a 0-1 per cent, solution 

 of sodium palmitate was required, but only a 0-004 per cent, solution of sodium 

 oleate. Lamar (1911) also noticed that sodium oleate, even in high dilution such 

 as 1 : 20,000, greatly accelerated the autolysis of pneumococci, and favoured their 

 lysis by normal or immune serum. Hsemolytic streptococci, meningococci, 

 gonococci, and diphtheria bacilli resemble pneumococci in their greater sensitivity 

 to soaps of the unsaturated fatty acids. On the contrary, the Gram-negative 

 bacilli of the coli-typhoid group are fairly susceptible to soaps of the saturated fatty 



