SALT ACTION 



121 



copper, silver, zinc, cadmium, mercury — found that, though in certain concentrations 

 they had a strong inhibitory action on bacterial growth, in lower concentrations 

 they had the reverse effect, actually stimulating growth. Winslow and Hotch- 

 kiss (1922) found that the same held true for some of the lighter metals (Table 7). 



TABLE 7 



Effect of different Concentrations of Salts on Growth of Bad. coli in 1 per cent. 



Peptone Water. 



A further point of interest was brought out by Sherman and Holm (1922), who 

 showed not only that NaCl stimulated growth in a concentration of 0-1 to 0-3 M, 

 but that it widened the range of H-ion concentration within which Bad. coli would 

 grow. Taking just visible turbidity of the culture as the sign of growth, they 

 obtained results set out in Table 8. 



TABLE 8. 



Growth of Bad. coli in Media of Different pH, in the Presence and Absence of NaCl. 



Thus the addition of 0-2 M NaCl to a 1 per cent, solution of peptone in distilled 

 water increased the rate of growth at unfavourable pH concentrations, and actually 

 enabled the organisms tor grow at pH 4-8 — a concentration at which in plain pep- 

 tone water they refused to grow at all. 



It must be pointed out that a salt which is bactericidal in an aqueous solution 

 may exert a stimulating effect when added in the same concentration to a nutrient 

 medium. 



Having referred to the favourable action of many weak solutions of salts on 

 the growth and on the survival of bacteria, we must now pass on to consider their 

 toxic action. This is the action which appealed particularly to the early workers. 

 Koch in 1881 drew attention to the toxic action of salts, especially to the salts 

 of the heavy metals, such as mercury and silver. There is no doubt that Koch 

 overestimated the germicidal effect of these substances. Geppert (1889) pointed 

 out that if the excess mercury or silver was removed at the end of the test by 

 bubbling HgS through the suspension, considerably higher concentrations of the 



