Studies on salt action. V. The influence of various salts upon bacterial growth

Abstract

Previous studies on salt action conducted in this laboratory have dealt with the effect of certain mineral salts upon the death rate of bacteria in water suspension. 1 The present investigation relates to the influence of various salts upon growth in a one per cent. pepton solution. The pepton used contained about 4 per cent. ash and the solution had a reaction of P_H 6.8-7.0. The salts studied were added in the form of chlorides in varying concentration. The solutions were inoculated with Bact. communis and incubated at 37° C., the rate of growth being determined by comparing the turbidity produced with standard suspensions of dead bacterial cells. Check determinations by the plate method indicated the substantial accuracy of this procedure.

Twenty-three salts in all were studied and the limiting toxicity determined as indicated below.

The results in general confirm those reported by Matthews 2 for Fundulus, and Eisenberg 3 for bacteria; and it is evident, as the former author pointed out, that there is a rough general relationship between toxicity and solution tension. We are making a further analysis of the relation between the toxic action of these salts and their other physico-chemical properties.

The new point brought out in our studies is the general occurrence of a definitely stimulating action, exerted by concentrations of salts below the inhibitive level. In the case of 15 out of the

23 salts studied we found a concentration which caused more rapid growth than occurred in the plain pepton solution, the stimulating salts including not only K, Na, NH₃, Li, Sr, Mg, Ca and Ba, but such toxic salts as those of Ti, Sn, Ni, Pb, Ce and Hg. The stimulating concentrations with the latter salts were of course exceedingly low (.00005 molar in the case of Pb, .oooor molar in the case of Ce, .000005 molar in the case of Hg) while with K and Na .25 molar concentrations were stimulating.