Studies on salt action. IV. The mutual influence of acidity and salt concentration upon bacteria

Abstract

Four years ago 1 we presented certain preliminary results of studies on the toxic influence of calcium and sodium salts upon Bact. communis; and two years ago one of us 2 called attention to the possible influence upon such salt effects of the reaction of the solution.

In our earlier experiments the reaction of the distilled water and salt solutions to which the bacteria were exposed was not controlled though the media were in general alkaline (P_H 8.0-9.0). Under these conditions we obtained the following results with various concentrations of NaCl and CaCl₂.

Later experiments, in which the hydrogen-ion content was maintained at various desired levels by repeated readjustment, gave quite different results, as indicated below.

These results (which are all based on averages of a sufficient number of tests to eliminate chance variations) were at first highly puzzling. In the unadjusted alkaline solution 5.0 isotonic NaCl was moderately toxic and I .o isotonic CaCl₂ highly toxic. At adjusted P_H values on the other hand 5.0 isotonic NaCl₂ remained at practically all reactions slightly more toxic than water but 1.o isotonic CaC1₂ entirely lost its toxicity and became actually stimulating, at least at P_H values between 5.0 and 7.0. It appeared therefore that neither 5.0 isotonic NaCl nor 1.0isotonic CaC1₂ is in itself toxic at adjusted PHvalues between 6.0 and 7.0; and that at an adjusted P_H much over 7.0 both salts, as well as distilled water, are definitely toxic. What conditions could have existed in the earlier work, at uncontrolled P_H, to make 1.0 isotonic CaCl₂ toxic and 5.0 isotonic NaCl nearly non-toxic?

The key to this riddle, first suggested by the experiments reported here two years ago, has been found in the influence of salt content upon the power of a bacterial suspension to regulate its reaction.