The Germicidal Action of the Hydrogen Ion and of the Lower Fatty Acids *

Although it has long been realized that the fatty acids have excellent germicidal powers, except for the work done by Reid5 little study seems to have been made of them as a group. In particular, the relative parts played by the dissociated and undissociated fractions have not been investigated in detail, although the existence of this dual action has been recognized. Furthermore, information on the concentration exponents of these acids seems to be limited to that given for acetic and butyric acds by Paul, Birstein, and Reuss.4 As Winslow and Lochridge6 demonstrated, acids such as hydrochloric exert their germicidal action almost entirely through the hydrogen ion and have a pronounced effect on most bacteria only at low pH values. Of the organic acids some are bactericidal at dilutions in which the hydrogen ion can be responsible for only a small part of the lethal effect, while others, as oxalic, undergo sufficient dissociation in their germicidal dilutions to make it seem probable that the hydrogen ion plays the most important part. In this paper are presented the results of some experiments undertaken to ascertain the relative importance of the H ion and of the undissociated molecule in the bactericidal action of the fatty acids against a strain of Staphylococcus aureus and one of Escherichia coli. The general technic of the end-point method employed has been described in an earlier paper.1 On the assumption that the hydrogen ion in high concentration has essentially the same germicidal action no matter what its source, the first experiments were carried out to determine the lethal action of this ion as furnished by hydrochloric acid. As in all of the experiments, to I 0 ml. of a distilled water dilution of germicide was added 0.02 ml. (about 50,000,000 cells) of a washed aqueous suspension of organisms. From this tube, held at 200 C., 0.02 ml. was removed at appropriate time intervals and inoculated into extract broth buf-