The Relation of pH Value of Medium to Selective Bacteriostatic Action of Dyes. ∗

Abstract

Several years ago experiments were carried out in this laboratory to determine whether changes in pH value of the medium with which the experiments were conducted would alter the character of the selective bacteriostatic activity of gentian violet. In these experiments, the results of which were never published, a series of divided plates with the following pH values were planted with B. coli, B. anthracis, Staphylococcus and B. prodigiosus: 5.4, 6.4, 6.6, 7.4, 7.6, 7.8, 8.8 and 9.3. The upper halves of the plates contained gentian violet in a strength of 1 to 200,000. On all the plates from pH 6.4 to pH 9.3 selective action of the dye took place exactly as on media of pH 7.2; growth of the Gram positives was inhibited, growth of Gram negatives was unaffected. At pH 5.4 no growth of any organism occurred even on the plain agar. Means were not then at hand for buffering the media in the alkaline range beyond 9.3 but in plates made of media to which large amounts of alkali had been added, certainly sufficient to give a pH well beyond 10, no growth occurred even on the plain agar. The conclusion was reached that, within the range of growth, pH of media was not a factor in determining the character of selective action of gentian violet.

The recent publication of Dubos 1 in which the suggestion is made that some of the inhibitory dyes owe their power of inhibition to the fact that they poise the media at an oxidation potential outside the range in which the inhibited organisms can grow, made it seem wise to repeat our earlier experiments. Divided agar plates, the upper halves containing gentian violet 1 to 200,000, were, therefore, made of media at the following pH values: 3.6, 3.8, 4.2, 4.4, 4.6, 3.8, 5.0, 5.2, 5.4, 5.6, 5.8, 6.0, 6.2, 8.5, 8.8, 9.3, 9.6, 10, 10.4, 11, 11.5, 12, 12.3, 12.7. To obtain the acid pH's HCl was added; to obtain the alkaline, sotlimn hydroxide. For the higher alkaline range M/10 CO₂ free sodium hydroxitle and M/10 glycin solution were used as buffers.