Synergistic Bactericidal Activity in Binary Mixtures of an Anionic Surfactant and Polyvalent Metal lons

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The bactericidal activity of the mixed solutions of sodium 1-octanesulfonate (NaOS) and metal ions (Na⁺, Mg²⁺, Al³⁺, La³⁺) againstEscherichia coilATCC 25922 and the leakage of intracellular materials from the organism treated with the mixtures were studied by a quantitative suspension test and turbidimetry, respectively. When the total concentrations of the systems were kept constant, a maximum in bactericidal activity was found in the viciniy of the molar ratio of 2: 1 for NaOS/divalent cations and 3: 1 for NaOS/trivalent cations. Monovalent cations were ineffective at the concentrations tested. In the case of NaOS/LaCl₃, the absorbance at 260 nm (A₂₆₀) also showed a maximum when plotted against the LaCl₃ mole fraction. This mole fraction at the maximum was almost similar to that of the maximum bactericidal activity. The interaction between NaOS and metal cations for the mixture systems was examined by a measurement of surface tension. The surface tension was found to be the lowest at a molar ratio similar to the results observed in the bactericidal test. This lowering of the surface tension seemed to be caused by a strong interaction between NaOS and polyvalent metal cations. These results suggest that the synergistic bactericidal activity in NaOS/polyva lent metal ion mixtures is closely associated with a hydrophobic complex formed between a I-octanesulfonicanion and a polyvalent cation, and with the disorganization of the cell membrane caused by that formation.