Controlling the antibacterial activity of surfactants by hydrotrope addition

Hydrotropes are amphiphiles that improve the water solubility of hydrophobic substances. In this study, we systematically evaluated the effects of eight hydrotropes on the antibacterial activity of fatty acids solubilized in a micellar system. The antibacterial activity was evaluated using minimum inhibitory concentration (MIC) as an indicator, and CMC was calculated for interfacial properties based on the measurement of the surface tension and the solubilization behavior of the oil‐soluble dye. The solubilization of lauric acid (12:0 FA) in the micelles of nonionic surfactants increased and decreased the antibacterial activities of the surfactants against Staphylococcus aureus (S. aureus) and Staphylococcus epidermidis, respectively. The addition of the hydrotropes decreased the antibacterial activity of the surfactants against both Staphylococci. However, on introducing small amounts of 12:0 FA, the surfactants exhibited selective antibacterial activity against S. aureus. These antibacterial phenomena were observed at concentrations above the critical micelle concentration of the surfactants. These findings suggest that the addition of hydrotropes affects the interfacial properties and antibacterial activity of micellar systems containing antibacterial ingredients.