Hydrophilicity of non-fatty acid moiety: significant determinant affecting antibacterial activity of lauric acid esters

Abstract

What affects to the antibacterial activities of lauric acid esters was investigated. Based on the results of antibacterial activity test evaluating minimum inhibitory concentration and minimum bactericidal concentration, sesamol laurate was found to have no effect on either Gram positive bacteria or Gram negative bacteria. On the other hand, erythorbyl laurate had antibacterial activity to Gram positive bacteria. To investigate why the antibacterial activities are shown differently between sesamol laurate and erythorbyl laurate, monolaurin, sucrose monolaurate, isoamyl laurate, and methyl laurate, erythorbyl laurate, and sesamol laurate were chosen based on hydrophilicity of non-fatty acid moiety. Minimum inhibitory concentration was assessed by broth micro-dilution method against Staphylococcus aureus, Listeria monocytogenes, Escherichia coli, and Salmonella Typhimurium. Monolaurin, erythorbyl laurate, and sucrose monolaurate showed antibacterial activities against Gram positive bacteria. On the other hand, isoamyl laurate, methyl laurate, and sesamol laurate had no inhibitory effect on both Gram positive and Gram negative bacteria even treated up to 1.0 mM. The mechanism of monolaurin, erythorbyl laurate, and sucrose monolaurate was investigated by measuring the released cell constituents at 260 nm using spectrophotometer and the lipid compositional changes using gas chromatography (GC). 260 nm absorbing materials of Staphylococcus aureus treated with monolaurin, erythorbyl laurate, and sucrose monolaurate were increased for 2 h, and membrane lipid composition was also changed. Octanol/water partition coefficient was calculated by atom/fragment contribution method. The partition coefficients indicating lipophilicity were 7.175, 5.284, and 5.717 for isoamyl laurate, methyl laurate, and sesamol laurate, respectively, whereas monolaurin, erythorbyl laurate, and sucrose monolaurate showed 3.670, -0.6858, and -4.122, respectively. The hydrophilic-lipophilic balance values of isoamyl laurate, methyl laurate, and sesamol laurate were 1.800, 3.700, and 4.835, respectively, while monolaurin, erythorbyl laurate, and sucrose monolaurate had the higher HLB value of 7.025, 15.25, and 16.09, respectively. These results suggested that lauric acid esters should retain proper hydrophilicity based on the log P value of lower than 4, and the HLB value of higher than 7 to incorporate into bacterial cell membrane as antibacterial agents.