Effect of isoquercitrin on membrane dynamics and apoptosis-like death in Escherichia coli

Minimum inhibitory concentration (MIC) is defined as the lowest concentration of a compound that completely inhibits microbial growth. Antibacterial mechanisms of compounds have been investigated at their sub-MICs as well as at their MIC. In this study, the effects of sub-MIC and MIC of isoquercitrin on Escherichia coli were investigated. The antibacterial effect of isoquercitrin was tested using the microdilution method. Sub-MICs of isoquercitrin induced the production of reactive oxygen species and depletion of glutathione. The oxidative effects induced by sub-MICs of isoquercitrin could be prolonged, finally resulting in apoptosis-like death. DNA fragmentation and phosphatidylserine externalization, which are regarded as the hallmarks of apoptosis, were evaluated using the TUNEL assay and Annexin V staining, respectively. Furthermore, isoquercitrin induced the peroxidation of membrane lipids and inner membrane permeabilization at both its sub-MIC and MIC. This suggested membrane damage in response to lipid oxidation. The uptake of membrane impermeable dyes, propidium iodide and calcein, demonstrated that isoquercitrin damaged the cell membrane at concentrations higher than its MIC. Thus, isoquercitrin induced apoptosis-like death and dysregulation of cell membrane dynamics.