Antibacterial activity and mechanism of action saponins from Chenopodium quinoa Willd. husks against foodborne pathogenic bacteria

The present study investigated the chemical composition of quinoa saponins and examined their possible antimicrobial modes of action against foodborne pathogenic bacteria. Six different compounds (Q(1)-Q(6)) were extracted from quinoa husks, separated/purified by column chromatography and identified by HPLC/MS and NMR. The anti-bactericidal effects against six types of bacteria, Staphylococcus aureus, Staphylococcus epidermidis, Bacillus cereus, Salmonella enteritidis, Pseudomonas aeruginosa, and Listeria ivanovii, were determined by the disk diffusion method and assessment of the MIC/MBC. The changes in membrane integrity were tested using a microplate reader, a flow cytometer, and a transmission electron microscopy. All the compounds exerted anti-bactericidal effects against S. aureus, S. epidermidis and B. cereus. Q(4) showed the strongest activity against S. aureus and S. epidermidis, with an MIC value of 0.0625 mg/mL and an MBC value of 0.125 mg/mL. The concentration of quinoa saponin Q(4) exhibited a dose-dependent relationship with its antibactericidal effect. In fact, a dose-effect relationship was found between the concentration of all quinoa saponins and their bacteriostatic effects. In addition, the release of nucleic acids and proteins from B. cereus increased gradually with increases in the Q(4) concentration, and the membrane structure of B. cereus was destroyed by Q(4). Notably, quinoa saponins caused severe damage to the tested bacteria through degradation of the cell wall followed by disruption of the cytoplasmic membrane and membrane proteins, which resulted in leakage of the cell contents.