Antibacterial activity of Sapindus saponins against microorganisms related to food hygiene and the synergistic action mode of Sapindoside A and B against Micrococcus luteus in vitro

Sapindus saponins are natural nonionic surfactants, and they have a broad-spectrum antibacterial effect, but few studies have elucidated the antibacterial mechanism of Sapindus saponins to date. Therefore, this study screened the antibacterial activity of Sapindus saponins singly and in combination against 7 bacteria, and investigated the synergistic antibacterial action via targeting cell membrane proteins. The only combination of Sapindoside A and B (SAB) with synergistic activity against Micrococcusluteus (M. luteus) was obtained, where the MICs of Sapindoside A and B were one fourth of their individual minimum inhibitory concentration (MIC). After treatment with SAB, the spectral features of protein in the cell membrane of M. luteus showed obvious changes based on Raman spectroscopy, and the constitutions of membrane proteins were changed seriously. Besides, changes in protein microenvironment was observed, and the contribution of Sapindoside A was greater than that of Sapindoside B with concentration below 2MIC. Molecular docking also demonstrated that Sapindoside A interacted with penicillin-binding protein 2, and showed higher binding energy than Sapindoside B, further indicating that the greater contribution in the synergistic action of SAB on membrane proteins. Collectively, these results showed that the synergistic antibacterial action of Sapindoside A and B against M. luteus could be achieved by attacking cell membrane proteins, and Sapindoside A played a major role, suggesting that SAB has the potential to be the natural antibacterial detergent additive in food industry, although it is limited in its scope of antimicrobial activity against foodborne bacteria.