Improved antibacterial effects of alkali-transformed saponin from quinoa husks against halitosis-related bacteria

Quinoa is a food crop native to the Andes. The process of dehulling quinoa can produce approximately 8–12% husk, which is often discarded because it contains bitter saponin. Saponin derived from quinoa has been reported to exhibit anti-inflammatory and antifungal activity. However, the antibacterial effects of quinoa saponin against halitosis-related bacteria are still unclear. In this study, quinoa saponin (QS) and alkali-transformed saponin (ATS) were separated by AB-2 resin to obtain QS-30, QS-80, ATS-30 and ATS-80. Halitosis-related bacteria included Porphyromonas gingivalis (P. gingivalis), Clostridium perfringens (C. perfringens) and Fusobacterium nucleatum (F. nucleatum). The MIC and MBC were determined using gradient dilutions in 96-well plates, and the saponins were identified by HPLC and mass spectrometry. The changes in membrane integrity were tested using a microplate reader, the membrane potential was tested by spectrofluorometry, and the morphological characteristics were examined using a transmission electron microscope to explore the antibacterial mechanisms. Antibacterial assays indicated that QS-80 and ATS-80 showed inhibitory activity. In addition, ATS-80 exerted a stronger inhibitory effect than QS-80, especially against Fusobacterium nucleatum, with a lower minimum inhibitory concentration (31.3 μg/mL) and a lower minimum bactericidal concentration (125 μg/mL). ATS-80 destroyed the bacterial membrane structure, leading to bacterial death. Based on the excellent antibacterial activity and economic prospects of quinoa husk, ATS-80 could be used as an antibacterial agent to treat halitosis.