Synergistic antimicrobial effects of thiamine dilauryl sulphate combined with the heartwood extract of <i>Pterocarpus marsupium</i> and antifungal mechanism in their combination

Thiamine dilauryl sulphate (TDS) exerts a bacteriostatic effect against Gram-positive bacteria and is used as a food additive. The heartwood extract of the Indian kino tree (Pterocarpus marsupium), a medicinal tree native to India, shows weak antimicrobial effects against bacteria and superficial dermatophyte-infective fungi. Herein, we report that the combination of TDS and P. marsupium extract exerts an additive or synergistic antimicrobial effect against various food spoilage microorganisms. Among the 13 species tested, this combination showed synergistic antimicrobial effects against Bacillus subtilis, Lactococcus lactis, Lactobacillus plantarum, and Candida albicans in an agar dilution method. We also found that the combination promoted the production of reactive oxygen species against a model yeast Saccharomyces cerevisiae using a fluorescnt 2',7'-dichlorofluorescein diacetate. Furthermore, the combination restricted drug efflux from yeast cells via efflux pumps in an efflux assay using a fluorescent rhodamine 6 G. These results suggest that the antifungal effect of TDS is improved in combination with P. marsupium extract via oxidative stress induction and drug efflux restriction. The combination of TDS and P. marsupium extract has a low environmental impact and toxicity when used in foods; therefore, it is also expected to be used in food and industrial products in the future.