Structure analysis of condensed tannin from rice straw and its inhibitory effect on Staphylococcus aureus

Abstract

To reduce the production cost of bioethanol and improve the utilization rate of straw resources, more high value-added products have to be found and applied. In this study, tannins were considered as a high-value by-product. The extraction of condensed tannins from rice straw was studied, and the tannins were characterized. The crude tannins consisted mainly of tannins, phenolic acids and monosaccharides. The crude tannins were separated and purified with AB-8 macroporous resin and Sephadex LH-20. The matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS) assay confirmed that catechin, epicatechin, epigallocatechin, epicatechin gallate and gallic acid were the major components in the purified tannins. The tannins exhibited strong inhibitory effects on growth of Staphylococcus aureus. Higher concentrations of crude tannins resulted in lower ethanol concentration and intercellular ATP content, and higher residual glucose. Moreover, the tannins could regulate morphology of biofilms. The images from atomic force microscopy and confocal laser scanning microscopy showed that cells hardly adhered to the plate surface, and large aggregations of cells were observed in culture medium containing tannins, and almost all cells were dead. This study provides the feasibility of tannins derived from rice straw as new antimicrobial agents with a different inhibitory mechanism.