Interaction of hydrolysable tannins with Streptococcus mutans lipid liposomes

Streptococcus mutans is one of the main pathogens responsible for dental caries, necessitating an urgent search for new effective strategies against this bacterium. Hydrolysable tannins, including 1,2,3,4,6-penta-O-galloyl-β-d-glucose (PGG), 1,2-di-O-galloyl-4,6-valoneoyl-β-d-glucose (dGVG), and 2-O-bis-digalloyl-4,6-valoneoyl-β-d-glucose (b-dGVG), exhibit antibacterial activity against S. mutans but their direct molecular interactions with bacterial lipids remain unclear. In this study, we investigated the binding mechanism of these tannins to liposomes prepared from lipids isolated directly from S. mutans using a biophysical approach. Fluorescence quenching and anisotropy measurements demonstrated that the tannins used interact with lipids and preferentially form complexes localized at the hydrophilic parts of the liposomes, with binding constants (K_b) in the range of 10³ to nearly 10⁵. Dynamic light scattering and zeta potential analyses showed that valoneoyl-containing tannins induce a shift towards more negative surface charges (e.g., down to -23.42 mV for b-dGVG) and minor aggregation. In contrast, the more flexible PGG primarily adsorbs onto the surface without significantly disrupting lipid packing. FT-IR spectra of dried lipid-tannin films, analyzed by spectral subtraction and Fourier self-deconvolution, indicated that hydrogen bonding between the tannins and lipid ester carbonyl groups is possible. Our results demonstrate that the tannins used interact with lipids, but with lower affinity than that observed for whole bacterial cells. This indicates that structural complexity of the intact bacterium, including the cell wall, surface polysaccharides, and membrane proteins, likely plays a major role in mediating its full antibacterial activity against S. mutans. These findings highlight the importance of targeting both lipid and protein membrane domains, offering a valuable molecular basis for designing novel, tannin-based anti-caries therapeutics.