Identification and characterization of antibacterial metal complexes in ZiHua DiDing (<i>Viola philippica</i>) aqueous extract

The excessive use of antibiotics leading to bacterial resistance poses a significant threat to global public health, particularly the serious problem of resistance in Staphylococcus aureus, which requires the development of new antibiotics. Metal complexes can combat bacterial resistance due to their unique mode of action and minimal tendency to induce resistance. Exploring new antibacterial agents from traditional Chinese medicine mainly focuses on organic components. We aimed to discover metal complexes with strong antibacterial activity and low drug resistance from ZiHua DiDing (Viola philippica) aqueous extract (ZDAE). We identified metal complexes in ZDAE and demonstrated that the addition of metal ions, particularly manganese (Mn2+), significantly affects the antibacterial activity of the extract. We also detected the heptasaponin manganese complex (ESM) in ZDAE. Subsequent synthesis and characterization of ESM showed that it is a 2:1 complex of esculetin (ES) and Mn2+, with significantly higher antibacterial activity against various bacteria. Activity against S. aureus reduced four-fold from 4800 μmol/L to 1200 μmol/L. The antibacterial mechanism of ESM against S. aureus includes inhibition of biofilm formation and protein synthesis, reduction of energy production, and disruption of cell wall structure. In addition, the minimum inhibitory concentration of ESM in the drug-containing medium remained stable for 20 generations, indicating that the extract of V. philippica produced potent metal complexes with enhanced antibacterial properties and lower ability to induce bacterial resistance.