Integrated in-silico and in-vitro evaluation of silybin as a potential quorum-sensing modulator against Acinetobacter baumannii and Staphylococcus aureus.

The emergence of multi-drug resistance in the class of ESKAPE pathogens such as Acinetobacter baumannii and Staphylococcus aureus has led to the search for anti-virulence mechanisms to overcome resistance posed by the existing antibiotics. Quorum sensing (QS), mediated by AbaI/AbaR circuits in A. baumannii and AgrA/AgrC circuits in S. aureus, regulates biofilm formation and virulence factor production in the two species, thereby serving as potential targets for phytocompound action. In the current study silybin, a flavonoid lignan from Silybum marianum, emerged as a potential QS modulator against both pathogens based on an integrated computational and experimental approach. Molecular docking studies predicted favourable interactions of silybin to the four regulatory proteins (-14.76 to -12.34 kcal/mol for AbaI and AbaR, -10.20 to -9.83 kcal/mol for AgrA and AgrC), binding functionally important amino acid residues in their active sites. Molecular dynamics simulations lasting 500 ns revealed that silybin binding decreased backbone and residue-level flexibility, increased compactness, reduced solvent-accessible surface area and restricted collective motion in comparison with their respective apo-proteins in all four targets. MM-GBSA calculations together with per-residue decomposition and contact analysis emphasized the role of van der Waals and electrostatic interactions contributed to the predicted binding energetics of the protein-ligand complexes. Silybin displayed in-vitro antibacterial activity, time-dependent inhibitory activity, and concentration-dependent antibiofilm activity up to 60% in A. baumannii and up to 70% in S. aureus. Downregulation was observed for genes involved in virulence in S. aureus, but not for abaI/abaR in A. baumannii. Overall, the findings support silybin as a potential QS- associated modulator with anti-biofilm activity against both pathogens. However, additional studies are required to establish QS-specific activity, selectivity, cytotoxicity, pharmacokinetic properties, and in-vivo efficacy.