Antimicrobial potential of Citrus australasica F. Muell. against methicillin-resistant Staphylococcus aureus supported by in silico analysis

The prevalence of methicillin-resistant Staphylococcus aureus (MRSA) has been steadily increasing over the past few decades, prompting an urgent need to develop novel antibiotic classes to combat the growing threat of multidrug-resistant bacteria. In this context, a phytochemical investigation of Citrus australasica F. Muell. (finger lime, Rutaceae) leaves yielded seven metabolites (1-7), including coumarins and flavonoid glycosides. These compounds were identified as marmin (1), xanthyletin (2), rutin (3), narcissin (4), lactic acid (5), glycerol (6), and β-sitosterol (7). The structures of these isolated compounds were elucidated using 1D and 2D NMR. The compounds were evaluated against the MRSA strain (ATCC 33591) using the agar well diffusion method, and compound 1 was the most active. Using a comprehensive protein-protein interaction (PPI) network constructed from the STRING database and visualized using Cytoscape software, we identified key proteins involved in MRSA pathogenesis and potential therapeutic targets. Molecular docking simulations assessed these compounds' binding interactions and affinities with PBP2a, a critical protein in MRSA resistance. Among the tested compounds, marmin exhibited a notable docking score of -6.488 kcal/mol and a low RMSD value of 0.956, indicating a strong and stable interaction. To validate the docking results, molecular dynamics (MD) simulations were conducted to provide insights into the stability and efficacy of these interactions over time. The MD simulations revealed that the protein-ligand complex of PBP2a and marmin (1) maintained stability throughout a 10-nanosecond simulation, with minor fluctuations indicating consistent binding interactions. Based on our findings, the compounds isolated from C. australasica showed promise as potential anti-MRSA therapeutic leads for future development.