Acalypha wilkesiana and Mirabilis jalapa extracts synergistically disrupt bacterial membranes and inhibit protein maturation/quorum sensing in MRSA and P. aeruginosa.

The increasing global threat of multidrug-resistant (MDR) skin pathogens, particularly methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa, necessitates novel therapeutic strategies beyond conventional single-target antibiotics. These bacteria are major contributors to difficult-to-treat infections, including surgical site infections and prosthetic joint infections (PJI), where mortality rates can be significant. Ethnopharmacology, leveraging traditional plant-based remedies, offers a promising avenue to discover new compounds with multi-targeting mechanisms to overcome resistance.

Researchers investigated the antimicrobial, synergistic, and molecular mechanisms of Acalypha wilkesiana and Mirabilis jalapa leaf extracts. Ethanol and aqueous extracts were prepared and purified via silica gel column chromatography. Antimicrobial efficacy was determined using Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) assays. Synergism was assessed by the checkerboard method, calculating the Fractional Inhibitory Concentration Index (FIC index). Membrane integrity was probed via K+ and protein leakage assays. Key phytochemicals, Quercetin and Chlorogenic Acid, were then docked against S. aureus Peptide Deformylase (PDF; PDB: 1Q1Y) and P. aeruginosa LasR (PDB: 2UV0), followed by 50-ns Molecular Dynamics (MD) simulations and MM-GBSA binding free energy calculations.

Ethanol extracts of A. wilkesiana showed potent activity, with an MIC of 12.5 mg/ml against P. aeruginosa ATCC 10145. When combined with M. jalapa extracts, strong synergy was observed, indicated by an FIC index of ≤ 0.25-0.50. Membrane leakage assays revealed significant cellular damage: K+ leakage measured 15.86 Cmol/kg and protein efflux was 3.83 Cmol/kg, suggesting catastrophic membrane failure. In silico, Quercetin exhibited high-affinity inhibition of S. aureus PDF, achieving a docking score of -9.802 kcal/mol and an MM-GBSA ΔGbind of -58.42 kcal/mol, with an RMSD of 1.45 ± 0.2 Å. Critical H-bonds at Gly49/Gln50 were maintained for 88% of the simulation frames. Similarly, Chlorogenic Acid bound effectively to the LasR autoinducer pocket, with a docking score of -9.7 kcal/mol and an MM-GBSA of -48.15 kcal/mol, showing an RMSD of 1.80 ± 0.3 Å. Hydrophobic contacts at Trp60 persisted for 95% of its trajectory. These findings suggest that Quercetin suppresses bacterial protein maturation via PDF inhibition, while Chlorogenic Acid disrupts quorum-sensing via LasR binding.

The combination of A. wilkesiana and M. jalapa extracts demonstrated strong synergy, achieving an FIC index of ≤ 0.25-0.50, indicating a highly effective multi-targeting approach against resistant bacteria.