α-Pinene and β-Pinene as Natural Adjuvants Against MRSA: Evidence from in vivo Models and Molecular Docking

Staphylococcus aureus is a major pathogen in Healthcare-Associated Infections (HAIs), with antibiotic resistance and virulence factors that hinder treatment and reduce therapeutic options. Natural terpenes such as α-pinene (APN) and β-pinene (BPN) have emerged as potential modulators of bacterial resistance. This study assessed the antibacterial and antivirulence activities of APN and BPN, alone and in combination with antibiotics, against S. aureus. The Minimum Inhibitory Concentrations (MICs) of the selected terpenes and antibiotics were defined by the broth microdilution method, and synergistic interactions were determined by checkerboard assays, followed by growth kinetics, time-kill curves, and antivirulence assays targeting biofilm formation, coagulase activity, and hemolysin production. In vivo toxicity, bacterial burden, and host survival were evaluated using the Galleria mellonella model. Both compounds exhibited MICs ranging from 64 to 4,096 µg/mL, with BPN showing pronounced synergy with amoxicillin (BPA) and oxacillin (BPO). The BPO combination reduced bacterial load by 82.1% and more effectively inhibited virulence factors than APN. In silico analyses supported these findings, revealing strong interactions between pinenes and key bacterial targets. No toxicity was observed in vivo. These results highlight BPN as a promising adjuvant candidate to enhance β-lactam efficacy against S. aureus, supporting integrated antibacterial and antivirulence strategies.