Terpenoids as principal bioactive compound of essential oils: enhancing synergistic efficacy with conventional antibiotics.

Background: The rise of antibiotic resistance imposes the search for novel antimicrobial strategies as natural products or its combination with antibiotics. This study investigates the synergistic effects of terpenoids from () essential oil in combination with antibiotics against () and (). The aims were to evaluate the antimicrobial efficacy, analyze functional group modifications and assess molecular interaction.

Methods: Essential oil was extracted from by hydro-distillation. The EO was analyzed for terpenoid content via Thin Layer Chromatography (TLC). Antimicrobial activity was assessed using the disc diffusion method and Minimum Inhibitory Concentration determinations (MIC) by broth dilution followed by bactericidal essay (Time-killing). FTIR and UV spectroscopy were employed to detect functional group modifications in terpenoid-antibiotic combinations. Molecular docking studies assessed interaction energies between terpenoids and antibiotics.

Results: TLC identified α-pinene, δ-carene, and caryophyllene in the EO. δ-Carene exhibited the highest synergy with antibiotics, showing the lowest MIC of 0.04 mg/mL against ATCC-43300 and 0.05 mg/mL against MTCC-739. Time-kill assays demonstrated that α-pinene, δ-carene, and caryophyllene achieved complete bacterial eradication by 4 hours in combination with amoxicillin against , and by 2 hours against in combination with erythromycin. FTIR analysis revealed peak shifts at 1599, 1774, and 2259 cm for amoxicillin + α-pinene, and new peaks at 1648 and 1287 cm for δ-carene + erythromycin. UV spectra indicated potential complex formations. Docking studies showed δ-carene's strong interaction with erythromycin and amoxicillin, with interaction energies of -96.10 and -87.75 kcal/mol, respectively.

Conclusion: Terpenoids from enhance the antimicrobial efficacy of antibiotics. Functional group modifications and complex formations suggest that these interactions may contribute to synergistic effects. These findings support the potential use of terpenoid-antibiotic combinations in overcoming antibiotic resistance and warrant further investigation into their mechanisms of action.