Lippia sidoides ESSENTIAL OIL-MEDIATED SILVER NANOPARTICLES: GREEN SYNTHESIS AND ANTIMICROBIAL ACTIVITY AGAINST RESISTANT Staphylococcus aureus

Antimicrobial resistance represents a growing challenge to food safety and animal production, highlighting the need for alternative strategies to control resistant food-associated pathogens. Essential oils are sources of bioactive compounds that can also act as reducing and stabilizing agents in the green synthesis of metallic nanoparticles. This study aimed to characterize the chemical profile of Lippia sidoides essential oil and evaluate silver nanoparticles (AgNPs) synthesized using this oil under different pH conditions for their antimicrobial activity against resistant Staphylococcus aureus. The chemical profile of the essential oil was determined by gas chromatography coupled to mass spectrometry (GC–MS) using an Agilent HP-6890 chromatograph coupled to an HP-5975 mass detector and an HP-5MS capillary column. Compounds were identified by comparison with the NIST-11 library and retention indices calculated using a C8–C24 homologous series. AgNPs were synthesized at pH 7, 8, 9, and 10, and nanoparticle formation was evaluated by UV–Vis spectroscopy based on surface plasmon resonance parameters. Antimicrobial activity was determined by minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against S. aureus groups comprising multidrug-resistant (MDR), mecA-positive, and mecC-positive isolates, in addition to the N315 control strain and a mixed pool of resistant isolates. GC–MS revealed a thymol-rich essential oil, with thymol (51.49%) as the major constituent, followed by p-cymene (11.66%), linalool (11.07%), trans-caryophyllene (6.55%), and carvacrol (3.94%). The synthesis pH influenced AgNP formation, with the highest formation index observed at pH 10 (0.0681 A.U./nm). AgNPs synthesized at pH 10 also showed the greatest overall antimicrobial activity, with an MIC of 1.55 mmol L⁻¹ against all S. aureus groups evaluated. The MBC was 1.55 mmol L⁻¹ for the mecA-positive, mecC-positive, N315, and mixed groups, whereas the MDR group showed an MBC of 3.10 mmol L⁻¹. These findings demonstrate that thymol-rich L. sidoides essential oil can be employed in the green synthesis of AgNPs active against S. aureus with different antimicrobial resistance profiles, while synthesis pH markedly influences nanoparticle formation and antimicrobial performance. The pH 10 condition was therefore selected as the most promising for further characterization and application. The authors acknowledge Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP; Process No. 2025/03175-3) for financial support.