The chemical composition and the preservative, antimicrobial, and antioxidant effects of Thymus broussonetii Boiss. essential oil: an in vitro and in silico approach
Authors: Boukhira Smahane , Amrati Fatima Ez-Zahra , Chebaibi Mohamed , Grafov Andriy , Mothana Ramzi A. , Al-Yousef Hanan M. , Bousta Dalila 
Published: July 2024

Introduction

The aim of this study was to evaluate the antioxidant, antimicrobial, and preservative efficacy of Thymus broussonetii Boiss. essential oil (EO) in a topically applied formulation using a challenge test.

Methods

The essential oil was extracted from the aerial part of T. broussonetii using hydrodistillation, and the obtained EO was further analyzed by gas chromatography/mass spectrometry (GC/MS). The antioxidant effect of the EO was evaluated using three methods: the inhibition of free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH), β-carotene–linoleic acid, and the ferric reducing antioxidant power (FRAP) methods. The antimicrobial activity and the minimum inhibitory concentration (MIC) of this EO were assayed by the disk-diffusion method and the broth microdilution method, respectively. The preservative efficacy of T. broussonetii EO was assayed at 1% and 2% (v/w) in a topical cream formulation using a challenge test against standard-specific microorganisms recommended by the European Pharmacopoeia. Furthermore, the identified phytochemical compounds were docked for their effect on nicotinamide adenine dinucleotide phosphate oxidase, human casein kinase 1 alpha 1 (CSNK1A1), glycogen synthase kinase 3, Staphylococcus aureus nucleoside diphosphate kinase, Escherichia coli beta-ketoacyl-[acyl-carrier protein] synthase, Pseudomonas aeruginosa LasR ligand-binding domain, and sterol 14-alpha demethylase (CYP51) from Candida albicans. The ADME/toxicity was predicted by analyzing the absorption, distribution, metabolism, and excretion parameters.

Results and discussion

chemical composition of the EO revealed the presence of thymol (63.09%), p-cymene (11%), and γ-terpinene (8.99%) as the major components. The antioxidant assays revealed that the essential oil exhibited strong antioxidant activity, as indicated by the minimum inhibitory concentration IC50 (IC50 = 210 ± 0.3 μg/mL for the DPPH assay, IC50 = 145 ± 0.1 μg/mL for the β-carotene assay, and IC50 = 84 ± 0.21 μg/mL for the FRAP assay) when compared to quercetin and butylated hydroxytoluene (BHT) as controls. The investigated essential oil exhibited important antimicrobial activity against all the tested microorganisms, and the MICs of the EO against bacteria and fungi were 0.02%–1%. Moreover, the EO of T. broussonetii evaluated at 2% (v/w) in a cream formulation succeeded in satisfying the A criteria for preservation efficacy against S. aureus, E. coli, and Aspergillus brasiliensis but exhibited less efficacy against P. aeruginosa (1.78 log reduction in the number of CFU/g after 7 days of evaluation) and C. albicans (1.09 log reduction in the number of CFU/g after 14 days of evaluation) when compared to the synthetic preservative phenoxyethanol 1% (v/w). In silico results showed that the antimicrobial activity of T. broussonetii EO is mostly attributed to thymol, terpinen-4-ol, and aromadendrene, while the antioxidant activity is attributed to thymol. These results indicate that the EO of T. broussonetii possesses important antimicrobial and antioxidant properties and can, therefore, be used as a natural preservative ingredient in the cosmetic industry.