Antimicrobial Activity of Essential Oils Evaluated In Vitro against Escherichia coli and Staphylococcus aureus

The spread of extended-spectrum β-lactamase-producing Escherichia coli and methicillin-resistant Staphylococcus aureus has caused a reduction in antibiotic effectiveness and an increase in mortality rates. Essential oils (EOs), known for their therapeutic efficacy, can be configured as novel broad-spectrum biocides. Accordingly, the bacteriostatic-bactericidal activity of Citrus Lemon (LEO), Pinus Sylvestris (PEO), Foeniculum Vulgaris (FEO), Ocimum Basilicum (BEO), Melissa Officinalis (MEO), Thymus Vulgaris (TEO), and Zingiber Officinalis Rosc. (GEO), at concentrations ranging from 1.25 to 40% (v/v), were tested in vitro against different E. coli and S. aureus strains using minimal inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs). The chemical compositions of the EOs were analyzed using GC/MS. The major components of all seven tested oils were limonene, α-pinene, anethole, estragole, citral, thymol, and zingiberene, respectively. We found that the bacteriostatic-bactericidal activity of the EOs was related to their chemotypes and concentrations, as well as the strain of the bacteria. A dose-effect correlation was found when testing GEO against S. aureus strains, whilst FEO was found to have no activity regardless of concentration. PEO, MEO, and BEO were found to have bactericidal effect with a MIC and MBC of 1.25% (v/v) against S. aureus strains, and LEO was found to have values of 1.25% (v/v) and 5% (v/v) against ATCC and clinical isolate, respectively. Interestingly, the antimicrobial activity of TEO was not related to oil concentration and the complete inhibition of growth across all E. coli and S. aureus was observed. Although preliminary, our data demonstrate the efficacy of EOs and pave the way for further investigations on their potential synergistic use with traditional drugs in the human and veterinary fields.