Antimicrobial impact of essential oils on human skin colonizers and pathogens: targeting Staphylococcus species

Background: Staphylococcus spp., one of the most abundant bacterial genera of the human skin microbiome, comprises commensal microorganisms – Staphylococcus epidermidis (S. epidermidis) – and opportunistic pathogens – Staphylococcus aureus (S. aureus). Staphylococcus-caused diseases are typically treated with antibiotics, but the antimicrobial resistance underscores the need for alternative treatments. Essential oils (EOs) are natural compounds made of terpenes and terpenoid molecules. Their antimicrobial activity results from the effect upon several cell targets, making the appearance of resistances unlikely. Objective: To study the antimicrobial activity of three EOs (eucalyptus, lavender, and thyme) on two Staphylococcus species (S. epidermidis and S. aureus). Methods: To assess the antimicrobial potential of EOs against the selected bacteria, the following assays were performed: i) determination of the minimum inhibitory (MIC) and minimum bactericidal (MBC) concentrations; ii) inhibition curves for each oil defined for the MIC, to understand how the bacteria respond to the oils; iii) antibiofilm formation assays using MIC, ½ MIC and ¼ MIC dilutions for each oil; and iv) evaluation of the membrane damage through flow cytometry, by staining the bacterial cells exposed to the oils, with propidium iodide (PI) and thiazole orange (TO). Results: The MIC and MBC values for the selected EOs ranged from 0.078% and 2.5% (v/v). Thyme presented the lower MIC for both bacteria. The inhibition curves corroborated the MIC results for all EOs. However, thyme was the one with a bactericidal effect on both bacteria. Thyme also presented higher antibiofilm activity compared to eucalyptus and lavender. The flow cytometry assay showed that all oils were able to damage the cell membrane of both bacteria (positive PI staining), but S. epidermidis showed less cell damage than S. aureus. Conclusion: The selected EOs presented antibacterial activity against S. epidermidis and S. aureus. Thyme EO was the one with the best antibacterial activity. S. epidermidis is more resistant to the oils than S. aureus.