In Vitro Evaluation of the Antimicrobial Activity of Eighteen Essential Oils Against Gram-Positive and Gram-Negative Bacteria in Two Different Growth Media

The rise in antimicrobial resistance and tolerance over time represents a significant threat to human and animal health. This has led to a notable increase in interest within the scientific community in the development of herbal-based therapeutic alternatives to antibiotics. The present study aimed at evaluating the in vitro antimicrobial activity of 18 essential oils (EOs) against a total of 17 strains belonging to both Gram-positive and Gram-negative bacteria, by employing the spot-on-agar method using two different culture media, Brain Heart Infusion (BHI) and Mueller–Hinton (MH). The antimicrobial properties of these essential oils were investigated, as well as their antimicrobial stability over a period of seven days. The overall efficacy of the EOs remained consistent over seven days, regardless of the solid medium used. However, the antimicrobial effects of the EOs were more pronounced in BHI than in MH for Gram-positive bacteria. While variations in antimicrobial activity were recorded among different species and strains, oregano EO proved to be the most effective against both Gram-positive and Gram-negative bacteria, followed by thyme and tea tree. The findings of this study support the notion that EOs could be employed as a promising therapeutic strategy to combat antibiotic-resistant bacteria, thereby enhancing the efforts aimed at addressing antibiotic resistance. Furthermore, the divergent antimicrobial effects exhibited by the two growth media employed here may facilitate the establishment of standardised protocols for the testing of EOs against bacteria.