Sustained antimicrobial activity of essential oils against MDR pathogens: Insights into resistance and biofilm suppression

Abstract

Essential oils (EOs) are widely recognized for their broad-spectrum antimicrobial properties. However, their impact on the development of antimicrobial resistance (AMR), particularly in multidrug-resistant (MDR) pathogens, remains underexplored. This study investigates the inhibitory potential of selected EOs and their influence on resistance development, quorum sensing (QS), and biofilm formation. Twenty-three EOs were screened for antimicrobial activity using a microdilution assay against strains of the ESKAPE pathogens. Five EOs with varying minimum inhibitory concentrations (MICs) were selected for resistance induction studies via serial passaging of Staphylococcus aureus ATCC 6538 under sub-inhibitory EO exposure. Anti-QS activity was assessed by quantifying violacein inhibition in Chromobacterium violaceum, while biofilm inhibition was evaluated using the crystal violet assay. Twelve EOs (52%) demonstrated noteworthy antimicrobial activity (MIC ≤ 1.00 mg/mL). Origanum vulgare and Thymus vulgaris exhibited strong inhibition (MIC ≤ 1.00 mg/mL), along with significant anti-QS and biofilm suppression (≥ 70% inhibition). Resistance induction studies revealed transient MIC fluctuations (p = 0.00 – 0.02) without stable resistance development. Notably, EO exposure led to heteroresistance to conventional antibiotics, but EO efficacy was retained across passages. The findings support the potential of EOs as sustainable antimicrobial agents against MDR pathogens. Their ability to inhibit QS and biofilm formation, coupled with the absence of long-term resistance development, underscores their promise in mitigating AMR. Further mechanistic studies and clinical validation are warranted to advance EO-based therapeutics.