Toward Essential Oil Stewardship: Strain-Resolved Evaluation of Thyme Oil Activity Against Pseudomonas aeruginosa

Malwina Brożyna^1*Zuzanna Stępnicka^1*Natalia Tymińska¹Bartłomiej Dudek¹Katarzyna Kapczyńska²Adam Matkowski¹Sun Fang³Michał Tomczyk⁴Adam Junka⁵

• ¹Wroclaw Medical University, Wrocław, Poland
• ²Instytut Immunologii i Terapii Doswiadczalnej im Ludwika Hirszfelda Polskiej Akademii Nauk, Wrocław, Poland
• ³Zhejiang Sci-Tech University, Hangzhou, China
• ⁴Uniwersytet Medyczny w Bialymstoku, Bialystok, Poland
• ⁵Uniwersytet Medyczny im Piastow Slaskich we Wroclawiu, Wrocław, Poland

The rising interest in essential oils (EOs) as antimicrobial agents demands evaluation frameworks that move beyond anecdotal efficacy and toward provide structured, reproducible assessments. In this study, we examined the strain-dependent response of Pseudomonas aeruginosa to Pharmacopoeia-grade Thyme Essential Oil (obtained from Thymus vulgaris L., TEO) or polyhexamethylene biguanide antiseptic (PHMB) using a panel of ten genetically diversified strains in planktonic and biofilm forms, and by complementary in vitro models. Chemical composition of TEO was assessed using Gas Chromatography Mass Spectrometry (GC–MS.), and the main components were thymol, p-cymene, and g-terpinene. Despite uniform test conditions, we observed striking inter-strain variability: TEO Minimal Inhibitory Concentrations (MICs) differed by up to 1000-fold, and biofilm susceptibility profiles ranged from full tolerance to near-complete eradication. Notably, strains with low metabolic activity and sparse cell populations—but high matrix biomass—exhibited reduced responsiveness to TEO, while susceptibility to PHMB was more consistent, though not absolute. These findings highlight the critical influence of both microbial phenotype and agent formulation on Kod pola został zmieniony This is a provisional file, not the final typeset article antimicrobial outcomes. Rather than framing EOs as superior or inferior alternatives, our results advocate for their integration into a stewardship paradigm—one that values standardization, model-based evaluation, and informed formulation. In this context, we position essential oil stewardship not as a constraint but as a necessary evolution for their credible inclusion in antimicrobial strategies.