Abstact: Background/Objectives: The rapid emergence of antibiotic-resistant Escherichia coli in food
and clinical environments necessitates new, clean-label antimicrobials. This study assessed
eight Greek native essential oils—oregano, thyme, dittany, rosemary, peppermint, lavender,
cistus and helichrysum—for activity against six genetically and phenotypically diverse
E. coli strains (reference, pNorm, mecA, mcr-1, blaOXA and O157:H7). We aimed to iden
tify oils with broad-spectrum efficacy and clarify the chemical constituents responsible.
Methods: Disk-diffusion assays measured inhibition zones at dilutions from 50% to 1.56%
(v/v). MIC and MBC values were determined by broth microdilution. GC–MS profiling
identified dominant components, and Spearman rank-order correlations (ρ) linked com
position to activity. Shapiro–Wilk tests (W = 0.706–0.913, p ≤ 0.002) indicated non-normal
data, so strain comparisons used Kruskal–Wallis one-way ANOVA with Dunn’s post hoc
and Bonferroni correction. Results: Oregano, thyme and dittany oils—rich in carvacrol
and thymol—exhibited the strongest activity, with MIC/MBC ≤ 0.0625% (v/v) against
all strains and inhibition zones > 25 mm at 50%. No strain-specific differences were de
tected (H = 0.30–3.85; p = 0.998–0.571; padj = 1.000). Spearman correlations confirmed that
carvacrol and thymol content strongly predicted efficacy (ρ = 0.527–0.881, p < 0.001). Oils
dominated by non-phenolic terpenes (rosemary, peppermint, lavender, cistus, helichrysum)
showed minimal or no activity. Conclusions: Phenolic-rich EOs maintain potent, strain
independent antimicrobial effects—including against multidrug-resistant and O157:H7
strains—via a multi-target mode that overcomes classical resistance. Their low-dose effi
cacy and GRAS status support their use as clean-label food preservatives or adjuncts to
antibiotics or bacteriophages to combat antimicrobial resistance.
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