Species-Specific Susceptibility of Planktonic and Biofilm Forming Candida Strains to Cyclodextrin-Encapsulated Essential Oils

Background/Objectives: Essential oils (EOs) have multi-target antifungal activity, but their translation is limited by volatility and poor aqueous dispersibility. Randomly methylated β-cyclodextrin (RAMEB) inclusion may enhance effective exposure and thereby alter susceptibility, stress responses, and biofilm outcomes in a species-dependent manner. This study quantified species-specific planktonic and biofilm susceptibility to four EOs and their RAMEB complexes across clinically relevant Candida species. Methods: Lavender (L), lemon balm (B), peppermint (P), and thyme (T) oils and their RAMEB complexes (RL, RB, RP, and RT) were tested against C. albicans and non-albicans Candida. Susceptibility thresholds were used to derive phase plasticity metrics. Functional inhibition was assessed via planktonic metabolism/viability and established biofilm metabolism/viability/biomass. Mechanistic signatures were captured by ROS/RNS measurements and a qPCR analysis of antioxidant genes (CAT1, GPX1, and SOD1) was performed. Mixed-effects models and multivariate/unsupervised and interpretable classification approaches (k-means, PCA, and CRT) were used to integrate endpoints and stratify response phenotypes. Results: Susceptibility thresholds were strongly species-structured (lowest MIC90/EC10 for C. albicans; higher thresholds and broader sublethal windows in non-albicans species). RAMEB complexation produced formulation-dependent shifts in efficacy, with RT emerging as the most consistent broad-spectrum inhibitory condition across compartments. Biofilm biomass was comparatively insensitive even when viability was suppressed, indicating a decoupling of structural biomass from biocidal activity. Mechanistic signatures were broadly conserved across species and linked to antioxidant-program engagement, with CAT1-related rules contributing to responder/tolerant classification. Conclusions: Integrating MIC/EC plasticity with functional and mechanistic markers supports the rational selection of EO formulations; RAMEB complexation, particularly RT, prioritizes candidates for further pharmaceutical optimization while highlighting species-specific vulnerabilities.