Vapor versus Condensed Phase Essential Oils: A Chemical and Biological Characterization Unveils a Phase-Dependent Behavior

The objective of this study was to compare the chemical composition and antimicrobial activity of essential oils (EOs) derived from five aromatic plants. Mentha spicata L., Thymus vulgaris L., Lavandula stoechas L., Origanum vulgare L., and Eucalyptus cinerea were subjected to steam-distillation, and both the volatile profile and antimicrobial activity of the obtained extracts were investigated. The volatilome was characterized using three complementary analytical approaches: Headspace solid-phase microextraction (HS-SPME), static headspace (HS), and liquid-phase (Liq) analysis. Antimicrobial activity was evaluated against two clinically relevant multidrug-resistant pathogens, methicillin-resistant Staphylococcus aureus (MRSA) and carbapenem-resistant Acinetobacter baumannii (CR-Ab), and the fungal pathogen Candida albicans (CA), by means of minimum inhibitory concentration (MIC) determination, disk diffusion, and vapor-phase exposure assays. Principal component analysis (PCA) was applied to identify compositional trends across EOs and sampling methods. The results revealed a significant phase-dependent chemical composition: the vapor phase was consistently enriched in monoterpenes relative to the liquid phase, which was dominated by oxygenated monoterpenoids. Origanum and thymus liquid EOs, for instance, contain a high abundance of phenolic compounds (i.e., thymol and carvacrol) which are virtually absent in the relevant vapor phase. Nevertheless, these EOs retained a significant activity in the gas phase, most probably due to the high abundance of the more volatile gamma-terpinene, which emerged as one of the most active compounds in vapor-phase assays, particularly against MRSA. These findings highlight the importance of phase-specific characterization in EO research, providing a rational basis for the optimization of vapor-phase EO applications in food safety and antimicrobial packaging.