Composition and Bioactivity of Alentejo Calamintha nepeta Essential Oil: The Impact of Seasonality and Climatic Stress on Antioxidant Capacity and MDR Antibacterial Potential.

Essential oils (EOs) from wild Calamintha nepeta (Portugal) populations collected in Portugal (Évora) were investigated in order to evaluate the impact of Mediterranean seasonal conditions on their phytochemical composition and biological activity. Essential oil GC-FID and GC-MS analyses revealed distinct seasonal chemotypes, with spring samples dominated by isopulegone/pulegone, whereas autumn samples contained higher proportions of isomenthone and menthol. Antioxidant activity was assessed through lipid peroxidation inhibition, DPPH radical scavenging and ferric reducing power assays, while antibacterial activity was evaluated against multidrug-resistant (MDR) clinical isolates. Seasonal differences were reflected in both EO chemical composition and bioactivity. Autumn samples displayed greater antioxidant potential, with Y1A showing the highest inhibition of lipid peroxidation (IC50 = 0.85 mg/mL) and Y2A exhibiting the highest ferric reducing power. Conversely, spring samples were more active against MDR bacteria. Among them, Y1S showed the broadest antimicrobial spectrum, with MIC values ranging from 465 to 1767 μg/mL. The unusually wet spring season coincided with marked isopulegone accumulation (≈50%), while warmer autumn conditions favoured higher levels of isomenthone and menthol in the EOs. These findings highlight the importance of seasonal environmental conditions in determining the phytochemical profile and bioactive potential of C. nepeta EOs, providing valuable insights for their standardisation and valorisation in pharmaceutical, food and conservation-related applications.