Impact of clove and thyme essential oils, and their main components, eugenol and thymol, on elderly gut microbiota in an artificial colon model

Abstract

Essential oils (EOs) derived from herbs and spices have multiple biological properties that support their potential as novel therapeutics for conditions including age-related gut microbiome alteration. This study aimed to assess the effect of EOs and their main compounds on the growth of a rationally-selected gut bacterial consortium (the S7), and to evaluate their impact on the whole gut microbiota of healthy and frail elderly donors in an in vitro colon model. EOs of clove (Syzygium aromaticum) and thyme (Thymus vulgaris) were extracted and characterised, yielding clove EO that was rich in eugenol (47.3%) with high antioxidant potential, and thyme EO with inferior thymol content (18.2%) and lower antioxidant capacity. The EOs obtained and their main compounds (eugenol and thymol, respectively), showed concentration-dependent inhibitory effects on the S7, with thyme EO exhibiting the strongest bactericidal effect, while EOs intermediate concentrations (1 mg/mL of clove EO and 0.1 mg/mL of thyme EO) enhanced the growth of one of the consortium species - Roseburia hominis. In frail subjects, clove EO significantly modified microbiota composition, affecting both alpha and beta diversity. Clove EO supplementation enhanced healthy-aging associated taxa including Lachnoclostridium and Faecalibacterium, while decreasing the relative abundance of Escherichia-Shigella group and Bacteroides. These results indicate the potential of clove EOs and eugenol as microbiota modulating ingredients for the design of functional foods and nutraceuticals aimed at mitigating age-related health conditions.