Evaluation of Bioactivity of Essential Oils: Cytotoxic/Genotoxic Effects on Colorectal Cancer Cell Lines, Antibacterial Activity, and Survival of Lactic Acid Bacteria.

Colorectal cancer (CRC) ranks among the most frequently diagnosed malignancies and is associated with a significantly high mortality rate. In recent years, increasing attention has been directed toward naturally derived substances with anticancer properties. In our study, we focused on determining the biological and antibacterial effects of selected essential oils (EOs)-peppermint, oregano, tea tree, lemon, lavender, frankincense, and oil blends (Zengest and OnGuard). Analyses were performed on human colon carcinoma cell lines (HCT-116 and HT-29). The cytotoxic (MTT assay), genotoxic effects (comet assay), and reactive oxygen species levels (ROS-Glo™ H₂O₂ Assay) of EOs and oil blends were determined. In our study, we found that all of the studied oils have the potential cyto/genotoxic effects on CRC cell lines after 24 h exposure. The results revealed that oregano, Zengest, and frankincense showed statistically the highest cytotoxic effects [IC₅₀ 0.05 µg/mL] compared to the other studied oils. These oils induced DNA damage and also increased ROS levels. On the other hand, peppermint was shown to have the lowest cytotoxic effect [IC₅₀ 0.67 µg/mL] on the HT-29 cell line. We also evaluated the antibacterial effects of oregano, tea tree, and the OnGuard blend, determining their impact on the viability of beneficial bacteria models, including Lacticaseibacillus rhamnosus, Lactiplantibacillus plantarum, Lacticaseibacillus paracasei, Lactobacillus brevis, Lactobacillus pentosus, and Weizmannia coagulans. Oregano exhibited strong antibacterial activity, with an inhibition zone of 31 mm, while tea tree and OnGuard showed inhibition zones ranging from 12 to 15 mm. The EOs (oregano, tea tree, OnGuard) demonstrated antibacterial effects, with MICs ranging from 0.05 to 0.5 µg/mL. Peppermint, lemon, lavender, frankincense, and the Zengest blend did not inhibit the growth of lactic acid bacteria or W. coagulans, and thus did not impact bacterial survival. On the other hand, they demonstrated potential anticancer effects.