Assessment of the Biological Activity of Thyme Essential Oil in the Presence of the Classic Antibiotic Tetracycline

The present study was purported to assess the activities: (i) antibacterial and synergistic against three types of both Gram-positive and Gram-negative bacteria that are susceptible to drug resistance and (ii) cytotoxic and synergistic on colorectal adenocarcinoma cells, of thyme essential oil (TEo) in combination with tetracycline (Tcyc). Chemical composition of thyme essential oil was evaluated by gas chromatography-mass spectrometry (GC-MS) method, antioxidant capacity by 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) assay, antibacterial and synergistic properties were determined by disk diffusion method and cytotoxic activity by quantifying viable cells by using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. Thyme essential oil has an elevated antioxidant activity, antibacterial potential against both Gram-positive and Gram-negative bacteria, especially on S. aureus and K. pneumoniae at the highest concentration tested (50µL/mL), also having a synergistic effect when combined with tetracycline (50µL/mL TEo with 10µg/mL Tcyc). Essential oil-treated cells showed a dose-dependent reduction in colorectal adenocarcinoma cell viability, while combination with tetracycline leads to a significantly attenuated decrease in viability.