Native oregano essential oil: Phytochemical composition, antibacterial and synergistic activities with oxytetracycline against multi-drug resistant (MDR) Bacteria

Abstract

Essential oil, particularly extracted from the aromatic plant Origanum vulgare (OEOs), exhibits antibacterial and anti-biofilm activity against multidrug-resistant (MDR) bacteria. In the current study, Origanum vulgare essential oil, a native herb of Pakistan, was tested against two novel MDR bacterial strains. These potential pathogens, isolated from the gut, have shown resistance to several antibiotics. The results indicated that the harvesting time significantly affected the oil yield, especially when the plant was collected in September 2024 during its full bloom. Extraction was performed using a modified Clevenger-type apparatus. The oil was tested against both strains: a Gram-positive bacterium, Bacillus licheniformis, and a Gram-negative bacterium, Pseudomonas azotoformans. Results demonstrated that oregano essential oil significantly inhibited the growth of both species, with a minimum inhibitory concentration of 1.25 μl/mL. The fractional inhibitory concentration index (FICI) was calculated after applying synergism using a checkerboard assay for both strains with the sensitive antibiotic oxytetracycline. Bacillus licheniformis exhibited a synergism value of 0.37, indicating strong synergism, whereas for Pseudomonas azotoformans, the FICI index was 0.50, reflecting successful synergism. This synergism helps to increase the efficacy of antibiotics and essential oils. Time-kill assays for oregano essential oil (OEO) demonstrate that within 7 h of incubation with OEO, it can completely kill Bacillus licheniformis, whereas for Pseudomonas azotoformans, the time required extends to 8 h. This approach could provide a sustainable solution for addressing the global antimicrobial resistance disease burden while also possibly reduce the economic burden for developing countries like Pakistan.