Antibacterial, antibiofilm, and gene expression assessment of ajwain (<i>Trachyspermum ammi</i>) essential oil on drug-resistant gastrointestinal pathogens and its combination effect with ampicillin

Essential oils are natural substances used as therapeutic agents and food preservatives to inhibit harmful microorganisms. This study aimed to assess the synergistic effect of Trachyspermum ammi essential oil and ampicillin on antibiotic-resistant gastrointestinal pathogens, including Escherichia coli, Enterococcus faecalis, Shigella flexneri, and Salmonella serotype Typhimurium. Using gas chromatography-mass spectrometry (GC-MS), the main components of T. ammi essential oil were identified as thymol, gamma terpenes, and cymene. The antibacterial and antibiofilm properties were evaluated by minimum inhibitory concentration (MIC), disk diffusion, and microtiter plate methods, revealing MIC values of 2, 1, 4, and 4 mg ml-1 for E. coli, E. faecalis, S. flexneri, and S. Typhimurium, respectively, and inhibition zones between 10 and 14 mm. Pathogens were examined for their biofilm-related virulence genes, including aggR, esp, icsA, and fliC, using real-time polymerase chain reaction (RT-PCR) in E. coli, E. faecalis, S. flexneri, and S. Typhimurium, respectively. The methyl thiazole tetrazolium (MTT) assay was used to evaluate the essential oil's effect on the viability of human embryonic kidney 293 (HEK293) cells, which showed cell viability of over 80%. The combination of T. ammi oil and ampicillin demonstrated a synergistic effect, and biofilm formation was inhibited. E. faecalis exhibited the greatest sensitivity, while S. flexneri exhibited the lowest sensitivity.