Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 16 4 2024 08 14 552 559 Shahriar Keyhani Department of Microbiology, Faculty of Medicine, Hamadan University of Medical Sciences, Hamadan, Iran Mohammad Yousef Alikhani Infectious Disease Research Center, Hamadan University of Medical Sciences, Hamadan, Iran Amin Doosti-Irani Department of Epidemiology, School of Public Health and Research Center for Health Sciences, Hamadan University of Medical Sciences, Hamadan, Iran Leili Shokoohizadeh Infectious Disease Research Center, Hamadan University of Medical Sciences, Hamadan, Iran 2023 11 12 2024 07 06 Background and Objectives: Today, medicinal plants and their derivatives are considered to reduce the prevalence of antibiotic resistance. The aim of this study was to investigate the effect of Mentha longifolia essential oil on oqxA efflux pump gene expression and biofilm formation in ciprofloxacin-resistant Klebsiella pneumoniae strains. Materials and Methods: A total of 50 clinical strains of K. pneumoniae resistant to ciprofloxacin were studied. The minimum inhibitory concentration (MIC) of M. longifolia essential oil and its synergistic effect with ciprofloxacin were determined using the microbroth dilution method and the fractional inhibitory concentration (FIC) method. Minimum biofilm inhibition concentration (MBIC) of M. longifolia essential oil was detected. The effect of essential oils on the expression level of the oqxA gene was detected by Real-time PCR. Results: M. longifolia essential oil showed inhibitory activity against ciprofloxacin-resistant strains of K. pneumoniae. When M. longifolia essential oil was combined with ciprofloxacin, the MIC was reduced 2-4 times. In 28% of the strains, M. longifolia with ciprofloxacin showed a synergistic effect. M. longifolia essential oil reduces the strength of biofilm formation and alters the biofilm phenotype. A significant decrease in oqxA gene expression was observed in all isolates after treatment with M. longifolia essential oil. Conclusion: Based on the results of this study, it was observed that supplementing M. longifolia essential oil can help reduce ciprofloxacin resistance and inhibit biofilm formation in fluoroquinolone-resistant K. pneumoniae strains. https://ijm.tums.ac.ir/index.php/ijm/article/view/4480 https://ijm.tums.ac.ir/index.php/ijm/article/download/4480/1700