Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 17 5 2025 10 14 773 782 Azadeh Ehsani Department of Microbiology and Microbial Biotechnology, Faculty of Life Sciences and Biotechnology, Shahid Beheshti University, Tehran, Iran Neda Soleimani Department of Microbiology and Microbial Biotechnology, Faculty of Life Sciences and Biotechnology, Shahid Beheshti University, Tehran, Iran Mohammadreza Modaresi Estehbanati Department of Pediatric Pulmonary and Sleep Medicine, Children's Medical Center Hospital, Tehran University of Medical Sciences, Tehran, Iran 2024 12 01 2025 08 07 Background and Objectives: Pseudomonas aeruginosa is a Gram-negative bacterium that causes respiratory infections in individuals with cystic fibrosis. Its level of virulence is primarily controlled through Quorum Sensing (QS), a communication mechanism that utilizes small signaling molecules. This study investigates P. aeruginosa antibiotic resistance in CF patients in Imam Khomeini Hospital and examines the presence of QS genes in resistant strains. Materials and Methods: Sixty-five P. aeruginosa samples were identified in CF patients in Imam Khomeini Hospital in Tehran. Antibiotic resistance was assessed using the disk diffusion method, and QS genes (rhlI, rhlR, lasI, lasR) were evaluated by applying PCR. Results: Approximately 61.5 % of P. aeruginosa strains were multiple-drug-resistant (MDR), with 30.7% classified as extensively drug-resistant (XDR). The highest resistance was observed against amoxicillin, amikacin, and cefepime. The most common QS gene in MDR and XDR strains was rhlR. Additionally, 78.9% of XDR isolates carried rhlI, rhlR, lasI, and lasR genes. Conclusion: The study specified that more than half of the P. aeruginosa strains exhibited resistance to five antibiotic classes, and effective antibiotics against P. aeruginosa were colistin, meropenem, ciprofloxacin, piperacillin/tazobactam, and cefotaxime. A noteworthy correlation was identified between MDR and XDR strains and the existence of QS genes in the strains. https://ijm.tums.ac.ir/index.php/ijm/article/view/5169 https://ijm.tums.ac.ir/index.php/ijm/article/download/5169/1823