Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 16 2 2024 04 19 166 175 EN Ingy El-Soudany Department of Microbiology and Immunology, Faculty of Pharmacy, Pharos University in Alexandria, Alexandria, Egypt Ibrahim Abdelwahab Department of Microbiology and Immunology, Faculty of Pharmacy, Pharos University in Alexandria, Alexandria, Egypt Marwa Yakout Department of Microbiology and Immunology, Faculty of Pharmacy, Pharos University in Alexandria, Alexandria, Egypt 2023 10 21 2024 01 10 Background and Objectives: Stenotrophomonas maltophilia is an opportunistic pathogen causing nosocomial infections. Diclofenac is an anti-inflammatory drug that is considered a non-antibiotic drug. This study assessed the antibacterial and antibiofilm effects of diclofenac and levofloxacin/diclofenac combination against levofloxacin resistant isolates. Materials and Methods: Minimum inhibitory concentration was determined using broth microdilution method for levofloxacin, diclofenac, and levofloxacin/diclofenac combination. Biofilm forming capacity and biofilm inhibition assay were determined. Relative gene expression was measured for efflux pump genes; smeB, and smeF genes and biofilm related genes rmlA, spgM, and rpfF without and with diclofenac and the combination. Results: Diclofenac demonstrated MIC of 1 mg/ml. The combination-with ½ MIC diclofenac- showed synergism where levofloxacin MIC undergone 16-32 fold decrease. All the isolates that overexpressed smeB and smeF showed a significant decrease in gene expression in presence of diclofenac or the combination. The mean percentage inhibition of biofilm formation with diclofenac and the combination was 40.59% and 46.49%, respectively. This agreed with biofilm related genes expression investigations. Conclusion: Diclofenac showed an antibacterial effect against Stenotrophomonas maltophilia. The combination showed in-vitro synergism, significant reduction in biofilm formation and in the relative level of gene expression. Furthermore, it can potentiate the levofloxacin activity or revert its resistance. https://ijm.tums.ac.ir/index.php/ijm/article/view/4452 https://ijm.tums.ac.ir/index.php/ijm/article/download/4452/1652