en Microbial pathogenesis Microbial pathogenesis Original article Original article <strong>Introduction: </strong>After <em>Staphylococcus aureus</em> and <em>Escherichia coli</em>, <em>Pseudomonas aeruginosa</em> is the third cause of hospital-acquired infection (HAI). This bacteria&#39;s ability to colonize in different environments, especially in hospitals and biofilm formation, has added to its impact as an HAI. The molecular mechanism of biofilm formation is not well understood, but several genes contribute to this phenomenon.&nbsp; This study investigates the frequency of <em>cbrA</em>, <em>cbrB</em>, <em>phoBR, </em>and<em> ndvB</em> genes in biofilm-forming <em>P. aeruginosa</em> isolates.<strong> Methods: </strong>Fifty <em>P. aeruginosa</em> clinical isolates were collected from various sources such as urine, ulcer, blood, secretions, and trachea in Milad Hospital, Tehran, from 2017 to 2018. Biofilm formation in the isolates was assessed by the microtiter plate assay, and the frequency of <em>cbrA</em>, <em>cbrB</em>, <em>phoBR</em>, and <em>ndvB</em> genes was investigated by PCR.<strong> Results: </strong>Among the 50 isolates, 44% were strong biofilm former, 34% moderate biofilm former, 12% weak biofilm former, and 10% did not form biofilms. PCR revealed a frequency of 94% for the <em>cbrA</em> gene, 78% for <em>cbrB</em>, 96% for <em>ndvB</em>, and 48% for <em>phoBR</em>.&nbsp; The coexistence of all four genes was 68% in strong biofilm former isolates, 41% in moderate biofilm former isolates, 37% in weak biofilm former, and zero in the isolates that formed no biofilm.<strong> Conclusion:</strong> The high frequency of <em>ndvB</em> and <em>cbrA</em> genes and the coexistence of <em>ndvB</em> and <em>cbrB</em> suggest the contribution of these genes in the biofilm formation of <em>P.&nbsp; aeruginosa. </em> Pseudomonas aeruginosa, Biofilm, cbrA, cbrB, phoBR, ndvB 97 102 http://jommid.pasteur.ac.ir/browse.php?a_code=A-10-290-2&slc_lang=en&sid=1 Mahla Tajmir Riahy Mahla@gmail.com 0000-0002-9909-8196 No Department of Microbiology, Islamic Azad University, Varamin- Pishva Branch, Varamin, Iran Sahar Honarmand Jahromi sahar.honarmand@yahoo.com 0000-0003-3060-0368 No Department of Microbiology, Islamic Azad University, Varamin- Pishva Branch, Varamin, Iran. Mansoor Khaledi mansoor.khaledi@yahoo.com 0000-0002-1576-5968 No Department of Microbiology, Faculty of Medicine, Shahed University, Tehran, Iran Hamed Afkhami Afkhami@gmail.com 0000-0002-1110-6447 No Department of Microbiology, Faculty of Medicine, Shahed University, Tehran, Iran Maryam Shafaati maryam.shafaati@gmail.com 0000-0002-7401-2237 No Department of Microbiology, Jahrom Branch, Islamic Azad University, Jahrom, Iran Hamid Lava Khamseh Hamaidh@gmail.com 0000-0002-2501-7541 No Microbiology Department, Kurdistan University of Medical Sciences, Sanandaj, Iran Shohreh Zare Karizi h.a.university.ac@gmail.com 0000-0002-0305-8588 Yes Department of Genetic, Islamic Azad University, Varamin- Pishva Branch, Varamin, Iran.