Moraxella catarrhalis is an opportunistic bacterium that has pathogenicity in the human respiratory tract. The rates of colonization and infection of this bacterium are high in the respiratory tract of children and the elderly. Methods: In this descriptive cross-sectional study conducted from April 2016 to March 2018, 400 samples were collected from patients with pharyngitis (n = 92), sinusitis (n = 85), otitis media (n = 43), respiratory failure (n = 60), and healthy individuals as the control

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... group (n = 120) in the Northern part of Iran. The isolates were identified by phenotyping methods. Finally, the virulence genes in the isolates were detected by polymerase chain reaction (PCR) technique. Results: Out of a total number of 400 samples, 32 samples (8%) were positive for M. catarrhalis. The frequency of this bacterium was found to be different in patients with pharyngitis as 5.44%, sinusitis as 8.24%, otitis media as 13.95%, respiratory failure as 15%, and in the control group as 4.16%. Since the P value from the Chi-square test was higher than 0.05, there was no relationship between the prevalence of M. catarrhalis and the type of the disease and control group. The PCR results showed that 100% of the isolates had mcaP, ompJ, and ompCD genes. However, the distribution of other virulence genes was varied. Conclusions: Overall, our findings verified the existence of M. catarrhalis in patients with respiratory diseases. Therefore, the rapid identification and successful treatment can play an important role in preventing their spread. In addition, the results verified the high prevalence of virulence genes in M. catarrhalis isolates from patients compared to controls. Regarding the results of this study and by comparing with similar studies, it can be concluded that the frequency of pathogenic isolates may be different even in regions that are not geographically different.