en Microbiology Microbiology Original Article Original Article <p dir="ltr" style="text-align: justify;"><span style="font-family: times new roman;"><span style="color: rgb(0, 0, 255);"><strong><em>Background: </em></strong></span>Treatment of chronic sinusitis is complicated due to increase of antibiotic-resistant bacteria. The aim of this study was to determine the multidrug resistance (MDR) pattern of the bacteria causing chronic sinusitis in north of Iran.</span></p> <p dir="ltr" style="text-align: justify;"><span style="font-family: times new roman;"><span style="color: rgb(0, 0, 255);"><strong><em>Methods:</em></strong></span> This cross-sectional study was carried out on patients with chronic sinusitis. Bacterial susceptibility to antimicrobial agents was determined according to the CLSI 2013 standards. Double-disk synergy (DDS) test was performed for the detection of extended-spectrum beta-lactamase (ESBL) producing bacteria; also methicillin-resistant Staphylococcus (MRSA) strains were identified by MRSA screen agar.&nbsp; The MDR isolates were defined as resistant to 3 or more antibiotics. Data were analyzed using SPSS 17 software. Descriptive statistics was used to describe the features of the data in this study.</span></p> <p dir="ltr" style="text-align: justify;"><span style="font-family: times new roman;"><span style="color: rgb(0, 0, 255);"><strong><em>Results:</em></strong></span> The rate of ESBL-producing bacteria was 28.75-37.03% among enterobacteriaceae and the rate of MRSA was 42.75%-60% among <em>Staphylococcus</em> strains. The most detectable rate of the MDR bacterial isolates was Gram-negative bacteria 39 (76.47%) and <em>Enterobacter </em>spp. 19(70.37%) was the most multidrug resistant isolate among Gram negative bacteria. Also 36 (73.46%) of the gram positive bacterial isolated were multidrug resistance and <em>Staphylococcus aureus</em> 9(90%) was the most MDR among Gram positive bacteria.</span></p> <p dir="ltr" style="text-align: justify;"><span style="font-family: times new roman;"><span style="color: rgb(0, 0, 255);"><strong><em>Conclusion:</em></strong></span> Antimicrobial resistance is increasing in chronic bacterial sinusitis. The emergence of MRSA and ESBL bacteria causing chronic sinusitis is increasing</span></p> Chronic sinusitis, Bacteria, Antibiotic resistance, MRSA, ESBL, MDR 114 119 http://caspjim.com/browse.php?a_code=A-10-387-1&slc_lang=en&sid=1 Mohammad-sadegh Rezai drmsrezaii@yahoo.com 10031947532846005286 10031947532846005286 No Mazandaran Pediatric Infectious Diseases Research Center, Mazandaran University of Medical Sciences, Sari, Iran. Rostam Pourmousa drmsrezaii49@gmail.com 10031947532846005289 10031947532846005289 Yes Mazandaran Pediatric Infectious Diseases Research Center, Mazandaran University of Medical Sciences, Sari, Iran. Roksana Dadashzadeh roksana.dadashzadeh0111@gmail.com 10031947532846005288 10031947532846005288 No Mazandaran Pediatric Infectious Diseases Research Center, Mazandaran University of Medical Sciences, Sari, Iran. Fatemeh Ahangarkani fkani63@gmail.com 10031947532846005287 10031947532846005287 No Antimicrobial Resistance Research Center, Department of Infectious Diseases, Mazandaran University of Medical Sciences, Sari, Iran.