<b>Background and Objective:</b> Detection of methicillin-resistant<i> S. aureus</i> have become a challenge in the presence of oxacillin-susceptible and <i>mecA</i>-positive <i>S. aureus </i>(OS-MRSA), concerning the misidentification events and therapeutic implications. This study aims to identify the OS-MRSA in clinical isolates of Post-viral acute rhinosinusitis, which, hopefully, can interfere with the therapeutic strategy. <b>Materials and Methods:</b> There were 60 patients diagnosed with Post-viral acute rhinosinusitis, recruited from an Ear, Nose and Throat (ENT) outpatient clinic. <i>Staphylococcus aureus</i> isolates were identified from the culture and were then tested for antibiotics susceptibility using a Kirby-Bauer disc diffusion test. The <i>mecA</i>, <i>mecC</i> and <i>blaZ</i> genes were determined using the Polymerase Chain Reaction (PCR) method. <b>Results:</b> <i>Staphylococcus aureus </i>was identified in 20 of the 60 samples from the patients (33.3%; 95% CI: 21.0-45.6). Of the 20 isolates, 19 isolates (95%) had a positive <i>mecA</i> gene, 19 (95%) had a positive <i>mecC</i> gene and 20 (100.0%) had a positive <i>blaZ </i>gene. The majority of the <i>mecA</i>-positive <i>S. aureus</i> showed an oxacillin-susceptible (85%) and 3 isolates (15.0%) were oxacillin-resistant toward the <i>S. aureus</i>. <b>Conclusion:</b> There was a high proportion of Oxacillin/cefoxitin-Susceptible <i>mecA</i>-positive <i>S. aureus</i> in the study population that indicate phenotypic susceptibility to antibiotics does not always indicate the absence of genes that carry resistant traits, thus allowing misidentification if the only phenotypic examination is carried out.
Keywords: BlaZ gene; Staphylococcus aureus; genotype; mecA gene; oxacillin-susceptible; phenotype; rhinosinusitis.