Methicillin-Resistant Staphylococcus aureus in Adult Conjunctivitis: Antimicrobial Resistance Patterns, Molecular Characteristics, and Association of Prior Antibiotic Exposure With Clinical Outcome.

BACKGROUND: Staphylococcus aureus is a major cause of bacterial conjunctivitis, and rising antimicrobial resistance, particularly methicillin-resistant S. aureus (MRSA), poses increasing therapeutic challenges. Understanding current clinical characteristics, resistance patterns, and molecular determinants is essential to guide appropriate management. PURPOSE: To evaluate the clinical features, antimicrobial susceptibility, and molecular resistance determinants of S. aureus isolates causing adult conjunctivitis, and to assess the association between previous antibiotic exposure, resistance, and clinical outcome. METHODS: In this observational cross-sectional study, 100 adult patients with culture-confirmed S. aureus conjunctivitis were evaluated clinically. Antimicrobial susceptibility testing was performed, and MRSA was identified using cefoxitin/oxacillin resistance and mecA detection. Fluoroquinolone resistance was assessed in conjunction with mutations in gyrA and parC. Biofilm genes (icaA, icaD) were detected using polymerase chain reaction. Clinical outcomes were recorded, and statistical analyses were performed. RESULTS: MRSA accounted for 40% of isolates, and fluoroquinolone resistance was detected in 20%. Erythromycin resistance was high (70%), whereas all isolates remained susceptible to linezolid. The mecA gene was present in 40% of isolates, icaD in 60%, icaA in 31%, and quinolone resistance-determining region mutations in 46%. Previous antibiotic exposure was significantly associated with MRSA infection (69.0% vs. 28.2%, P < 0.001) and treatment failure (37.9% vs. 8.5%, P = 0.001). Fluoroquinolone resistance showed a nonsignificant increasing trend with prior antibiotic use. CONCLUSIONS: S. aureus conjunctivitis in adults demonstrates a substantial burden of MRSA, high macrolide resistance, and frequent presence of biofilm and resistance genes. Prior antibiotic exposure is a major risk factor for MRSA and poor outcomes. These findings highlight the need for antimicrobial stewardship, culture-guided therapy, and ongoing surveillance to optimize management in the era of rising resistance.