Exogenous Methicillin-Resistant Endophthalmitis is Caused by Multidrug-Resistant Lineages that are Associated with Poor Outcomes.

Purpose: To investigate the genomic epidemiology of methicillin-resistant (MRSA) endophthalmitis and correlate it with the presenting clinical features and outcomes.

Methods: Nine patients presenting with MRSA endophthalmitis from 2014 to 2022 were included. Phenotypic and genomic tests were used for strain characterization. Demographics, clinical presentation, treatment and outcomes were reviewed.

Results: The MRSA population was dominated by multidrug-resistant (MDR) strains within the clonal complex 5 (CC5) carrying an SCC type II genetic element (USA100-like strains). These strains carried genes that confer resistance to five antibiotic classes, in addition to mutations in topoisomerase genes ( and ) that resulted in resistance to all fluoroquinolones tested. Patients were mostly male (56%), with a median age of 82.7 years, and most had no recent history of extensive healthcare exposure. All cases were exogenous following ocular surgery (67%) or intravitreal injection (33%). The main exam findings were visual acuity ≤ hand motion, hypopyon (89%), and vitreous opacity (89%). Five patients (56%) showed improvement in visual acuity at 1 month following presentation, three (33%) at 3 months, and two (22%) at 6 months. Complications included evisceration ( = 1) and phthisis ( = 1). Patients who had pars plana vitrectomy within 48 hours of presentation had better clinical outcomes compared to those who did not.

Conclusion: Exogenous MRSA endophthalmitis is caused by MDR strains that resemble the hospital-acquired lineage USA100. These strains cause severe endophthalmitis in patients with no recent hospital/healthcare exposure.