Ocular Microbiology and Immunology Group
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2025 OMIG Abstract

Clinical and Genomic Profile of Staphylococcus aureus ST398 in Ocular Infections

Humza Qureshi, Lucas Sejournet, and Paulo J. M. Bispo

Department of Ophthalmology, Massachusetts Eye and Ear, Harvard Medical School, Boston, Massachusetts

Purpose: ST398 S. aureus has emerged as a major cause of acute severe infections in individuals with close contact to livestock and was later found to be disseminated in populations without livestock contact. This study aims to analyze ocular isolates of the ST398 strain in order to define its clinical characteristics and determine whether the livestock-associated or human-originated strain lineage is implicated.

Methods: Whole genome sequencing of ocular S. aureus isolates collected between 2014 and 2024 at Massachusetts Eye and Ear was performed to identify circulating genotypes. Assembled S. aureus genomes were screened using the Center for Genomic Epidemiology’s (CGE) MLST tool for strain identification. Virulence factors were identified using the CGE VirulenceFinder tool together with the annotation and BLAST features within Geneious. Resistomes were predicted using the CGE ResFinder and Comprehensive Antibiotic Resistance Database (CARD) Resistance Gene Identifier tools. Resistance phenotypes were verified with broth microdilution testing. Retrospective chart review of clinical data was performed for patients whose isolates were analyzed.

Results: 10 ocular isolates from 8 patients were identified as S. aureus ST398. The average patient age was 52.3 ± 12.0 years, 6 (75%) patients were female, 4 (50%) had Medicaid or were unhoused, and none worked with livestock. 3 (37.5%) patients were diagnosed with keratitis, 3 (37.5%) with conjunctivitis, 1 (12.5%) with endophthalmitis, and 1 (12.5%) with preseptal cellulitis. The average best corrected visual acuity at presentation was 1.25 ± 1.04 (logMAR) and 0.88 ± 1.09 after treatment. 3 (37.5%) patients required surgery including corneal patch graft, orbitotomy, and vitrectomy. Genes encoding relevant virulence factors in the isolates included the hemolysin genes hlgA (100%) hlgB (100%) and hlgC (100%), and hld (90%). All isolates lacked mecA for methicillin resistance while possessing erm(T) for erythromycin resistance and norC which facilitates fluoroquinolone resistance. 8 isolates (80%) possessed blaZ for penicillin resistance. In-vitro susceptibility testing showed 100% agreement with predicted erythromycin and penicillin resistance profiles. Presence of the norC gene was insufficient for resistance to the fluoroquinolone moxifloxacin in all isolates.

Conclusions: Human-originated methicillin-susceptible S. aureus ST398 seems to be well-adapted to the ocular niche and can cause severe infections despite being treatable with moxifloxacin. Additionally, low socioeconomic status may be a risk for ST398 ocular infection.