From colonizer to culprit: genomic and clinical insights into S. epidermidis from post-surgical endophthalmitis.

Purpose: To describe the clinical characteristics of exogenous episodes of endophthalmitis from which Staphylococcus epidermidis was isolated by vitreous cultures. We also explored the genomic traits of these S. epidermidis isolates and their relatedness to S. epidermidis originating from carriers and from prosthetic joint infections in the same geographical region.

Methods: S. epidermidis isolated from cases of exogenous endophthalmitis (n = 33) were genome sequenced. Clinical features were retrospectively collected from medical records. The isolates were compared with previously sequenced S. epidermidis isolates from the nares of healthy individuals (n = 151) and from prosthetic joint infections (n = 138).

Results: The most common ophthalmological procedure preceding the endophthalmitis was a posterior segment surgery (76%; 25/33), mainly intravitreal injection (70%; 23/33). These patients displayed a significantly shorter time to symptoms compared to those with an anterior segment surgery (median 3 vs. 9 days; p < 0.001), and significantly less phenotypic methicillin resistance (8%, n = 2/25 vs. 50%, n = 4/8; p = 0.02). Most isolates of S. epidermidis originating from endophthalmitis cases did not belong to known healthcare-associated lineages and did not cluster with isolates from prosthetic joint infections. Rather, they were more similar to isolates from the nares of healthy individuals.

Conclusions: Genomic data suggest that the S. epidermidis isolated from the vitreous of Swedish cases of postsurgical endophthalmitis may originate from the commensal flora of the individual, and not from the healthcare facilities. The type of preceding surgical procedure (anterior vs. posterior segment) may influence both symptom delay and the presence or absence of methicillin resistance.