Abstract
Purpose: :
S. aureus is a normal commensal of the human skin and nasopharynx, yet S. aureus infection appears to be predominantly caused by only a subset of these organisms. This is an epidemiologic analysis of the phenotypic and genotypic characteristics of S.aureus clinical isolates that have been associated with keratitis.
Methods: :
All S. aureus clinical isolates were prospectively collected over a 24 month period at the MEEI (2006-2008). The diagnosis of clinical keratitis and associated risk factors was by medical record review. The S. aureus-associated keratitis strains were assessed for: 1) antibiotic susceptibility data by CLSI standards, 2) biofilm robustness by Gentian violet staining using an in vitro microtiter plate assay, and 3) species lineage by multi-locus sequence typing (MLST).
Results: :
26 cases of keratitis were identified from the 600 S. aureus clinical isolates. Risk factors associated with S.aureus keratitis included trauma, prior surgery, soft contact lens wear, and the presence of a foreign body. Ocular surface disease does not appear to be an independent risk factor. All 26 isolates were tetracycline- and trimethoprim-sulfamethoxazole- sensitive. Prior antibiotic usage did correlate strongly with methicillin-resistance (9/26 strains), and all the MRSA strains were found to be ciprofloxacin-resistant (10/26). More than one-third of all the S.aureus keratitis-associated isolates, and more than one-half of medically-related foreign body infections, were caused by a single clone, ST5. Both sensitive and resistant S. aureus strains were represented within ST5.
Conclusions: :
These results suggest that there may be specific S.aureus lineages which possess phenotypic and genotypic characteristics that enable them to more effectively cause sight-threatening keratitis. Future work will examine their virulence traits and a comparison to commensal S.aureus strains.
Keywords: keratitis • Staphylococcus • clinical (human) or epidemiologic studies: biostatistics/epidemiology methodology