Background: Staphylococcus aureus is a frequent cause of different types of eye infections with isolates exhibiting increased antimicrobial resistance to commonly prescribed antibiotics. S. aureus produces several virulence determinants which may be disease specific. This study aimed to determine the antimicrobial susceptibility patterns by using broth microdilution and the virulence determinants by using PCR.
Methods: .63 S. aureus strains from the USA and Australia were evaluated; 14 from niCIE, 26 from conjunctivitis and 23 from infectious keratitis (IK).
Results: The susceptibility of isolates decreased in the following order: vancomycin (100%). gentamicin (98%), chloramphenicol (76%), oxacillin (74%), ciprofloxacin (46%), ceftazidime (11%), azithromycin (8%) and polymyxin B (8%). 87% of all isolates were multidrug resistant and 4 isolates from infectious keratitis were extensively drug resistant. The infectious keratitis strains from Australia were usually susceptible to ciprofloxacin (57%) and oxacillin (93%), conversely a minority of infectious keratitis isolates from the USA were susceptible to ciprofloxacin (11%; p=0.04) and oxacillin (11%; p=0.02). More conjunctivitis strains from Australia (75%) were susceptible to ciprofloxacin than the USA isolates (36%; p=0.2). Only 55% of infectious keratitis isolates from the USA were susceptible to chloramphenicol and this was less than the USA conjunctivitis strains (95%; p = 0.01). No infectious keratitis isolate possessed clfA; conversely all strains form non-infectious CIE possessed clfA (p=0.001). 72% of conjunctivitis strains from the USA possessed clfA, but no conjunctivitis strains from AUS possessed this gene (p=0.01). pvl, which is associated with community-acquired MRSA, was present in 50% of Australian infectious keratitis and 100% of Australian conjunctivitis isolates but was absent in non-infectious CIE isolates (p = 0.001). Australian conjunctivitis isolates (100%) were more likely to possess pvl than the USA conjunctivitis isolates (36%; p = 0.031).
Conclusions: Knowledge of the rates of resistance to antibiotics may be important in decisions on treating these diseases. The differences in possession of virulence genes may be related to the pathogenesis of these conditions