Background: Staphylococcus aureus is a frequent cause of eye infections, with isolates exhibiting increased antimicrobial resistance to commonly prescribed antibiotics. S. aureus can target the cornea, tear ducts, eyelids and conjunctiva. The aim of this study was to analyse the antimicrobial susceptibility patterns of S. aureus strains from USA and Australia derived from infectious and non-infectious ocular surface adverse events.
Methods: 51 strains of S. aureus from different ocular conditions (11 microbial keratitis [MK] strains, 26 conjunctivitis strains and 14 strains from non-infectious contact lens corneal infiltrative events (niCIEs), were analysed for susceptibility to antibiotics commonly used to treat these conditions. Strains isolated from niCIEs were also tested for susceptibility to multipurpose disinfectant solutions. The broth dilution method was used for all testing.
Results: All the strains in study were sensitive to vancomycin (100%) and gentamicin (98%). The susceptibility to other antibiotics decreased in the following order chloramphenicol (80%), oxacillin (70%), ciprofloxacin (45%), ceftazidime (13%), azithromycin (10%) and polymyxin B (2%). All MK strains from Australia were susceptible to ciprofloxacin and oxacillin whereas only 11% MK strains from the USA were susceptible to these antibiotics (p = 0.107). 75% of conjunctivitis strains from Australia were susceptible to ciprofloxacin compared to 37% of conjunctivitis strain from USA (p=0.278). Most (90%) Australian niCIE and USA conjunctivitis (96%) strains were susceptible to chloramphenicol. Only 55% of MK strains from USA were susceptible to chloramphenicol and this was significantly different to the frequency for USA conjunctivitis strains (p = 0.0036). 84% of all strains were multi-drug resistant. All niCIE strains were sensitive to all four MPDSs at 100% concentration. The MPDS OPTI-FREE PureMoist was most active against all strains, followed by Renu advanced formula, Complete Revitalens and Biotrue.
Conclusions: All S. aureus strains remained sensitive to vancomycin and gentamicin. However, MK strains from USA were more likely to be resistant to oxacillin and ciprofloxacin. As gentamicin and ciprofloxacin are commonly used to treat keratitis, these results should be borne in mind when therapeutic decisions are made.