Purpose: To determine the virulence of Staphylococcus aureus strains in the rabbit conjunctiva.
Methods: Three strains of methicillin-sensitive S. aureus (8325-4, Newman, and UMCR1) and two strains of methicillin-resistant S. aureus (70490 and MW2) were analyzed. Rabbit bulbar conjunctivas (n ≥ 6 per group) were injected with 10(5) colony forming units (CFU) in 10 µl. Eyes were photographed and analyzed for pathology at 20 hr postinfection (PI) using slit lamp examination (SLE) to measure five parameters on a scale from 0 (normal) to 4 (severe): injection, chemosis, iritis, corneal edema, and pinpoint conjunctival hemorrhages. The parameter grades were added to produce a SLE score. Bacteria were enumerated and histopathological analysis was done at 20 hr PI. Myeloperoxidase assays were performed on conjunctival swabs (n ≥ 3 per strain) at 0 and 20 hr PI.
Results: Conjunctivas injected with 8325-4 or Newman had SLE scores of 1.67 ± 0.12 and 0.81 ± 0.16, respectively. Strain 70490 produced an average SLE score of 2.94 ± 0.47, whereas MW2 produced a score of 5.04 ± 0.73. UMCR1 produced severe conjunctivitis having a SLE score of 13.25 ± 0.80. Only strain UMCR1 grew in the conjunctiva showing a 2.7 log increase in CFU; all other strains remained near the inoculated numbers or decreased as much as 1.85 logs. Myeloperoxidase activity was greatest in the tear film of UMCR1 infected eyes with over one million PMN present at 20 hr PI.
Conclusions: Only one S. aureus strain, UMCR1, was able to cause a reproducible severe conjunctivitis. This conjunctival infection could be used to test new antimicrobials and to help understand the pathogenesis of conjunctivitis, especially in terms of overcoming the host defenses.