Staphylococcus corneal virulence in a new topical model of infection

Abstract

Purpose: To develop a topical inoculation model of Staphylococcus aureus keratitis in which scarification, contact lenses, and spermidine are used to inhibit the host defenses and to investigate the role of alpha-toxin in this infection.

Methods: An alpha-toxin-positive parent strain (8325-4), its isogenic alpha-toxin-negative mutant (DU1090), and a genetically rescued form of the mutant (DU1090/pDU1212) were bound to rabbit-specific contact lenses, treated with spermidine (50 mM), and applied to scarified rabbit corneas. Eyes were treated topically with spermidine before and after lens application. Eyes were graded for disease by slit lamp examination (SLE) every 6 hours until 24 hours PI (PI), and erosion diameters were measured. Histopathologic changes and colony forming units (CFUs) of bacteria were determined.

Results: Spermidine treatment and inoculation of eyes with Staphylococcus on contact lenses resulted in significant increases in both CFUs per cornea (P = 0.0041) and SLE score (P <or= 0.0001), compared with eyes inoculated without spermidine treatment. The CFUs in eyes infected with 8325-4, DU1090, or DU1090/pDU1212 demonstrated a similar (P >or= 0.1959) multilog increase in CFUs over the inoculum at 24 hours PI. The alpha-toxin-producing strains, 8325-4 and DU1090/pDU1212, caused significantly more disease than the alpha-toxin-deficient mutant DU1090 at 24 hours PI (P <or= 0.0001). Histopathology revealed bacteria in scarified regions of the corneas and, for 8325-4 and DU1090/pDU1212, extensive epithelial sloughing and severe inflammation.

Conclusions: A new topical model of infection has been developed, and alpha-toxin is an important virulence factor in this model.