Staphylococcus corneal virulence in a new topical model of infection

PURPOSE. To develop a topical inoculation model of Staphylococcus aureus ke ratitis in which scarification, contact lenses, and spermidine are used to inhibit the host defenses and to investigate the role of alpha -toxin in th is infection. METHODS. An alpha -toxin-positive parent strain (8325-4), its isogenic alph a -toxin-negative mutant (DU1090), and a genetically rescued form of the mu tant (DU1090/pDU1212) were bound to rabbit-specific contact lenses, treated with spermidine (50 mM), and applied to scarified rabbit corneas. Eyes wer e 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 cha nges and colony forming units (CFUs) of bacteria were determined. RESULTS. Spermidine treatment and inoculation of eyes with Staphylococcus o n contact lenses resulted in significant increases in both CFUs per cornea (P = 0.0041) and SLE score (P less than or equal to 0.0001), compared with eyes inoculated without spermidine treatment. The CFUs in eves infected wit h 8325-4. DU1090, or DU1090/pDU1212 demonstrated a similar (P greater than or equal to 0.1959) multilog increase in CFUs over the inoculum at 24 hours PI. The alpha -toxin-producing strains. 8329-4 and DU1090/pDU1212, caused significantly more disease than the alpha -toxin-deficient mutant DU1090 at 24 hours PI (P less than or equal to 0.0001). Histopathology revealed bact eria 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.