Corneal Pathogenesis of Staphylococcus aureus Gamma-toxin

Abstract

Abstract: : Purpose: To determine the extent of corneal damage mediated by Staphylococcus aureus gamma-toxin or alpha-toxin in a rabbit model. Methods: Rabbit corneas were intrastromally injected with bacterial cultures (100 CFU) or culture supernatants of Staphylococcus aureus strains Newman (gamma-toxin producing), Newman Δhlg (gamma-toxin-deficient), Newman Δhla (alpha-toxin-deficient) or Newman Δhlg-Δhla (gamma- and alpha-toxin-deficient). Ocular inflammation was scored by slit lamp examination (SLE), histological examination of corneal tissue, and corneal epithelial erosions were measured at 15, 20, and 25 hour after injection of bacteria or at 1, 2, 4, 6, and 24 hours after injection of supernatant. Bacterial replication (CFU/cornea) was determined by quantitatively culturing corneal homogenates in triplicate. Results: All strains produced equivalent numbers of CFU/cornea (approximately 7 logs) at 25 hour postinfection (P ≥ 0.05). Strains Newman Δhlg, Newman Δhla, or Newman Δhlg-Δhla had lower SLE scores than the parent strain (Newman) at 20 and 25 hours postinfection (P ≤ 0.0074). Histopathological examination demonstrated reduced corneal infiltrate and an intact cornea of eyes infected with alpha-toxin-deficient mutants. SLE scores also revealed that intrastromal injection of supernatants from Newman Δhlg, Newman Δhla, or Newman Δhlg-Δhla, relative to Newman, the parent strain, produced significantly less ocular damage and inflammation at 4, 6, and 24 hours (P ≤ 0.0001). Intrastromal injection of culture or culture supernatants of strain Newman, Newman Δhlg, Newman Δhla, or Newman Δhlg-Δhla caused multiple ocular changes including severe iritis, scleral edema, injection, and corneal epithelial erosion. Only strains of Newman with a functional alpha-toxin gene produced epithelial erosions at any time point. Conclusions: Gamma-toxin is capable of producing inflammation and tissue damage to the rabbit cornea. Alpha-toxin causes epithelial erosions, as well as the same types of ocular inflammation mediated by gamma-toxin.

Keywords: keratitis • Staphylococcus • microbial pathogenesis: experimental studies