Cytotoxic strains of Pseudomonas aeruginosa can damage the intact corneal surface in vitro

Purpose: Although the corneal epithelial cell layer is believed to serve as a barrier against moat types of bacteria, certain strains of P. aeruginosa have been shown to kill corneal epithelial cells in primary cultures. The aim of this study was to test whether these strains could damage epithelia on uninjured whole corneas. Methods: Five-week-old Sprague-Dawley rats were sacrificed end their eyes were placed in organ culture. The corneal surface of each eye was incubated with 8 μL of media alone or a bacterial suspension containing 8x10⁶ cfu of one of 12 cytotoxic and noncytotoxic strains of P. aeruginosa for 3 hours at 35°C. Trypan blue was then added to visualize surface epithelial cell injury. A masked observer examined each cornea under a dissecting microscope and assigned s score of between 1 end 3 to describe the extent of injury. Results: Cytotoxic P. aeruginosa injured the surface epithelium. The extent of injury induced by the various strains correlated with previously published in vitro measures of cytotoxic capacity toward cultured corneal epithelial cells. Cytotoxicity required at least 2 hours of bacterial contact and was dependent upon ExsA, a transcriptional activator of several genes in P. aeruginosa, including the gene encoding exoenzyme S. Conclusions: Cytotoxic P. aeruginosa strains can damage epithelia on an uninjured corneal surface providing there is prolonged bacterial contact. Stagnation of cytotoxic bacteria against the corneal surface may contribute to the pathogenesis of infection associated with the use of soft contact lenses.