Characterization of Infectious Crystalline Keratitis Caused by a Human Isolate of Streptococcus mitis

Streptococcus mitis isolated from a human with infectious crystalline keratitis was injected intrastromally into corneas of adult New Zealand white rabbits that were treated with tetracycline hydrochloride, methylprednisolone acetate, or a combination of tetracycline and methylprednisolone. Animals were followed up for up to 44 days; untreated corneas and those treated with tetracycline developed no disease or “fluffy” stromal infiltrates with overlying epithelial defects representing an abscess. Corneas treated with the combination of tetracycline and corticosteroid usually developed crystalline stromal opacities that on histopathologic examination were shown to be intrastromal aggregates of cocci. Transmission electron microscopy of crystalline lesions within 10 days of infection revealed typical cocci intermixed with a fibrillar material having periodicity characteristic of fibrinogen or fibrin, and immunoperoxidase staining for fibrinogen was positive. By 1 month, electron microscopy revealed aggregates of degenerated bacteria that were surrounded by cellular processes of activated keratocytes. Our studies demonstrate a model for crystalline keratitis in which organisms are seen to reside within the stroma for up to 44 days without an inflammatory response. Periocular corticosteroids appear to be necessary to create this model. It is possible that the organisms are isolated from the host response by fibrin or by keratocytes.