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Characterization of infectious crystalline keratitis caused by a human isolate of Streptococcus mitis
P J McDonnell1, S Kwitko, J M McDonnell
1Doheny Eye Institute, Los Angeles, CA 90033.
Abstract:
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.
Insights
This study models infectious crystalline keratitis in rabbits, finding that corticosteroids are essential for developing the condition. The bacteria Streptococcus mitis appear shielded from immune responses by fibrin or keratocytes.
Area of Science:
- Ophthalmology
- Microbiology
- Immunology
Background:
- Infectious crystalline keratitis is a severe corneal infection.
- The role of host immune response in its pathogenesis is not fully understood.
Purpose of the Study:
- To establish an animal model for crystalline keratitis.
- To investigate the interaction between Streptococcus mitis and corneal tissue under different treatment conditions.
Main Methods:
- Intrastromal injection of Streptococcus mitis into rabbit corneas.
- Treatment with tetracycline, methylprednisolone, or combination therapy.
- Histopathological and transmission electron microscopy analysis.
Main Results:
- Corticosteroid treatment combined with tetracycline induced crystalline stromal opacities.
- These opacities contained aggregates of bacteria and fibrin.
- Bacteria persisted for up to 44 days, shielded by fibrin or keratocytes, with minimal inflammation.
Conclusions:
- Periocular corticosteroids are crucial for developing this crystalline keratitis model.
- Fibrin and keratocytes may facilitate bacterial sequestration, evading host immune response.
- This model provides insights into the pathogenesis of infectious crystalline keratitis.