This award plans to use the infrastructure of the NEI-sponsored Steroid for Corneal Ulcers Trial (SCUT) to examine whether adding topical steroids improves clinical outcomes of subgroups of bacterial corneal ulcers. There is ongoing controversy regarding the use of steroids in treating bacterial corneal ulcers. Additional data, including results in specific subgroups, are needed to help clinicians understand the risks vs benefits. Steroid for Corneal Ulcer Treatment (SCUT) is an NEI-funded, trial enrolling patients at the Proctor Foundation at UCSF, Arvind Eye Hospital in South India, and Dartmouth-Hitchcock Medical Center. The main objective of SCUT is to study the effect of topical steroids on bacterial corneal ulcers, as measured by outcomes including best corrected visual acuity and scar size at 3 months. The investigator proposes to perform ancillary studies comparing baseline characteristics and clinical outcome by organisms (Pseudomonas aeruginosa vs. all others) by genetic difference (Invasive via cytotoxic P. aeruginosa) and by antibiotic susceptibility. Three specific aims will be addressed: Aim 1: to determine the effect of adjunctive topical steroid therapy on clinical outcomes in P. aeruginosa keratitis compared to bacterial keratitis due to the other organisms. Aim 2: To determine whether the invasive and cytotoxic subtypes of P. aeruginosa have different clinical signs at enrollment, or have a different response to treatment. Aim 3: To determine whether there is a correlation between antibiotic susceptibility and the clinical outcome in bacterial keratitis. The data and samples necessary for these aims are already being collected as part of the SCUT trial. As part of this research proposal, P.aeruginosa isolates will be subtyped for Aim 2 and the minimum inhibitory concentration of each bacterial isolate to the study antibiotic moxifloxacin will be determined for Aim 3. Regression models will be utilized to determine the impact of various factors, including treatment with steroids, bacterial isolate type, P.aeruginosa virulence factors, and minimum inhibitory concentration, on baseline characteristics and clinical outcomes in bacterial keratitis. Bacterial keratitis is a major cause of visual impairment worldwide, affecting both patients' quality of life and functional status. Our goal is to provide data to allow clinicians to risk-stratify patients with bacterial corneal ulcers in an evidence-based manner.