Efficacy of topical chloramphenicol and tobramycin ophthalmic solutions in preventing severe Staphylococcus aureus keratitis in rabbits: Current Eye Research

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Abstract
Various marketed chloramphenicol opthalmic solutions were compared with various dilutions of Tobrex Ophthalmic Solution were tested for effectiveness in a Staphylococcus aureus rabbit keratitis model. Anesthetized rabbits were each infected intracorneally with 104 Staphylococcus aureus ATCC 29737 cells. Treatment groups consisted of five or six rabbits (10 or 12 eyes) per group. One group of rabbits was infected but not treated (Positive Control Group). Topical dosing of commercially available ophthalmic solutions was accomplished by depositing 0.1 mL of a color-coded test solution into the lower cul-de-sac of each eye. Dosing begin one hour after the mid-infection time and continued for a total of nine hourly treatments. Twenty four hours after infection the rabbit eyes were graded (masked) using standard slit-lamp scoring procedures. The slit-lamp scores for five of eight ocular parameters were used to calculate an eye score value for each rabbit eye. The five ocular parameters were selected, based on previous Stepwise Discriminant Computer Analysis of over 300 rabbit eyes infected with Staphylococcus aureus ATCC 29737 and treated with various antibiotics. The eye score values for each group were averaged and the treatments were compared for significant differences in efficacy using the nonparametric, Wilcoxon-Mann-Whitney Rank Sum Test. This work has shown that first, tobramycin is about 15 times more effective than chloramphenicol in vitro against a representative, sensitive strain of Staphylococcus aureus (ATCC 29737); second, there were no significant differences in efficacy among various commercially available chloramphenicol ophthalmic solutions in a rabbit keratitis model infection; and third, tobramycin (0.1%) solution was significantly superior in efficacy to chloramphenicol (0.5%) solution C at a tobramycin concentration (0.1%) five times lower than the chloramphenicol concentration (0.5%) in the rabbit keratitis model.