Experimental Lysostaphin therapy in methicillin resistant Staphylococcus aureus induced keratitis in Rabbits: A comparative evaluation

Objective: A comparative evaluation of topical lysostaphin therapy with vancomycin and azithromycin in methicillin-resistant Staphylococcus aureus (MRSA) keratitis in a rabbit model. Methods: This study was conducted on 18 eyes of 18 rabbits. All rabbits were divided into 3 groups with 6 rabbits each. Right eyes were experimental eyes. Keratitis was induced by aseptically injecting the corneal stroma with 100 colony forming unit (CFU) of MRSA strain (MU-50). Each group was treated 5 hours post injection with one drop of 5% vancomycin, 0.28% lysostaphin or 1% azythromycin, every half an hour for 5 hours. Seven ocular parameters (conjunctival injection, chemosis, corneal infiltrate, corneal edema, flare in the anterior chamber, hypopyon formation, and iritis) were graded on a scale of 0 to 4 on slit lamp examination of rabbit eyes. Rabbits were sacrificed and corneas were excised and cultured to determine the number of CFU per cornea. Results: Lysostaphin significantly lowered the total slit lamp examination scoring of the experimental eye as compared to vancomycin and azithromycin (p-value 0.014) group. None of the eyes treated with vancomycin and azithromycin were sterile whereas four out of six eyes became sterile in lysostaphin treated group. There was significant lowering of mean log CFU/cornea in lysostaphin group as compared to vancomycin and azithromycin (p-value 0.008). Also, there was no statistically significant difference in slit lamp examination scoring between vancomycin and azithromycin groups. Conclusion: Lysostaphin has a greater potency to treat methicillin-resistant Staphylococcus aureus induced keratitis as compared to azithromycin and vancomycin.