ID50 in a Rabbit Lasik Model

Abstract

Abstract: : Purpose: To determine the number of contaminating bacteria (S. epidermidis) necessary to cause infection (ID₅₀) in a "Lasik Model" of keratitis, and evaluate the effect of prophylactic antibiotic therapy. Methods: Corneal pockets were created in one rabbit eye (n=25). The rabbits were separated into groups of five and received either 2 µl of phosphate buffered saline (PBS) or PBS plus 10², 10³, 10⁴, or 10⁵ colonies of S. epidermidis in the corneal pocket. The corneas were examined once daily to monitor corneal infiltrate. In parallel, another set of rabbits received PBS or PBS plus inoculations of 10³, 10⁴, 10⁵ or 10⁶ organisms. Immediately following the inoculation, one drop of 0.3% gatifloxacin (Zymar) was applied to the corneal surface and continued four times daily for 10 days. Results: In the study group receiving inoculation and no antibiotic, significant infiltrate developed in 3 of 5 animals receiving 10² organisms, and in all animals receiving larger inocula (ID₅₀= 10² organisms). In the group receiving concomitant inoculant plus antibiotic, significant infiltrate developed in 4 of 5 animals receiving 10⁴ organisms, and in all others receiving larger inocula (ID₅₀=10⁴ organisms). Follow–up in animals receiving antibiotic showed that in all but one rabbit, corneal infections cleared; however, of animals not receiving antibiotics, only infections in the 10² group resolved. Conclusions: Inoculations containing as low as 10² S. epidermidis organisms produce active infection in a rabbit corneal model. Corneas receiving inoculations higher than 10² organisms overwhelm innate immune defenses and produce infiltrates that persist. Inoculated corneas concomitantly treated with gatifloxacin require more than 10 times the organisms than control to cause initial infection. Infections resolve with treatment in greater than or equal to 95 percent of infected rabbits.

Keywords: antibiotics/antifungals/antiparasitics • keratitis • refractive surgery: LASIK