Topical antibiotic therapy for the treatment of experimental Staphylococcus aureus keratitis

A rabbit model of Staphylococcus aureus keratitis was developed to study the chemotherapeutic efficacy of ciprofloxacin, vancomycin, and cefazolin. Intrastromal injection of 100 colony forming units of log phase S. aureus ATCC strain 25923 resulted in rapid growth in the cornea, peaking at 10(7) cfu/cornea by 12 hr post-infection. Slit-lamp examination revealed that infected eyes reached 30% of maximum inflammation by 10 hr and 60% by 22 hr post-infection. Antibiotic therapy (one drop every 15 min for 5 hr) was initiated at 4 hr post-infection (experiment 1) or 10 hr post-infection (experiment 2). Another group was initiated at 10 hr post-infection and treated for 10 hr (experiment 3). In experiment 1, treatment from 4-9 hr post-infection with 0.3% ciprofloxacin drops decreased the cfu per cornea 6.1 logs, compared to placebo-treated controls (P = 0.0001), and rendered 50% of inoculated eyes sterile. Vancomycin (5.0%) and cefazolin (5.0%) each lowered the cfu per cornea 4.6 logs (P = 0.0187) but did not sterilize any eyes. In experiment 2, therapy from 10-15 hr post-infection with 0.3% ciprofloxacin reduced the cfu per cornea 0.9 logs (P = 0.0001). Vancomycin (5.0%) and cefazolin (5.0%) decreased the cfu per cornea 0.2 logs (P = 0.3973) and 0.3 logs (P = 0.1307), respectively. In experiment 3, therapy from 10-20 hr post-infection with 0.3% ciprofloxacin reduced the cfu per cornea 3.9 logs (P < 0.0001). In this keratitis model, ciprofloxacin was more effective than vancomycin or cefazolin in killing S. aureus.