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Regis P Kowalski, Eric G Romanowski, Kathleen A Yates, John Eric Romanowski, Arman Grewal, Richard Anthony Bilonick; Topical Penicillin Treatment of Staphylococcus aureus with Elevated MICs to Penicillin in a Rabbit Keratitis Model. Invest. Ophthalmol. Vis. Sci. 2018;59(9):2660.
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© ARVO (1962-2015); The Authors (2016-present)
Penicillin is seldomly used to topically treat Staphylococcus aureus (SA) keratitis, but penicillin has renewed potential as a potent topical antibiotic to treat SA keratitis based on current lower penicillin minimum inhibitory concentrations (MICs). We evaluated topical 6% penicillin to treat SA keratitis using isolates with varying MICs to penicillin.
Bilateral rabbit corneas were infected intrastromally (2000 CFU) by SA with varying penicillin MICs (0.12, 0.25, 0.5, 16, 32, and 64 µg/ml). The corneal epithelium was abraded in the left eye to mimic ulceration. After 4 hours, four rabbits for each strain were sacrificed prior to topical treatment to determine the onset colony counts. The remaining eyes from each SA strain (8/treatment) were treated topically with 6% penicillin, 2.5% vancomycin, and saline every 15 minutes for 5 hours. One hour afterwards, the rabbits were sacrificed, the corneas were removed, homogenized, and standard colony counts were performed. The data was analyzed to determine bactericidal effect, penicillin efficacy (linear mixed effects model), and a tentative susceptibility standard.
For both intact and abraded corneas, a bactericidal effect was noted for SA with MICs of 0.12, 0.25, and 16 µg/ml. For the abraded epithelium group, the colony counts of the penicillin group was significantly lower than vancomycin (p=0.014), but not for the intact epithelium group (p=0.17). Colony count differences (p=0.00001) indicated that the corneal epithelium hindered the penetration of penicillin. A tentative SA keratitis susceptibility standard could be 16 µg/ml.
Penicillin appears to be a potent antibiotic to topically treat SA keratitis with elevated MICs higher than the serum standard of 0.12 µg/ml. Penicillin may be an alternative SA keratitis therapy, but anaphylaxis will remain an issue.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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