Rose Bengal and Riboflavin Mediated Photodynamic Antimicrobial Therapy Against Selected South Florida Nocardia Keratitis Isolates

PURPOSE: To examine and compare the efficacy of in vitro growth inhibition using rose bengal and riboflavin photodynamic antimicrobial therapy (PDAT) for Nocardia keratitis isolates. METHODS: Nocardia asteroides complex, Nocardia amikacinitolerans, and Nocardia farcinica species were isolated from patients with confirmed Nocardia keratitis. Isolates were tested against three experimental groups: (1) no photosensitizer/no irradiation, (2) photosensitizer/no irradiation, and (3) photosensitizer/irradiation. Each isolate was prepared in suspension to a concentration of 1.5 × 10(8) CFU/mL. Bacterial suspensions were mixed with water or prepared 0.1% photosensitizer solution for a final bacterial concentration of 1.5 × 10(7) CFU/mL. Aliquots of 1 mL were plated on 5% sheep blood agar. Rose bengal and riboflavin PDAT plates were irradiated for 15 minutes with a 525- or 375-nm custom 6-mW/cm(2) powered light source for a total fluence of 5.4 J/cm(2). All experimental groups were repeated in triplicate. Plates were incubated in a 35°C non-CO(2) incubator for 96 hours and photographed. Percent inhibition was evaluated using LabVIEW-based software. RESULTS: All strains of Nocardia tested with 0.1% rose bengal and irradiated for 15 minutes demonstrated statistically significant inhibition of growth (P < 0.05). No other experimental groups displayed any bacterial inhibition. CONCLUSIONS: Rose bengal is superior to riboflavin PDAT against selected Nocardia isolates. In vivo testing is warranted to investigate the utility of rose bengal PDAT for severe Nocardia keratitis. TRANSLATIONAL RELEVANCE: In vitro results for three clinical strains of Nocardia support the possible use of rose bengal PDAT as a complementary treatment of Nocardia keratitis.