Efficacy of the combination of flashlight (365 nm) and riboflavin in reducing microorganisms isolated from canine corneas: in vitro study Ultraviolet Flashlight for Riboflavin Photoactivation

The photoactivation of riboflavin induces genetic and structural damage to microbial cell membranes, potentially leading to the death of bacteria, viruses, and fungi. However, the conventional approach requires the use of a Photoactivated Chromophore for Corneal Collagen Crosslinking (PACK-CXL), a high-cost device with limited availability in clinical practice. As a low-cost alternative, this study evaluated photoactivation using a portable ultraviolet flashlight (365 nm) against isolates obtained from cases of canine ulcerative keratitis. Riboflavin at different concentrations (0.025, 0.05, 0.1, 0.2, 0.4, and 0.8%) were tested and exposed to UVA radiation (365 nm) at a distance of 3 cm for 15 or 30 minutes, followed by analysis using the time-kill curve method. Results showed that the flashlight was able to activate riboflavin and completely inhibit the growth of Staphylococcus pseudintermedius at a concentration of 0.4% for both exposure periods, and also caused effects at 0.2% after 15 minutes and at 0.05% after 30 minutes of UVA exposure. For Streptococcus canis , significant reductions were observed at 0.4% after 15 minutes of exposure, although these effects were insufficient to kill the bacterium completely. Klebsiella pneumoniae showed reduced growth at 0.2% (after 30 minutes), whereas Pseudomonas aeruginosa did not respond to any tested concentrations. The comparative analysis between exposure times (15 vs. 30 min) revealed no significant differences (p > 0.05). The handheld ultraviolet flashlight (365 nm) was able to photoactivate riboflavin, and this low-cost approach may represent a potential alternative in the absence of conventional PACK-CXL therapy, although further studies are warranted.