To evaluate the antibacterial efficacy of photo-activated riboflavin using Ultraviolet A (UVA) on three bacterial strains commonly detected in keratitis.

Three bacterial strains (Staphylococcus epidermidis, Staphylococcus aureus and Pseudomonas aeruginosa) were cultured on blood/hematin–agar plates and dispersed in PBS. Dispersion was done of 10 μl of bacterial stock-solutions in 90 μl of RPMI, where different riboflavin molarities had been added, to achieve a bacterial concentration of 1-4 × 10 ⁴/ml. Riboflavin end molarities before illumination were 0, 100, 200, 300 and 400 μM. Each solution had a negative control. The solutions were illuminated with UVA (365 nm) for 30 minutes (5.4 J/cm²) and then continued for a total time of 60 minutes (10.8 J/cm²). A count of CFU was conducted after incubation and results compared.

In all tested strains, a slight decrease of bacteria was seen when exposed to UV for 30 minutes. A doubling of the UV dose showed a marked decrease of bacterial count in all bacteria tested. The combination of UV and riboflavin showed a more extensive reduction of CFU, confirming an interaction effect between UV and riboflavin.

Riboflavin photo-activation using UVA (365 nm) can achieve an extensive eradication of bacteria, and the combination is more potent in reducing bacterial number than UV alone.