In Vitro Effectiveness Of Photodynamic Therapy Against Multi-resistant Pathogens

Abstract

Purpose: : Infectious keratitis is a common disease in Ophthalmology that is often caused by bacteria such as Staphylococcus aureus (SA) and Pseudomonas aeruginosa (PA), and the fungus Candida albicans (CA). While bacterial and fungal induced corneal infections are usually treated with high doses of topical antibiotics and antifungals, respectively, these classical antimicrobial therapies start to become ineffective due to the emergence of multi-resistant variants of SA, PA, and CA in recent years.

Methods: : One putative new option in the treatment of corneal infections is photodynamic therapy (PDT), in which a photosensitizing agent is activated by light to produce highly reactive singlet oxygen (¹O₂) that can interact with and inactivate a variety of biomolecules. Here we report about the in vitro effectiveness of the photosensitizer chlorin e6 (Ce6) in combination with red light (670 nm and 24 J/cm³) against 20 multi-resistant clinical isolates of SA, PA, and CA. The microbial isolates were tested in liquid culture against different concentrations of the photosensitizer (1-512 µM) in combination with and without light (30 min of incubation followed by 10 min of illumination).

Results: : Incubating SA with light-activated Ce6 concentrations ≥128 µM reduced the viability of all tested cell cultures by more than 99.99%. CA was slightly more susceptible to this PDT, with the majority of isolates (16/20) being killed by more than 99.99% by Ce6 concentrations between 32 and 64 µM. Concentrations ≥128 µM, however, were less effective in reducing the viability of the CA cultures (<99% killing), albeit of an increased binding/incorporation of the photosensitizer, suggesting either an absorption effect or that higher amounts of Ce6 might induce a resistance mechanism in this fungus. A similar phenomenon was observed for PA, which was less sensitive to this PDT protocol than SA and CA. Only 5 out of the 20 tested PA isolates were killed by the PDT treatment by >99.99% with Ce6 concentrations of 128 and 256 µM, respectively, while high concentrations of the photosensitizer (512 µM) were less effective against this bacterium (<99% killing).

Conclusions: : PDT with red light-activated chlorin e6 might be useful in treating corneal infections caused by multi-resistant S. aureus and C. albicans, but its effectiveness seems to be limited in terms of P. aeruginosa keratitis.

Keywords: bacterial disease • fungal disease • photodynamic therapy