In vitro photodynamic eradication of Pseudomonas aeruginosa in planktonic and biofilm culture

Photodynamic disinfection (PDD) is a nonantibiotic approach to treating drug-resistant bacterial infections. Pseudomonas aeruginosa , an opportunistic pathogen, is problematic because of its propensity to develop antibiotic resistance and its ability to secrete a protective biofilm matrix. This study examined the ability of PDD to eradicate planktonic and biofilm cultures of P. aeruginosa in vitro . Planktonic P. aeruginosa cultures were briefly exposed to a methylene blue-based photosensitizer formulation and subjected to energy doses ranging from 1.7 to 20.6 J cm⁻² using a 670 nm nonthermal diode laser. Biofilms were grown for 24 and 48 h and exposed to photosensitizer for 30 s before illumination with 13.2 or 26.4 J of energy. A single exposure of planktonic P. aeruginosa to photosensitizer at >15.5 J cm⁻² resulted in 100% eradication (>7 log₁₀ reduction from control), an effect that could be decreased significantly in the presence of the singlet oxygen quenchers l -tryptophan and sodium azide. Decreasing the energy dose below this threshold by varying both power density and illumination duration resulted in a dose-dependent decrease in bacterial kill. In addition, 24 h biofilm viability was reduced by 99% with single exposure and 99.9% with double exposure, while 48 h biofilm viability was reduced by >99.999% with both single and double exposures. This study shows that PDD is effective in eradicating planktonic and biofilm cultures of P. aeruginosa, supporting the concept that translation into clinical practice for indications such as otitis externa and wound disinfection is a viable option.