The photodynamic antibacterial effects of a new class of Ru(II)-based photosensitizers (TLD1411 and TLD1433) were evaluated against a strain of S. aureus (ATCC 25923) and a methicillin-resistant strain of S. aureus (MRSA, ATCC 33592). Bacterial samples were dosed with a range of photosensitizer concentrations (0.3-12 渭M) and exposed to 530 nm light (90 J cm鈭?) in normoxic conditions (ambient atmosphere) and in hypoxic conditions (0.5% O2).
Both photosensitizers exerted photodynamic inactivation (PDI) of the microorganisms in normoxia, and this activity was observed in the nanomolar regime. TLD1411 and TLD1433 maintained this PDI potency under hypoxic conditions, with TLD1433 becoming even more active in the low-oxygen environment.
The observation of activity in hypoxia suggests that there exists an oxygen-independent, Type I photoprocess for this new class of compounds in addition to the typical Type II pathway mediated by singlet oxygen. The intrinsic positive charge of the Ru(II) metal combined with the oxygen independent activity demonstrated by this class of photosensitizers presents a new strategy for eradicating both gram-positive and gram-negative bacteria regardless of oxygenation level.