Synergistic Antibacterial Efficacy Enables Determination of Optimal aPDT-Disinfectant Combinations

Objectives: Antimicrobial photodynamic therapy (aPDT) is a promising antibacterial approach. Due to different mechanisms of action of aPDT and disinfectants, the combined use of aPDT and a disinfectant may lead to a synergistic antibacterial efficacy. We investigated treatment concentration combinations of aPDT (TMPyP) and each one of the disinfectants Chlorhexidine-digluconate (CHX) or Benzalkonium-chloride (BAC) used in daily clinical practice.
Methods: The antimicrobial photodynamic effect of light-activated TMPyP (5,10,15,20-Tetrakis(1-methyl-4-pyridinio)porphyrin-tetra(p-toluenesulfonate), 15.3mW/cm², 9.2J/cm², λ_em:380-480nm) against Staphylococcus aureus (ATCC25923), Escherichia coli (ATCC25922), and Enterococcus faecalis (ATCC4083) was determined by plating of treated and untreated broth cultures and compared to the effect of disinfectants BAC and CHX. Minimal inhibitory concentration of CHX or BAC alone and in combination with aPDT were determined using the checkerboard method (n= 6) to evaluate optimal treatment concentrations of combinations from aPDT and each disinfectant. A concentration of such combination was defined optimal, if (i) all bacteria could be eradicated, (ii) each component (aPDT, disinfectant) of this combination was not higher than the single concentration necessary to eradicate the bacteria alone, and (iii) a certain decrease of one of these concentrations caused a higher increase of the other concentration compared to all other treatment concentration combinations that eradicated all bacteria.
Results: Optimal synergistic treatment concentration combinations could be detected in all cases. Results for E. faecalis see Table
Conclusions: The existence of an optimal treatment concentration combination of aPDT and standard disinfectants on E. coli, E. faecalis, and S. aureus showed (i) an antibacterial effect of the combined treatment, (ii) that the concentration of the treatment combination was lower than the concentration necessary for eradication by the respective single treatment (synergistic effect), and (iii) that a substitution of one concentration by the other causes higher changes for all other combinations eradicating all bacteria.