Date of Completion

2023

Document Type

Thesis

Degree Name

Bachelor of Science in Pharmacy

Keywords

photodynamic activity, aloe-emodin, sodium thiocyanate, Staphylococcus aureus, reactive oxygen species

Abstract

Antibacterial photodynamic therapy (aPDT) provides an alternative solution to address antimicrobial resistance due to its unique mechanism of action involving the production of reactive oxygen species (ROS) such as hydroxyl radicals (●OH) and singlet oxygen (1O2) via light activation of photosensitizers (PS). To enhance the PDT of a PS, inorganic salts can be added to form ROS which are longer-lived and can diffuse deeply into the bacteria. This study aimed to enhance the activity of aloeemodin (AE) through antimicrobial photodynamic combination therapy with sodium thiocyanate (NaSCN) against Staphylococcus aureus. NaSCN and irradiation significantly enhanced the PDT of AE (1000 uM AE, 12.2 J/cm2 at 488 nm) against S. aureus resulting in a percentage inhibition of 57.9419% to a maximum of 113.1939% at 500 uM (p > 0.05). The checkerboard assay showed an indifferent activity (FIC = 1.125) between AE and NaSCN resulting in a failure to detect a synergistic activity between the two. ROS (SO3 - , CN-) generation during the photodynamic reactions was also independent of the increasing concentrations of AE and NaSCN which were determined using colorimetric assays (o-phenanthroline method and Prussian blue assay). Results showed that AE may act as limiting reactant in the AE-NaSCN photodynamic reaction due to the consistent low sulfite concentrations at high AE concentrations (1000 uM). Meanwhile, no significant correlation was observed between ROS generation and percentage inhibition. A time-kill curve assay showed that higher log reduction values were observed for the mixture of AE and NaSCN (0.7672log10 to 1.2392log10) compared to AE alone (0.4354log10 to 1.119log10). Increasing the time of irradiation for the mixture of AE and NaSCN showed greater bacterial inhibition compared to AE alone. These results suggest that although NaSCN significantly improved AE-PDT against S. aureus due to the formation of SO3 - and CN- , its photodynamic reaction with AE should further be examined due to the system’s dependence on O2 concentrations. Shifting the focus towards exploring other assays to quantify the ROS levels generated during the photodynamic reactions is also recommended.

First Advisor

Sigfredo B. Mata

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