Antimicrobial effect of dissolved curcuminoid in natural deep eutectic solvents (NADES) to Escherichia coli and Staphylococcus aureus: A promising candidate for antimicrobial photodynamic therapy (aPDT)

An antimicrobial photodynamic therapy (aPDT) is a local antimicrobial treatment which utilizes a photosensitizer dye, visible light, and oxygen. It is considered as an alternative treatment for bacterial or fungal resistance. In this treatmetn, a pure, stable and non-toxic natural photosensitizer compound as a host cell which soluble in water and capable of producing reactive photoproducts is required. Curcumin as a natural yellow-orange photosensitizer dye with anti-inflammatory, anti-carcinogenic, anti-bacterial, and anti-infection activities is believed to be safe for human consumption. Combining curcuminoids as a photosensitizer dye with NADES as solvent instead of solving the low solubility drawback of curcuminoids in water, as well as becoming a potential candidate of aPDT. However, an antimicrobial effect of dissolved curcuminoids in NADES need to be studied first. Antimicrobial tests of curcuminoids to both of Escherichia coli and Staphylococcus aureus using minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) methods were conducted. Three NADES i.e. malic acid-sucrose-water (MAS-H2O = 1:1:18); fructose-glucose-water (FG-H2O = 1:1:1); and fructose-sucrose-water (FS-H2O = 2:1:15) were tested by applying nine different concentrations of curcuminoids (2.00-4.00 mM). A blank of sample (no dissolved curcuminoids) as well as a pure solution of each constituent compounds of NADES such sucrose, malic acid, fructose, and glucose were also applied. Bacterial suspension approx. 10⁸ cells/mL of 1 mL (24 h incubated at 37^oC) was used for the test. MAS-H2O (1:1:18) shown the most effective antimicrobial activity compared to both of FG-H2O (1:1:1) and FS-H2O (2:1:15). The toxicity of MAS-H2O (1:1:18) to both E. coli and S. aureus may due to the low pH condition of NADES itself since malic acid has high acidity (pH <3). Meanwhile, both other NADES contains sugars, i.e. fructose, glucose, and sucrose, showing lower pH value (pH >5). Both on the concentration of the curcuminoids and bacteria effects the observed toxicity.