Investigation of the antimicrobial effect of natural deep eutectic solvents (NADES) as solvents in antimicrobial photodynamic therapy

•The NADES CS and/or MFG (diluted ≤ 1:200) possessed antibacterial properties.•The eutectic network of CS and MFG appeared to remain upon dilution ≤ 1:200.•THPP in PBS, CS or MFG induced similar phototoxicity in a Gram-positive bacterium.•THPP in CS or MFG induced higher phototoxicity than in PBS in Gram-negatives.

Natural deep eutectic solvents (NADES) are a third class of liquids, separate from water and lipids. Some NADES, especially those containing organic acids, have been suggested to possess antimicrobial properties. Such properties may be advantageous when NADES are used as solvents in e.g. antimicrobial photodynamic therapy. However, to control the toxicity of acid-containing NADES, they must retain their specific qualities upon dilution. Hence, the aims of this study were to investigate the effect of dilution on the acid-containing NADES network, their antimicrobial activity on different planktonic microorganisms, and their influence on phototoxicity when used as solvents for a photosensitiser. Four bacteria and one fungus were exposed to the NADES, CS (citric acid:sucrose) and MFG (malic acid:fructose:glucose) (molar ratios 1:1 and 1:1:1, respectively), at ≤ 1:200 dilution. Additionally, the antimicrobial properties of the NADES were studied in Escherichia coli in terms of pH and chelating effects. In investigations of phototoxicity, the microorganisms were exposed to the photosensitiser meso-tetra(p-hydroxyphenyl)porphine (THPP; 1 nM) dissolved in diluted NADES combined with blue light (27 J/cm2). The eutectic network appeared to remain upon dilution ≤ 1:200. CS (1:200) was less toxic than an equal concentration of citric acid in the Gram-negative bacteria Klebsiella pneumoniae and E. coli (p < 0.05). A higher degree of phototoxicity was induced in E. coli (~ 1% survival) when THPP was dissolved in CS or MFG than in phosphate buffer (~ 61% survival; p < 0.05). No conclusion could be drawn as to whether the observed toxicity in E. coli exposed to NADES was due to the pH of the solutions or chelation of outer membrane-bound cations.