Antimicrobial activity of polymeric ionic liquids

Polymeric ionic liquids (PILs) bearing imidazolium group have been extensively studied as antimicrobial compounds due to their broad spectrum of activity and because do not easily induce bacterial resistance with low toxicity toward mammalian cells. With the aim to develop new antimicrobial compounds, we synthesized new ionic and polyionic liquids by metathesis reaction of 1-hexadecyl-3-methylimidazolium bromide, previously prepared by alkylation of 1-methylimidazole with 1-bromohexadecane [1], with different anions. In order to obtain PILs, two copolymers have been produced by reaction of styrene and sodium styrene sulfonate [2] mixed at different molar ratios (75:25 and 90:10) and then used as anions for the synthesis of polyionic liquids. The following compounds have been synthesized: 1-hexadecyl-3-methylimidazolium dimethyl-5-sulfoisophtalate (Hdmim+ DMSIP-), 1-hexadecyl-3-methylimidazolium styrene sulfonate (Hdmim+ SS-), 1-hexadecyl-3-methylimidazolium poly(styrene-styrene sulfonate75:25 (Hdmim+ PSS- 75:25) and 1-hexadecyl-3-methylimidazolium poly(styrene-styrene sulfonate 90:10) (Hdmim+ PSS- 90:10). All the synthesized compounds have been characterized by 1HNMR, MALDI, DSC and TGA analysis. The antimicrobial activity, tested on Nutrient Agar plates by the disk diffusion method, was evaluated by measuring the amplitude (mm) of the inhibition zone (lack of growth of the tested microorganism) around cotton gauze samples impregnated with ethanol solutions, at different concentrations (50, 100, 200 and 500 ppm), of PILs and ILs. The samples tested showed considerable antibacterial activity, although with variable efficacy depending on the bacterial strain and the concentration of solutions used. In particular, a strong antimicrobial activity (inhibition halo > 3 mm) was highlighted using gauze containing ILs and PILs at the highest concentration against Listeria innocua, Pseudomonas fluorescens and Bacillus subtilis, respectively. The synthesized compounds induced a good antimicrobial activity against both Gram positive (L. innocua and B. subtilis) and Gram negative (Ps. fluorescens) bacterial pathogens, thus resulting promising antimicrobial agents for future applications in different fields (medical, alimentary, textile, etc.).