Ionic Liquids as Biocompatible Antibacterial Agents: A Case Study on Structure-Related Bioactivity on Escherichia coli

Abstract

The potential of ionic liquids (ILs) to be used as antimicrobial agents for biomedical applications has been hindered by the fact that most of them are cytotoxic toward mammalian cells. Understanding the mechanism of bacterial and mammalian cellular damage of ILs is key to their safety design. In this work, we evaluate the antimicrobial activity and mode of action of several ILs with varying anions and cations toward the clinically relevant Gram-negative Escherichia coli. Langmuir monolayer technique was used to evaluate if the IL's mode of action was related to the bacterial cell membrane interaction for an effective E. coli killing. 1-Decyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide [DMIM][TFSI] and trihexyltetradecyl phosphonium bis(trifluoromethylsulfonyl) imide [P_6,6,6,14][TFSI] were surface-active and induced bacterial cell lysis, through a membrane-disruption phenomenon on bacteria, in a mechanism that was clearly related to the long alkyl chains of the cation. 1-Ethyl-3-methylimidazolium hydrogen sulfate [EMIM][HSO₄] was highly antimicrobial toward E. coli and found suitable for biological applications since it was harmless to mammalian cells at most of the tested concentrations. The results suggest that the imidazolium cation of the ILs is mostly responsible not only for their antimicrobial activity but also for their cytotoxicity, and the inclusion of different anions may tailor the ILs' biocompatibility without losing the capacity to kill bacteria, as is the case of [EMIM][HSO₄]. Importantly, this IL was found to be highly antimicrobial even when incorporated in a polymeric matrix.

Keywords: Escherichia coli; antimicrobial; biocompatible; ionic liquids; surface activity.

Figure 1

Zone of inhibition (ZoI) of ILs in contact with E. coli. (A) Agar plate denoting the effect of changing the anion, (B) quantitative measurement of all ZoI, and (C) agar plate denoting the effect of changing the anion.

Figure 2

(A) Concentration-dependent antimicrobial activity (the X-axis is presented in log₁₀ scale) and (B) details of the inhibition of E. coli in contact with 5%, 0.5%, and 0.05% v/v ILs and corresponding MICs calculated in μmol/mL.

Figure 3

(A) Kinetic adsorption resulting from the incorporation of ILs (0.05% v/v) (final concentration) into the air–water interface of E. coli PE:PG monolayer at a surface pressure of 33 mN/m and schematic representation of the incorporation of the ILs in the subphase beneath the phospholipid monolayer. (B) Release of cytoplasmic β-galactosidase from the E. coli cells in contact with the ILs at 0.05% v/V and the control (PBS) and respective schematic representation of the enzyme release and its further detection using ONPG.

Figure 4

IL ζ potential, which is indicative of the overall charge of these compounds, measured at a physiological pH of 7.4.

Figure 5

Dose-dependence cell viability of ILs in contact with preosteoblast for 24 h. The X-axis is presented in a log₁₀ scale.

Figure 6

IL-containing polymeric (PVDF-TrFE) films. (A) Cross section by SEM (bar represents 30 μm) and (B) antimicrobial activity against E. coli.