Ecology & Safety, Volume 13, 2019

Abstract: Ionic liquids - low melting point salts - are attractive for a wide range of applications, from material science to medicinal chemistry. The vast number of possible combinations of different cations and anions enables researchers to fine-tune the physico-chemical and/or biological properties (e.g., hydrophobicity or toxicity). This paper systematically analyses the antimicrobial potency of a library of 24 L-phenylalanine derived surface-active ionic liquids (SAILs; C2-C16) and provides EC50, MIC and MBC values for these compounds towards two clinically relevant pathogenic bacterial models – Escherichia coli and Staphylococcus aureus using standard broth microdilution method ISO 20776-1:2006. We demonstrated that the antimicrobial potency of SAILs containing different cationic headgroups (pyridinium, imidazolium, and cholinium) increased with the length of the alkyl ester chain from C2 to C12 and then similar values obtained for C14 followed by a decreased toxicity for C16. This trend was not dependent on the type of headgroup. The minimum inhibitory concentration (MIC) ranged from 8000 mg/L (C2 SAILs) down to 4 mg/L (C12 SAILs). Most potent were C12 SAILs that inhibited the growth of S. aureus and E. coli at concentration of few milligrams per liter: EC50 values ~2 and ~15 mg/L (or ~4 and ~27 μM), respectively. These data are comparable to the antimicrobial efficiency of benzalkonium chloride, which is widely used antimicrobial compound. As a rule, gram-positive S. aureus was 7-fold more susceptible to the SAILs than gram-negative E. coli. In addition, the results obtained in this study on medically relevant bacteria were in agreement with our previous data on Vibrio fischeri – a naturally luminescent marine bacterium. We hypothesize that the toxic effects of studied SAILs was manifested via disturbing the bacterial membranes. To summarise, the SAILs are promising antimicrobials which toxicity towards bacteria can be tuned by modifying the alkyl ester chain properties.