Organic ammonium and phosphonium salts exert excellent antimicrobial effects by interacting lethally with bacterial membranes. Particularly, quaternary ammonium lipids have demonstrated efficiency both as gene vectors and antibacterial agents. Here, aiming at finding new antibacterial devices belonging to both classes, we prepared a water-soluble quaternary ammonium lipid (6) and a phosphonium salt (1) by designing a synthetic path where 1 would be an intermediate to achieve 6. All synthesized compounds were characterized by Fourier-transform infrared spectroscopy and Nuclear Magnetic Resonance. Additionally, potentiometric titrations of NH3+ groups 1 and 6 were performed to further confirm their structure by determining their experimental molecular weight. The antibacterial activities of 1 and 6 were assessed first against a selection of multi-drug-resistant clinical isolates of both Gram-positive and Gram-negative species, observing remarkable antibacterial activity of both compounds against Gram-positive isolates of Enterococcus and Staphylococcus genus. Further investigations on a wider variety of strains of these species confirmed the remarkable antibacterial effects of 1 and 6 (MICs = 4-16 and 4-64 µg/mL, respectively), while 24 h-time-killing experiments carried out with 1 on different S. aureus isolates evidenced a bacteriostatic behavior. Moreover, both compounds 1 and 6, at the lower MIC concentration, did not show significant cytotoxic effects when exposed to HepG2 human hepatic cell lines, paving the way for their potential clinical application.
Keywords: Gram-positive and Gram-negative bacteria; MICs determination; cytotoxicity studies; membrane permeabilization; multi-drug-resistant bacteria; phosphonium salt (1); quaternary ammonium lipid (6); time-killing experiments.