Quorum Sensing Signals Form Complexes with Ag⁺ and Cu²⁺ Cations

Quorum sensing (QS) regulates important bacterial behaviors such as virulent protein production and biofilm formation. QS requires that molecular signals are exchanged between cells, extracellularly, where environmental conditions influence signal stability. In this work, we present a novel complexation between metal cations (Ag⁺ and Cu²⁺) and a QS autoinducer signal, N-hexanoyl- L-homoserine lactone (HHL). The molecular interactions were investigated using mass spectrometery, attenuated total reflectance-Fourier transform infrared spectroscopy, and computational simulations. Results show that HHL forms predominantly 1:1 complexes with Ag⁺ ( K_d = 3.41 × 10^-4 M) or Cu²⁺ ( K_d = 1.40 × 10^-5 M), with the coordination chemistry occurring on the oxygen moieties. In vivo experiments with Chromobacterium violaceum CV026 show that sublethal concentrations of Ag⁺ and Cu²⁺ decreased HHL-regulated QS activity. Furthermore, when Ag⁺ was preincubated with HHL, Ag⁺ toxicity to CV026 decreased by an order of magnitude, suggesting HHL:metal complexes alter the bioavailability of the individual constituents.