In vitro susceptibility and cellular uptake for a new class of antimicrobial agents: dinuclear ruthenium(II) complexes

Abstract

Objectives: To determine the in vitro susceptibility and cellular uptake for a series of dinuclear ruthenium(II) complexes [{Ru(phen)2}2{μ-bbn}]4+ (Rubbn), and the mononuclear complexes [Ru(Me4phen)3]2+ and [Ru(phen)2(bb7)]2+ against Staphylococcus aureus, methicillin-resistant S. aureus, Escherichia coli and Pseudomonas aeruginosa.\ud \ud Methods: The in vitro susceptibility was determined by MIC and MBC assays, and time–kill curve experiments, while the cellular uptake was evaluated by monitoring the fluorescence of the complexes remaining in the supernatant of the cultures after incubation for various periods of time, flow cytometry and confocal microscopy.\ud \ud Results: Rubb12 and Rubb16 are highly active, with MIC and MBC values of 1–2 mg/L (0.5–1 μM) for the two Gram-positive strains and 2–4 mg/L for E. coli and 16–32 mg/L for P. aeruginosa. Rubb16 showed equal or better activity (on a molar basis) to gentamicin and ampicillin for all strains apart from P. aeruginosa. The relative MBC to MIC values indicated that Rubb12 and Rubb16 are bactericidal, and from the time–kill curve experiments, the ruthenium complexes can kill the bacteria within 2–6 h. The cellular uptake studies demonstrated that the observed antimicrobial activity is correlated with the level of uptake of the ruthenium complexes. Confocal microscopy confirmed the cellular uptake of Rubb16, and tentatively suggested that the ruthenium complex is localized in the bacteria.\ud \ud Conclusions: The inert dinuclear ruthenium(II) complexes Rubb12 and Rubb16 have potential as new antimicrobial agents. The structure of the dinuclear ruthenium complexes can be readily further modified in order to increase their selectivity for bacteria over human cells