The antimicrobial activity of inert oligonuclear polypyridylruthenium(II) complexes against pathogenic bacteria, including MRSA

The minimum inhibitory concentrations (MIC) of a series of synthetic inert polypyridylruthenium(II) complexes against four strains of bacteria – Gram positive Staphylococcus aureus (S. aureus) and methicillin-resistant S. aureus (MRSA), and Gram negative Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa) – have been determined. The results demonstrate that for the dinuclear ruthenium(II) complexes ΔΔ/ΛΛ-[{Ru(phen)2}2{μ-bbn}]4+ {where phen = 1,10-phenanthroline; bbn = bis[4(4′-methyl-2,2′-bipyridyl)]-1,n-alkane (n = 2, 5, 7, 10, 12 or 16)} the complexes linked by the bb12, bb14 and bb16 ligands are highly active, with MIC values of 1 μg mL−1 against both S. aureus and MRSA, and 2–4 and 8–16 μg mL−1 against E. coli and P. aeruginosa, respectively. The mononuclear complex [Ru(Me4phen)3]2+ showed equal activity (on a mole basis) against S. aureus compared with the Rubb12, Rubb14 and Rubb16, but was considerably less active against MRSA and the two Gram negative bacteria. For the dinuclear Rubbn family of complexes, the antimicrobial activity was related to the octanol–water partition coefficient (logP). However, the highly lipophilic mononuclear complex Δ-[Ru(phen)2(bb16)]2+ was significantly less active than Rubb16, highlighting the importance of the dinuclear structure. Preliminary toxicity assays were also carried out for the ΔΔ isomers of Rubb7, Rubb10, Rubb12 and Rubb16 against two human cells lines, fresh red blood cells and THP-1 cells. The results showed that the dinuclear ruthenium complexes are significantly less toxic to human cells compared to bacterial cells, with the HC50 and IC50 values 100-fold higher than the MIC for the complex that showed the best potential – ΔΔ-Rubb12