In vitro antibacterial activity of dinuclear thiolato-bridged ruthenium(II)-arene compounds

Abstract

The in vitro assessment of diruthenium(II)-arene compounds against Escherichia coli, Streptococcus pneumoniae, and Staphylococcus aureus showed a significant antibacterial activity of some compounds against S. pneumoniae, with minimum inhibitory concentration (MIC) values ranging from 1.3 to 2.6 µM, and a medium activity against E. coli, with MIC of 25 µM. The nature of the substituents anchored on the bridging thiols and the compounds molecular weight appear to significantly influence the antibacterial activity. Fluorescence microscopy showed that these ruthenium compounds enter the bacteria and do not accumulate in the cell wall of gram-positive bacteria. These diruthenium(II)-arene compounds exhibit promising activity against S. aureus and S. pneumoniae and deserve to be considered for further studies, especially the compounds bearing larger benzo-fused lactam substituents.

Keywords: Escherichia coli; ICP-MS; MIC; Staphylococcus aureus; Streptococcus pneumoniae; benzo-fused lactams; fluorescence; ruthenium complexes; uptake.

Fig 1

Structure of compounds 1–8 forming Family 1.

Fig 2

Structure of diruthenium conjugates 9–12 forming Family 2.

Fig 3

Structure of diruthenium-BODIPY conjugates 13–16 forming Family 3.

Fig 4

Structure of compounds 17–22 forming Family 4. 23 is the benzo-fused lactam thiol ligand used in compounds 20 and 21 and tested against various bacteria.

Fig 5

Synthesis of mixed trithiolato diruthenium complexes 17–20 and 22.

Fig 6

Synthesis of the symmetric trithiolato diruthenium complex 21.

Fig 7

Growth curves of S. aureus 20 after the addition of the diruthenium compounds at MIC when OD₆₀₀ = 0.5 is reached (dotted lines). (A) S. aureus treated with compounds 1–2 and 5–6 at MIC upon reaching OD₆₀₀ = 0.5. (B) S. aureus treated with compounds 8, 11, and 12 at MIC upon reaching OD₆₀₀ = 0.5. (C) S. aureus treated with compounds 3 and 7 at MIC upon reaching OD₆₀₀ = 0.5. Treatment with penicillin at four times its MIC was used as a bactericidal control.

Fig 8

Fluorescence microscopy images of S. aureus 20 cells stained with DAPI after 3 h of incubation, with or without treatment with diruthenium conjugates 9 and 15. (A) Untreated control sample stained with DAPI. (B) The sample was treated with 9 for 3 h and stained with DAPI for visualization. (C) The sample was treated with 15 for 3 h and stained with DAPI for visualization. 15 is visible after excitation at 440/470 nm with emission at 525/550 nm. White arrows indicate cells with overlapped fluorescence of DAPI and BODIPY diruthenium conjugate 15. Green arrows indicate emission in 525/550 nm (corresponding to conjugate 15) that do not overlap with DAPI nor were cells visible under Brightfield.