In Vitro Antibacterial Activity of Dinuclear Thiolato-Bridged Ruthenium(II)-Arene Compounds
The antibacterial activity of 22 thiolato-bridged dinuclear ruthenium(II)-arene compounds was assessed in vitro against Escherichia coli, Streptococcus pneumoniae and Staphylococcus aureus. None of the compounds efficiently inhibited the growth of the three E. coli strains tested and only compound 5 exhibited a medium activity against this bacterium (MIC (minimum inhibitory concentration) of 25 M). However, a significant antibacterial activity was observed against S. pneumoniae, with MIC values ranging from 1.3 to 2.6 M for compounds 1-3, 5 and 6. Similarly, compounds 2, 5-7 and 20-22 had MIC values ranging from 2.5 to 5 M against S. aureus. The tested diruthenium compounds have a bactericidal effect significantly faster than that of penicillin. Fluorescence microscopy assays performed on S. aureus using the BODIPY-tagged diruthenium complex 15 showed that this type of metal compound enter the bacteria and do not accumulate in the cell wall of gram-positive bacteria. Cellular internalization was further confirmed by inductively coupled plasma mass spectrometry (ICP-MS) experiments. The nature of the substituents anchored on the bridging thiols and the compounds molecular weight appear to significantly influence the antibacterial activity. Thus, if overall a decrease of the bactericidal effect with the increase of compounds molecular weight is observed, however the complexes bearing larger benzo-fused lactam substituents had low MIC values. This first antibacterial activity screening demonstrated that the thiolato-diruthenium compounds exhibit promising activity against S. aureus and S. pneumoniae and deserve to be considered for further studies.