Is Bismuth Really the “Green” Metal? Exploring the Antimicrobial Activity and Cytotoxicity of Organobismuth Thiolate Complexes

Antimicrobial resistance is becoming an ever-increasing threat for human health. Metal complexes and, in particular, those that incorporate bismuth offer an attractive alternative to the typically used organic compounds to which bacteria are often able to develop resistance determinants. Herein we report the synthesis, characterization, and biological evaluation of a series of homo- and heteroleptic bismuth(III) thiolates incorporating either one (BiPh2L), two (BiPhL2), or three (BiL3) sulfur-containing azole ligands where LH = tetrazolethiols or triazolethiols (thiones). Despite bismuth typically being considered a nontoxic heavy metal, we demonstrate that the environment surrounding the metal center has a clear influence on the safety of bismuth-containing complexes. In particular, heteroleptic thiolate complexes (BiPh2L and BiPhL2) display strong antibacterial activity yet are also nonselectively cytotoxic to mammalian cells. Interestingly, the homoleptic thiolate complexes (BiL3) were shown to be completely inactive toward both bacterial and mammalian cells. Further biological analysis of the complexes revealed the first insights into the biological mode of action of these particular bismuth thiolates. Scanning electron microscopy images of methicillin-resistant Staphylococcus aureus (MRSA) cells have revealed that the cell membrane is the likely target site of action for bismuth thiolates against bacterial cells. This points toward a nonspecific mode of action that is likely to contribute to the poor selectivity's demonstrated by the bismuth thiolate complexes in vitro. Uptake studies suggest that reduced cellular uptake could explain the marked difference in activity between the homo- and heteroleptic complexes.