Antibacterial performance of heterocyclic organobismuth (III) complexes based on bidentate C,O‐coordinating ligands: Synergism of ligand identity and coordination number

A series of heterocyclic organobismuth (III) complexes based on bidentate C,O-coordinating ligands were designed and synthesized as antimicrobials. Antibacterial assays showed that complexes of this type are more effective for Gram-positive bacteria (Staphylococcus aureus, Staphylococcus epidermidis, and Enterococcus faecalis) than Gram-negative ones (Escherichia coli and Pseudomonas aeruginosa). Their activities are especially relevant to the synergism of lipophilicity, geometry, and stability, which depends on both the identity of coordinating ligands and the coordination number at the bismuth center. By comparison, the hypervalent 14-Bi-6 species diarylbismuth nitrate (8) was found to exhibit the most potent inhibitory effect, together with a high degree of selectivity, which gives an IC50(LO2)/MIC(Staphylococcus aureus) ratio of up to 23.08. Time-kill analysis demonstrated that complex 8 is bacteriostatic at low concentrations while displaying significant bactericidal activity at high doses. The results of drug resistance experiments suggested that complex 8 can inhibit the formation of bacterial biofilm and consequently delay or prevent the development of drug resistance. Furthermore, complex 8 also showed high inhibition efficiency against several drug-resistant Staphylococcus aureus, and the MIC values are within the range of 0.39-1.56 mu M, thus indicating the lack of cross-resistance between this organometallic compound and commonly used antibiotics.