The antibacterial activity and selectivity of bismuth(III) tris(8-hydroxyquinolinates)

The series of bismuth(III) tris(8-hydroxyquinolinates); [Bi(Q")₃] (1), [Bi(Q'Cl)₃] (2), [Bi(QCl₂)₃] (3), [Bi(QBr₂)₃] (4), and [Bi(QI₂)₃] (5) (where Q"-H = C₉H₇NO; Q'Cl-H = C₉H₆NOCl, QCl₂-H = C₉H₅NOCl₂; QBr₂-H = C₉H₅NOBr₂; and QI₂-H = C₉H₅NOI₂) were synthesised, fully characterised, and evaluated for their antibacterial activity towards three Gram-positive bacteria (vancomycin-resistant E. faecalis, S. aureus, methicillin-resistant S. aureus), and four Gram-negative bacteria (A. baumannii, P. aeruginosa, K. pneumoniae, and E. coli) and also their cytotoxicity towards mammalian cells. New crystallographic data on 4 indicates it is dimeric in the solid state through 'Bi₂O₂' bridging which is consistent with data previously reported for 5. The five complexes (1-5) all exhibited good but variable antibacterial activity and selectivity. Complexes 2 and 5 showed significant activity towards Gram-positive bacteria with MIC (minimum inhibitory concentration) values ranging from 0.78 μM - 3.13 μM and selectivity indices of 6.2 - ≥16.0. For Gram-negative species, complexes 3 and 4 exhibited highly selective activity towards multi-drug resistant strains of A. baumannii with a range of MIC values 0.39-1.56 μM and selectivity indices of 3.14-7.23 respectively. While some of the 8-hydroxyquinolines themselves show reasonable antibacterial activity this is generally enhanced through complexation to bismuth(III).