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).