A compound comprising a metal species and a biologically acceptable ligand, wherein the biologically acceptable ligand comprises a biguanide moiety, and wherein the biologically acceptable ligand forms a complex with the metal species in which the metal species is stabilised in an oxidation state greater than 1+. Compositions and medical devices comprising the compound. A method for the treatment or prophylaxis of microbial, including bacterial, infections, comprising the use of such compounds, compositions or medical devices.