A compound according to formula (I) or formula (Ia), and a composition comprising the compound for use as an antimicrobial: wherein, X1, X2, X3 and X4 are each independently selected from: N, O, S; Y1 and Y2 are each independently selected from: N, O, S, C(Ra); M1 and M2 are each a metal centre; R1, R2, R3, R4 and Ra are each independently selected from: hydrogen, alkyl, alkenyl, aryl, halogen, haloalkyl, haloalkenyl, haloaryl, hydroxy, alkoxy, carboxylic acid, amino, amido, nitro or combination thereof; A1, A2, A3 and A4 are each bidentate ligands; and rings D1 and D2 are each independently comprise one or more heteroatoms selected from N, O, S, C(Ra); wherein said compound is for use as an antimicrobial.