The invention provides the use of a compound of formula (I), or metallated derivative thereof, for killing, inhibiting or preventing the growth of a microbial biofilm: wherein X1, X2, X3, X4, Y1, Y2, Y3, Y4 and Z have meanings given in the description. The biofilm may be on a living or inert support. Preferably, the microorganisms are selected from the group consisting of bacteria and fungi.
