A collection of structurally closely related furanones was synthesized to identify the most important structural elements in brominated furanones for inhibiting the formation of bacterial biofilms. The results suggest that a conjugated exocyclic vinyl bromide on the furanone ring is the most important structural element for the non-toxic but inhibition activity for Escherichia coli biofilm formation. Furanones bearing monosubstituted bromide groups on saturated carbons were found to have a toxic effect that attenuates the bacterial growth.