Modulation of quorum sensing in Gram-negative bacteria, particularly strains of Pseudomonas which form biofilms, by compounds including those of formula I and formula II: where: AR is optionally substituted phenyl, cycloalkyl or cycloalkenyl or heterocyclic, and R1 is optionally substituted alkyl, alkenyl, alkoxyalkyl, or alkylthioalkyl or alkyl substituted at the omega position with optionally substituted phenyl, cyclohexyl or cyclohexenyl. In particular compounds inhibit quorum sensing and biofilm formation. Pharmaceutical compositions for treatment of bacterial infections and methods of treatment of such infections are provided.