Compounds of formula (I) have antibacterial activity, wherein Q represents -N(OH)CH(=O) or -C(=O)NH(OH); Y represents -C(=O)-, - C(=S)-, -S(=O)-, or -SO2-; R1 represents hydrogen, C1-C6 alkyl or C1-C6 alkyl substituted by one or more halogen atoms, or, except when Q is a radical of formula -N(OH)CH(=O), a hydroxy, Cl-C6 alkoxy, Cl-C6 alkenyloxy, halogen, amino, Cl-C6 alkylamino, or di-( C1-C6 alkyl)amino group; R2 represents a substituted or unsubstituted C1-C6 alkyl, C1-C3 alkyl-O-Cl-C3 alkyl, Cl-C3 alkyl-S-C1-C3 alkyl, cycloalkyl(C1-C3 alkyl)-, aryl(C1-C3 alkyl)-, heterocyclyl(C1-C3 alkyl)-, or R1R2N-C1-C3 alkyl group wherein R1 represents hydrogen or Cl-C3 alkyl and R2 represents Cl-C3 alkyl, or R1R2N- represents a cyclic amino group; R3 and R5 independently represent hydrogen or a substituted or unsubstituted C1-C6 alkyl group or R3 and R5 taken together with the carbon and nitrogen atoms to which they are respectively attached form a saturated heterocyclic ring of from 5 to 7 ring atoms, which may be fused to a second carbocyclic or heterocyclic ring, either of which rings may optionally be substituted; R4 represents hydrogen or a substituted or unsubstituted Cl-C6 alkyl, C2--C6 alkenyl, C2-C6 alkynyl, cycloalkyl, aryl, heterocyclyl, Cl-C3 alkyl-O-CI-C3 alkyl, C1--C3 alkyl-S-C1-C3 alkyl, C1-C3 alkyl-NH-(CI-C3 alkyl)-, cycloalkyl(C1-C3 alkyl)-, heterocyclic(C1-C3 alkyl)- or aryl(C1-C3 alkyl)- group; and A represents a primary, secondary or tertiary amino group or a group -R6, -OR6, wherein R6 is a substituted or unsubstituted Cl-C6 alkyl, C2-C6 alkenyl, C2-C6 alkynyl, cycloalkyl, aryl, heterocyclyl, C1-C3 alkyl-O-(C1-C3 alky)-l, C1-C3 alkyl-S-(C1-C3 alkyl)-, Cl-C3 alkyl--NH-(C1-C3 alkyl)-, cycloalkyl(C1-C3 alkyl)-, heterocyclic(C1-C3 alky)-l or aryl(C1-C3 alkyl)- group.