Compounds of formula (II) have antibacterial activity: wherein: Q represents a radical of formula -N(OH)CH(=0) or formula -C(=O)NH(OH); R 1 represents hydrogen, methyl or trifluoromethyl or, except when Q is a radical of formula -N(OH)CH(=0), a hydroxy, halo or amino group; R 2 represents a group R 10 -(D) n -(ALK) m - wherein R 10 represents hydrogen or an optionally substituted C 1 -C 6 alkyl, C 2 -C 6 alkynyl, C 2 -C 6 alkynyl, cycloalkyl, aryl, or heterocyclyl group and ALK represents a straight or branched divalent C 1 -C 6 alkylene, C 2 -C 6 alkynylene, or C 2 -C 6 alkynylene radical, and may be interrupted by one or more non-adjacent -NH-, -O- or -S­linkages, D represents -NH-, -O- or -S-, and m and n are independently 0 or 1; R 4 represents the side chain of a natural or non-natural alpha amino acid; ring A represents an optionally substituted monocyclic heterocyclic ring containing from 5 to 7 ring atoms, one of which is the nitrogen atom shown, the remaining ring atoms being selected from compatible combinations of carbon, oxygen, sulfur and nitrogen; X is oxygen or sulfur; Y is oxygen, sulfur or -NH-; R is 0, 1, 2 or 3; and ring B represents an optionally substituted carbocyclic or heterocyclic ring system.