Compounds of formula (II) are antibacterial agents wherein Q represents a radical of the formula: --N(OH)CH(.dbd.O) or the formula: --C(.dbd.O)NH(OH); R.sub.1 represents hydrogen, C.sub.1 C.sub.6 alkyl or C.sub.1 C.sub.6 alkyl substituted by one or more halogen atoms, or, except when Q is a radical of the formula: --N(OH)CH(.dbd.O), a hydroxy, C.sub.1 C.sub.6 alkoxy, C.sub.1 C.sub.6 alkenyloxy, amino, C.sub.1 C.sub.6 alkylamino, or di-(C.sub.1 C.sub.6 alkyl)amino group; R.sub.2 represents a substituted or unsubstituted C.sub.1 C.sub.6 alkyl, cycloalkyl(C.sub.1 C.sub.6 alkyl)- or aryl(C.sub.1 C.sub.6 alkyl)-group; and A represents a group of formula (IIA), or (IIB) wherein R.sub.4 represents the side chain of a natural or non-natural alpha amino acid, and R.sub.5 and R.sub.6 when taken together with the nitrogen atom to which they are attached form a saturated heterocyclic first ring of 5 to 7 atoms as specified in the description.