Compounds of formula (I) are antibacterial or antiprotozoal agents for treatment of infections in humans and non-human mammals, wherein: Z represents a radical of formula --N(OH)CH(.dbd.O) or formula C(.dbd.O)NH(OH); R.sub.1 represents hydrogen, methyl or trifluromethyl, or except when Z is a radical of formula --N(OH)(.dbd.O), a hydroxy, halo or amino group; R.sub.2 represents a group R.sub.10--(X).sub.n-(ALK).sub.m-- wherein R.sub.10, ALK, X, m and n are as defined in the specification; R.sup.3 represents hydrogen, (C.sub.1 C.sub.6) alkyl or phenyl(C.sub.1 C.sub.6)alkyl-; and R.sub.5 and R.sub.6 are as defined in the specification. ##STR00001##