Compounds of formula (I) are antibacterial agents:
wherein Z represents a radical of formula —N(OH)CH(═O) or of formula —C(═O)NH(OH), and R1-R4 are as defined in the specification. A method for the treatment of bacterial or protozoal infections in humans and non-human mammals, which comprises administering to a subject suffering such infection an antibacterially or antiprotozoally effective dose of a compound of formula (I) or a pharmaceutically or veterinarily acceptable salt, hydrate or solvate thereof.