There are provided compounds of formula I: ##STR1## wherein R.sup.1 is hydrogen or a C.sub.1 -C.sub.4 alkyl group either unsubstituted or substituted by one or more substituents chosen from a free or protected hydroxy or halogen atom;R.sup.2 is a free or esterified carboxy group or carboxylate anion;R.sup.3 and R.sup.4 are each independently hydrogen or organic group,X is --O--, ##STR2## --S--, Q is either(a") free or protected hydroxy, or(b") a C.sub.1 -C.sub.1 acyloxy, or(c") carbamoyloxy OCONH.sub.2, or(d") an optionally substituted heterocyclylthio group, or(e") an optionally substituted imido group, or(f") an optionally substituted quaternary ammonium group or(g") a halogen atom,A and the pharmaceutically or veterinarily acceptable salts thereof. There are provided also methods for preparing compounds of formula I. The compounds of formula I are useful as antibacterial agents.