Abstract: Antibacterial agents have the formula (I) or are pharmaceutically acceptable salts or in vivo hydrolysable esters thereof: ##STR1## wherein R.sup.3 is phenyl, substituted phenyl, cyclohexenyl, cyclohexadienyl or an optionally substituted 5- or 6-membered heterocyclic ring containing up to three hetero atoms selected from oxygen, sulphur or nitrogen, R.sup.4 is hydrogen or formamido, R.sup.5 is hydrogen, C.sub.1-6 alkyl, substituted alkyl, aryl or aralkyl, R.sup.6 and R.sup.7 are the same or different and represent hydrogen, C.sub.1-6 alkyl, substituted alkyl, halogen, amino, phenyl, substituted phenyl, hydroxyl or C.sub.1-6 alkoxy or R.sup.6 and R.sup.7 form the residue of an optionally substituted 5- or 6-membered carbocyclic ring or a 5- or 6- membered heterocyclic ring containing up to three hetero atoms selected from oxygen, sulphr or nitrogen, and X represents ##STR2## or tautomers thereof wherein R.sup.8 and R.sup.9 are the same or different and each represents hydroxy, or protected hydroxy.