Compounds of formula (I) or salts thereof: wherein R.sup.1 is hydrogen, methoxy or formamido; R.sup.2 is an acyl group; CO.sub.2 R.sup.3 is a carboxy group or a carboxylate anion, or R.sup.3 is a readily removable carboxy protecting group or a pharmaceutically acceptable salt-forming group or in vivo hydrolysable ester group; R.sup.4, R.sup.5, R.sup.6 and R.sup.7 independently represent hydrogen or a bond or a substituent group or may form a cyclic system. These compounds have antibacterial activity.