A compound of the formula (I) ##STR1## wherein R is a group ##STR2## R.sup.1 is hydrogen, phenyl, C.sub.1-20 alkyl, C.sub.2-8 alkenyl or C.sub.2-8 alkynyl each of which may optionally be substituted; or C.sub.3-7 cycloalkyl, X is a divalent group --Y--C.dbd.C--, and Y is oxygen or sulphur, have antibacterial and/or antimycoplasmal activity.