A compound of the formula: ##STR1## wherein R.sup.1 represents hydrogen or an alkyl group, X represents oxygen or sulfur or a group of formula --NR.sup.2 (where R.sup.2 is hydrogen or an alkyl group and in the case of alkyl, it may form a ring jointed with R.sup.1), and Y represents acetoxy group or a group of formula --SR.sup.3 (where R.sup.3 is a nitrogen-containing heterocyclic group), or a pharmaceutically acceptable salt thereof, is found to have a broad antimicrobial spectrum and, in particular, effective against gram-negative bacteria including Escherichia coli, Klebsiella pneumoniae, Proteus vulgaris, Proteus morganii, as well as gram positive ones. Thus, these compounds may be used for antimicrobial agents in therapeutical purposes.