The invention relates to an antimicrobial compound of the formula: ##STR1## in which R.sup.1 is carboxy or protected carboxy,R.sup.2 is hydroxy(lower)alkyl or protected hydroxy(lower)alkyl,R.sup.3 and R.sup.4 are each hydrogen or lower alkyl, andR.sup.5 is saturated 4 to 6-membered heteromonocyclic group containing 1 to 4 nitrogen atom(s), saturated 5 or 6-membered heteromonocyclic group containing 1 to 2 oxygen atom(s) and 1 to 3 nitrogen atom(s) or saturated 5 or 6-membered heteromonocyclic group containing 1 to 2 sulfur atom(s) and 1 to 3 nitrogen atom(s), wherein said aliphatic heterocyclic group may be substituted by one or more suitable substituent(s) selected from a group consisting of hydroxy, protected hydroxy, hydroxy(lower)alkyl, protected hydroxy(lower)alkyl, halogen, lower alkoxy, lower alkyl, lower alkoxy(lower)alkyl, imino, protected imino, lower alkylamino, protected lower alkylamino, mono(or di)-(lower)alkylcarbamoyloxy, lower alkylidene, lower alkanimidoyl and imino-protective group, or pharmaceutically acceptable salts thereof.