The present invention relates to a compound having the formula: ##STR1## wherein R is a hydrogen atom, a carboxyl-protecting group or a negative charge and Q is a hydrogen atom, a halogen atom, a hydroxyl group, an acetoxy group, a carbamoyloxy group, an azide group, a substituted or unsubstituted quarternary ammonio group or a substituted or unsubstituted heterocyclic thio group having at least one hetero atom selected from the group consisting of a nitrogen atom, an oxygen atom, and a sulfur atom, or a non-toxic salt or physiologically hydrolyzable non-toxic ester thereof; a process for producing it; and an antibacterial agent comprising it as an active ingredient.Further, the present invention also relates to a compound having the formula: ##STR2## wherein R.sup.3 is a hydrogen atom or an amino-protecting group, and each of R.sup.4 and R.sup.5 which may be the same or different is a hydrogen atom or a carboxyl-protecting group, or a salt thereof; and a process for producing it.