The subject of the present invention is an isolated antigen being an immunogenic form of the common enterobacterial antigen (ECA) of Gram-negative bacteria of the family Enterobacteriaceae: ECA combined with lipopolysaccharide (ECALPS) a glycoconjugate of inactivated form of this antigen with a protein, as well as compositions and vaccines containing such an antigen/glycoconjugate designed for the prevention and treatment infections caused by Gram-negative bacteria.