The present invention relates to a synthetic oligosaccharide of general formula (I): T*-[(—Ux+4—Ux+3—Ux+2—Ux+1—Ux)m—(Vx+2—Vx+1—Vx)1-m-T-O-L-E that is related to Klebsiella pneumoniae serotype O3, O3b and/or O5 lipopolysaccharide and conjugate thereof. Said synthetic oligosaccharide, said conjugate and pharmaceutical composition containing said synthetic oligosaccharide or said conjugate are useful for prevention and/or treatment of diseases associated with Klebsiella pneumoniae. Furthermore, the synthetic oligosaccharide of general formula (I) is useful as marker in immunological assays for detection of antibodies against Klebsiella pneumoniae serotype O3, O3b and/or O5 bacteria.