Method for labeling specifically living Gram negative bacteria and Gram positive bacteria in a sample comprising bacteria, the method comprising the steps of: a) incubating said bacteria of said sample with at least one modified monosaccharide compound comprising a first reactive chemical group capable to chemically react with a second reactive group, so that a monosaccharide residue bearing said first reactive group is incorporated into the envelope of said living bacteria, and b) contacting said residue incorporated into the envelope of said living bacteria, with a labeling molecule comprising a said second reactive group, for generating the chemical reaction of said first reactive group of said residue incorporated within said living bacteria with said second reactive group of said labeling molecule, resulting in a covalent link, characterized in that the said residue is not an endogenous ulosonic acid residue of the envelope of said bacteria, said modified monosaccharide compound has the following formula, or a salt thereof: Wherein - A is H, OH, NH 2, OH and NH 2 being substituted or not by said protecting group selected in the group consisting of alkyl, hydroxyalkyl, acyl, formyl or imidoyl groups, and - B is OH, NH 2, OH and NH 2 being substituted or not by protecting group selected in the group consisting of alkyl, hydroxyalkyl, acyl, formyl or imidoyl groups and - C is OH, NH 2, OH and NH 2 being substituted or not by said protecting group, and - D is a C 2 or C 4 alkyl, each carbon being substituted or not by OH or NH 2, OH and NH 2 being substituted or not by protecting groups selected in the group consisting of alkyl, hydroxyalkyl, acyl, formyl and imidoyl groups, and - D group is substituted by said first reactive group, wherein the first reactive group Ra is selected among groups consisting in or bearing the group azido and groups consisting in or bearing the group alkyne, the said first reactive group being preferably the group azido, and the said second reactive group is selected among groups consisting in or bearing respectively the groups alkyne and azido, the said second reactive group being preferably the group alkyne, reacting the said azido reactive group with the said alkyne reactive group is carried out in performing an azide alkyne cycloaddition. a) incubating said bacteria of said sample with at least one modified monosaccharide compound comprising a first reactive chemical group capable to chemically react with a second reactive group, so that a monosaccharide residue bearing said first reactive group is incorporated into the envelope of said living bacteria, and b) contacting said residue incorporated into the envelope of said living bacteria, with a labeling molecule comprising a said second reactive group, for generating the chemical reaction of said first reactive group of said residue incorporated within said living bacteria with said second reactive group of said labeling molecule, resulting in a covalent link, - A is H, OH, NH 2, OH and NH 2 being substituted or not by said protecting group selected in the group consisting of alkyl, hydroxyalkyl, acyl, formyl or imidoyl groups, and - B is OH, NH 2, OH and NH 2 being substituted or not by protecting group selected in the group consisting of alkyl, hydroxyalkyl, acyl, formyl or imidoyl groups and - C is OH, NH 2, OH and NH 2 being substituted or not by said protecting group, and - D is a C 2 or C 4 alkyl, each carbon being substituted or not by OH or NH 2, OH and NH 2 being substituted or not by protecting groups selected in the group consisting of alkyl, hydroxyalkyl, acyl, formyl and imidoyl groups, and - D group is substituted by said first reactive group,