Fig. 1.

Structure of sialic acids and overview of bacterial sialic acid utilisation. a The most common sialic acid, N-acetylneuraminic acid. An asterisk represents positions that are modified to give variation. These include acetyl groups or the less common lactyl, phosphate, succinyl and methyl groups. b Lactone derivative, the third highest detected sialyl metabolite in the human gut. c 9-O-Acetylated derivative, the second highest detected sialyl metabolite in the human gut. d N-Glycolylneuraminic acid. Differing at the C-5 acetamido group, this sialyl metabolite is common in most mammals, but is not synthesised by humans