Sialic acids are highly abundant in vertebrate tissues where they are involved in many different biological processes. Originally discovered within the Deuterostome lineage of animals, sialic acids can be actually considered as a subset of a more ancient family of 9-carbon backbone monosaccharides called nonulosonic acids (NulOs), which more recently have been also found in Eubacteria and Archaea.
Many pathogenic bacteria incorporate sialic acid into cell surface features like their lipopolysaccharide or capsule polysaccharides, which helps them to evade the innate immune response of the host.
A recent genome level study examined a large set of sequenced microbial genomes, which indicated that biosynthetic pathways to produce nonulosonic acids (NulOs) are far more widely distributed across the phylogenetic tree of life, than previously realized. This finding is moreover supported by recent lectin staining studies and a molecular level survey on prokaryotic nonulosonic acids, showing that also many non-pathogenic and purely environmental strains produce bacterial sialic acids (NulOs). Some (anammox) bacteria produce NulOs that in addition to the very acidic alpha-keto acid group also display (neutralizing) basic groups (free amines). Comparable cell surface sialic acids have been produced by chemical remodelling to manipulate the cell surface charge by producing a free amine at C5, which neutralizes the negatively charged carboxyl group at C1.