Sialic acids are α-keto acids that chemically derive from neuraminic acid, with N-acetylneuraminic acid (Neu5Ac) being the most important member. They play a wide variety of biological roles in nature. Their presence in bacteria is usually associated to a higher virulence, and are therefore regarded as determinants of pathogenicity (virulence factors). They occur in lipo-polysaccharide and/or in the capsule of certain pathogenic bacteria, in the form of homo- or hetero-polysaccharide that mimic some glycosphingolipids and glycoproteins present in animal cells. Such mimicry may serve bacteria to evade host immune defenses. The metabolic processes associated to Neu5Ac comprise its external uptake, its de novo biosynthesis and its catabolism. The biosynthetic pathway of Neu5Ac in bacteria proceeds through the following reactions: a) synthesis of ManAc-6-P, b) synthesis of ManNAc, c) synthesis of Neu5Ac, d) activation of the monomer to CMP-β-Neu5Ac, and e) transfer of Neu5Ac to the acceptor structure. In the catabolism of this compound, the neuraminidase and N-acetyl-neuraminidate-lyase activities play important roles.