Sialic acids are found at all cell surfaces of vertebrates and some invertebrates, and also at certain bacteria that interact with vertebrates.
Many viruses such as the Ad26 serotype of adenoviruses (Adenoviridae), rotaviruses (Reoviridae) and influenza viruses (Orthomyxoviridae) can use host-sialylated structures for binding to their target host cell. Sialic acids provide a good target for these viruses since they are highly conserved and are abundant in large numbers in virtually all cells. Unsurprisingly, sialic acids also play an important role in several human viral infections. The influenza viruses have hemagglutinin activity (HA) glycoproteins on their surfaces that bind to sialic acids found on the surface of human erythrocytes and on the cell membranes of the upper respiratory tract. This is the basis of hemagglutination when viruses are mixed with blood cells, and entry of the virus into cells of the upper respiratory tract. Widely used anti-influenza drugs (oseltamivir and zanamivir) are sialic acid analogs that interfere with release of newly generated viruses from infected cells by inhibiting the viral enzyme neuraminidase.
Some bacteria also use host-sialylated structures for binding and recognition. For example, evidence indicates that free sialic acids can behave as a signal to some specific bacteria, like Pneumococcus. Free sialic acid possibly can help the bacterium to recognize that it has reached a vertebrate environment suitable for its colonization. Modifications of Sias, such as the N-glycolyl group at the 5 position or O-acetyl groups on the side chain, may reduce the action of bacterial sialidases.