Part of the tripartite ATP-independent periplasmic (TRAP) transport system SiaPT involved in the uptake of sialic acid (N-acetyl-beta-neuraminate). N-Acetylneuraminate can then be incorporated into the lipooligosaccharides (LOS) as a terminal non-reducing sugar, protecting the bacterium from complement-mediated killing by normal human serum. The complex comprises the extracytoplasmic solute receptor protein SiaP, and the fused transmembrane protein SiaT. Cells lacking this gene are incapable of transporting sialic acid and thus unable to sialylate its LOS, making the organism unable to survive when exposed to human serum and causing reduced viability in biofilm growth. Is essential for lipooligosaccharides (LOS) sialylation and serum resistance. In the N-terminal section; belongs to the TRAP transporter small permease family. In the C-terminal section; belongs to the TRAP transporter large permease family.