Catalyzes the proton-dependent transport of sialic acid (PubMed:22167185, PubMed:23848303). Can transport the common sialic acid N-acetylneuraminic acid (Neu5Ac) and the related sialic acids N-glycolylneuraminic acid (Neu5Gc) and 3-keto-3-deoxy-D-glycero-D-galactonononic acid (KDN) (PubMed:23848303). Functions as a bidirectional transporter in vitro (PubMed:22167185). H(+)(in) + N-acetylneuraminate(in) = H(+)(out) + N-acetylneuraminate(out) Negatively regulated by the transcriptional repressor NanR. Induced by N-acetylneuraminate, via inactivation of NanR. Deletion mutant is unable to grow in the presence of Neu5Ac, Neu5Gc or KDN. Belongs to the major facilitator superfamily. Sialate:H(+) symporter (SHS) (TC 2.A.1.12) family. Extended N-terminus. Extended N-terminus.