Characterization of a novel sialic acid transporter of the sodium solute symporter (SSS) family and in vivo comparison with known bacterial sialic acid transporters

The function of sialic acids in the biology of bacterial pathogens is reflected by the diverse range of solute transporters that can recognize these sugar acids. Here, we use an Escherichia coliΔnanT strain to characterize the function of known and proposed bacterial sialic acid transporters. We discover that the STM1128 gene from Salmonella enterica serovar Typhimurium, which encodes a member of the sodium solute symporter family, is able to restore growth on sialic acid to the ΔnanT strain and is able to transport [14C]-sialic acid. Using the ΔnanT genetic background, we performed a direct in vivo comparison of the transport properties of the STM1128 protein with those of sialic acid transporters of the major facilitator superfamily and tripartite ATP-independent periplasmic families, E. coli NanT and Haemophilus influenzae SiaPQM, respectively. This revealed that both STM1128 and SiaPQM are sodium-dependent and, unlike SiaPQM, both STM1128 and NanT are reversible secondary carriers, demonstrating qualitative functional differences in the properties of sialic acid transporters used by bacteria that colonize humans.