Bacterial Sialic Acid Catabolism at the Host–Microbe Interface

Sialic acids consist of nine-carbon keto sugars that are commonly found at the terminal end of mucins. This positional feature of sialic acids contributes to host cell interactions but is also exploited by some pathogenic bacteria in evasion of host immune system. Moreover, many commensals and pathogens use sialic acids as an alternative energy source to survive within the mucus-covered host environments, such as the intestine, vagina, and oral cavity. Among the various biological events mediated by sialic acids, this review will focus on the processes necessary for the catabolic utilization of sialic acid in bacteria. First of all, transportation of sialic acid should be preceded before its catabolism. There are four types of transporters that are used for sialic acid uptake; the major facilitator superfamily (MFS), the tripartite ATP-independent periplasmic C4-dicarboxilate (TRAP) multicomponent transport system, the ATP binding cassette (ABC) transporter, and the sodium solute symporter (SSS). After being moved by these transporters, sialic acid is degraded into an intermediate of glycolysis through the well-conserved catabolic pathway. The genes encoding the catabolic enzymes and transporters are clustered into an operon(s), and their expression is tightly controlled by specific transcriptional regulators. In addition to these mechanisms, we will cover some researches about sialic acid utilization by oral pathogens.