Diversity of microbial sialic acid metabolism

Abstract

Sialic acids are structurally unique nine-carbon keto sugars occupying the interface between the host and commensal or pathogenic microorganisms. An important function of host sialic acid is to regulate innate immunity, and microbes have evolved various strategies for subverting this process by decorating their surfaces with sialylated oligosaccharides that mimic those of the host. These subversive strategies include a de novo synthetic pathway and at least two truncated pathways that depend on scavenging host-derived intermediates. A fourth strategy involves modification of sialidases so that instead of transferring sialic acid to water (hydrolysis), a second active site is created for binding alternative acceptors. Sialic acids also are excellent sources of carbon, nitrogen, energy, and precursors of cell wall biosynthesis. The catabolic strategies for exploiting host sialic acids as nutritional sources are as diverse as the biosynthetic mechanisms, including examples of horizontal gene transfer and multiple transport systems. Finally, as compounds coating the surfaces of virtually every vertebrate cell, sialic acids provide information about the host environment that, at least in Escherichia coli, is interpreted by the global regulator encoded by nanR. In addition to regulating the catabolism of sialic acids through the nan operon, NanR controls at least two other operons of unknown function and appears to participate in the regulation of type 1 fimbrial phase variation. Sialic acid is, therefore, a host molecule to be copied (molecular mimicry), eaten (nutrition), and interpreted (cell signaling) by diverse metabolic machinery in all major groups of mammalian pathogens and commensals.

FIG. 1.

Structures of sialic acids and related keto sugars. Note that the numbers in panel A indicate the relative carbon atoms in Neu5Ac and each of the succeeding ulosonic acids.

FIG. 2.

Encapsulated E. coli K1. The PSA capsule of E. coli K1 was stabilized with monospecific antibody, fixed, thin sectioned, and examined by transmission electron microscopy (30, 153). Note the microcapsule surrounding the outer surface of the bacterium and the dense, granular cytoplasm.

FIG.3.

Genetic organization of nan systems in gram-positive and gram-negative bacteria. The criteria for assigning functions to the various open reading frames are described in the text. Accession numbers (National Center for Biotechnology Information) are given in parentheses in the following. (A) E. coli (NC000913); (B) H. influenzae Rd (NC000907); (C) P. multocida Pm70 (NC002663); (D) V. cholerae N16961 (NC002505); (E) Y. pestis C092 (NC003143); (F) S. pneumoniae TIGR4 (NC003028); (G) S. pyogenes MGAS8232 (NC003485); (H) C. perfringens Str.13 (NC003366), (I) F. nucleatum ATCC25586 (NC003454), (J) S. aureus MW2 (NC003923); (K) L. plantarum WCFS1 (NC004567). Open reading frames are identified by the gene locus designation assigned to them in the database. Colored arrows indicate the known or probable gene products as follows: transcriptional regulators (magenta); aldolases (orange); transporters (yellow); epimerases (green); kinases (blue); sialidases (grey), and unknown (plum). Slashes (nagB) or dots (nagA) represent the nagBA homologues. Bronze-yellow open reading frames encode putative polypeptides with no known function but are associated with TRAP transporters as described in the text. Bent arrows indicate known or potential transcriptional start sites.

FIG. 4.

Bacterial solute transporters. Solute (solid circles) channels (solid rectangles) for uptake through the cytoplasmic membrane (CM) from the extracellular environment (Out) to the cytoplasm (In) are indicated by the large rectangles representing integral membrane transport proteins. (A) Facilitated diffusion. (B) ABC transporter, where intermediate-size open circles indicate the periplasmic binding component and the hatched circles indicate the ATPase. (C) Symporter of the major facilitator superfamily of membrane transporters, where the small circles indicate protons or metal ions coupled to solute uptake. (D) TRAP transporter, indicating features in common with ABC and secondary transporters and the addition of a second (hatched rectangle) membrane protein of unknown function that is necessary for solute uptake.

FIG. 5.

Phylogeny of selected sialic acid aldolase (NanA), epimerase (NanE), and kinase (NanK) polypeptides involved in microbial sialic acid catabolism. Colored blocks indicate phylogenetic relationships of the respective gene products as defined in the legend to Fig. 3. Primary structures were multiply aligned by using ClustalW and analyzed with the Phylogenetic Analyses and Reconstruct Phylogeny tools in MacVector version 7.2. The tree was constructed by using the neighbor-joining algorithm and absolute number of differences. Bootstrap values for 1,000 replicates are given as percentages at major nodes. Polypeptides are identified by their gene designation as they appear in Fig. 3.

FIG. 6.

Hydropathy analysis of NanT. E. coli NanT was analyzed for hydropathy (H) by using the Kyte-Doolittle (KD) index. Predicted membrane-spanning domains are indicated beginning from the N terminus.

FIG. 7.

Secondary structural model of NanT. The secondary structure of NanT was modeled on the basis of conserved residues in the major facilitator superfamily as described in the text. Amino acid residues (circles) are given as their single-letter designations, and where applicable the charge is also indicated. Black circles indicate conserved residues. Shaded rectangles indicate the 12 membrane-spanning domains in common with those of most superfamily members. The central, unshaded rectangles indicate those unique to NanT (82). Triangles indicate the site of TnphoA insertions with units of alkaline phosphatase activity.

FIG. 8.

Proposed pathways of sialic acid synthesis and degradation in E. coli. Metabolites of sialic acid degradation or synthesis are defined in the text. Note that cosubstrates and by-products are not indicated but can be found in reference . Abbreviations not defined in the text are as follows: ECA, enterobacterial common antigen; GlmS, GlcN-6-P synthase (l-glutamine:d-fructose-6-P amidotransferase); GlmU, GlcNAc-1-P uridyltransferase; RffE, UDP-GlcNAc epimerase. Dashed lines indicate that multiple reactions catalyze the indicated syntheses. The figure was adapted from reference .

FIG. 9.

Accumulation of intracellular PSA in an E. coli K1 neuE null mutant. In contrast to the case for the wild type (Fig. 2), the defect caused by loss of NeuE disrupts the export of PSA, which is found to accumulate in the cytoplasm, predominantly at one pole of the cells. This phenotype is observed in other E. coli K1 mutants with export defects.