Masquerading microbial pathogens: capsular polysaccharides mimic host-tissue molecules

Abstract

The increasing prevalence of antibiotic-resistant bacteria portends an impending postantibiotic age, characterized by diminishing efficacy of common antibiotics and routine application of multifaceted, complementary therapeutic approaches to treat bacterial infections, particularly multidrug-resistant organisms. The first line of defense for most bacterial pathogens consists of a physical and immunologic barrier known as the capsule, commonly composed of a viscous layer of carbohydrates that are covalently bound to the cell wall in Gram-positive bacteria or often to lipids of the outer membrane in many Gram-negative bacteria. Bacterial capsular polysaccharides are a diverse class of high molecular weight polysaccharides contributing to virulence of many human pathogens in the gut, respiratory tree, urinary tract, and other host tissues, by hiding cell surface components that might otherwise elicit host immune response. This review highlights capsular polysaccharides that are structurally identical or similar to polysaccharides found in mammalian tissues, including polysialic acid and glycosaminoglycan capsules hyaluronan, heparosan, and chondroitin. Such nonimmunogenic coatings render pathogens insensitive to certain immune responses, effectively increasing residence time in host tissues and enabling pathologically relevant population densities to be reached. Biosynthetic pathways and capsular involvement in immune system evasion are described, providing a basis for potential therapies aimed at supplementing or replacing antibiotic treatment.

Keywords: bacterial pathogens; capsular polysaccharides; combating antibiotic resistance; glycosaminoglycans; immune system evasion; polysialic acid.

Fig. 1

Schematic cross-sectional representation of layers constituting the bacterial cell wall of a typical Gram-negative bacterium. The thick external CPS layer conceals the bacterium to prevent desiccation, bacteriophage infection, complement-mediated killing, and opsonophagocytosis. The black and white inset (top left) shows a quick-freeze, deep-etch scanning electron micrograph of the Gram-negative organism Bacteroides thetaiotaomicron (Martens et al., 2009); this SEM image was originally published in The Journal of Biological Chemistry. Martens EC, Roth R, Heuser JE & Gordon JI. Cover image. J Biol Chem. 2009; 284(27):cover. © the American Society for Biochemistry and Molecular Biology.

Fig. 2

Glycan-centric schematic of typical Gram-negative cell wall components. Membrane proteins and other cell-wall constituents are neglected for simplicity. (a) Cell wall cross-section. (b) Lipopolysaccharide. (c) Capsular polysaccharide. Abbreviations are as follows: Lyso-PG = lyso-phosphatidylglycerol, GlcNAC = N-acetylglucosamine, MurNAc = N-acetylmuramic acid, GalNAc = N-acetylgalactosamine, KDO = 3-deoxy-D-mannooctulosonic acid, PPEtn = Pyrophosphoethanolamine, GlcA = glucuronic acid.

Fig. 3

Symbolic representations and chemical structures of glycans described in this review. Non-immunogenic bacterial CPSs and structurally related animal glycans exhibited side-by-side to demonstrate similarity between backbones. In the case of chondroitin sulfate (CS) and heparan sulfate/heparin, bacterial CPS structures are identical to precursors of the mature human glycans depicted here. Of note, a related GAG known as dermatan sulfate also shares the unsulfated chondroitin backbone as a biosynthetic precursor, but, unlike CS, some glucuronic acid residues in the chain are epimerized to iduronic acid. CS type B possesses iduronic acid residues, so it is sometimes classified as dermatan sulfate. Conversely, HA and PSA structures are identical in microbial CPS and mature human GAGs. *R^2,4,6 = H or SO₃^{-; †}R^2,3,6 = H or SO₃^-, Y = SO₃^- or Ac (Ac = COCH₃). Detailed disaccharide structures have been reported elsewhere (Sugahara & Mikami, 2007; Chang et al., 2012b).

Fig. 4

CPS gene loci in Gram-negative bacteria expressing ABC-transporter dependent CPS assembly pathways. (a) Gene loci encoding enzymes and transport proteins required for assembly of Group 2 E. coli K-antigens K1 (polysialic acid), K4 (chondroitin), and K5 (heparosan). Genes encoded by Regions 1 and 3 are well-conserved within Group 2 E. coli strains and encode enzymes required for CPS translocation across the cell wall, while Region 2 encodes CPS-specific glycosyltransferases and other biosynthetic enzymes. (b) Gene locus encoding proteins required for assembly of N. meningitidis serogroup B CPS. Region A encodes CPS-specific biosynthetic enzymes and varies between serogroups, while Regions B-E are highly conserved in all N. meningitidis serogroups. Regions B and C encode CPS translocation proteins homologous to genes in Regions 1 and 3 of Group 2 E. coli (homologous genes connected with gray bands), Regions D and D′ encode LPS assembly genes, and Region E has no known function. (c) Gene loci encoding P. multocida type A, D, and F CPS biosynthetic and transport proteins. Region 1 and 3 encode translocation genes whose functions are relatively well conserved in P. multocida, and Region 2 encodes CPS biosynthetic enzymes unique to the serotype specified. Homologous inter-species transport genes are color-coded and connected by bands of matching colors. Genes encoding glycosyltransferases are illustrated as white arrows with black outline and glycosyltransferase activity denoted within (bifunctional glycosyltransferases are labeled as found in nature, with N-terminal domain displayed at 5′ end of gene and C-terminal domain displayed at 3′ end of gene). UDP-glucose dehydrogenase is frequently encoded in CPS-specific biosynthetic clusters and is indicated here with diagonal lines. Other genes with known and putative homologs are designated with matching numbers. Note: genes and operons are not drawn to scale.

Fig. 5

Central biosynthetic pathway for CPS precursor production in E. coli. This metabolic model is representative of early CPS biosynthesis for many bacteria, including those of interest in this review.

Fig. 6

Biosynthetic pathway for PSA production in E. coli K1.

Fig. 7

Biosynthetic pathway for production of K4 CPS, chondroitin, in E. coli K4.

Fig. 8

Biosynthetic pathway for production of K5 CPS, heparosan, in E. coli K5.

Fig. 9

Strategies to combat capsule-bearing pathogens. Probiotic or engineered strains are designated in blue, and pathogenic strains are colored red with or without an orange capsule. (a) Ingestion of wild-type probiotic or engineered strains that outcompete pathogenic bacteria. (b) Small molecule inhibitors of CPS biosynthetic enzymes (upper) or CPS translocation proteins (lower). Triangles represent small-molecule inhibitors; purple circular (“Pac-Man”) symbol represents polysaccharide glycosyltransferase. (c) Treatment with probiotic bacteria engineered to secrete lyases or glycosidases (drawn as scissors) with activity against pathogen's CPS (upper) or treatment with purified enzyme (lower). (d) Treatment with natural or engineered bacteriophage to lyse bacteria bearing specific CPS type.