Surface Glucan Structures in Aeromonas spp

Abstract

Aeromonas spp. are generally found in aquatic environments, although they have also been isolated from both fresh and processed food. These Gram-negative, rod-shaped bacteria are mostly infective to poikilothermic animals, although they are also considered opportunistic pathogens of both aquatic and terrestrial homeotherms, and some species have been associated with gastrointestinal and extraintestinal septicemic infections in humans. Among the different pathogenic factors associated with virulence, several cell-surface glucans have been shown to contribute to colonization and survival of Aeromonas pathogenic strains, in different hosts. Lipopolysaccharide (LPS), capsule and α-glucan structures, for instance, have been shown to play important roles in bacterial-host interactions related to pathogenesis, such as adherence, biofilm formation, or immune evasion. In addition, glycosylation of both polar and lateral flagella has been shown to be mandatory for flagella production and motility in different Aeromonas strains, and has also been associated with increased bacterial adhesion, biofilm formation, and induction of the host proinflammatory response. The main aspects of these structures are covered in this review.

Keywords: Aeromonas; LPS; O-antigen; capsule polysaccharide; glycosylation; α-glucan.

Figure 1

Schematic representation of flagellin structure and arrangement into the flagellar filament. Flagellins are arranged into 11 protofilaments in a way that domains D0 (yellow) and D1 (grey) remain in the interior of the flagellar filament, while domains D2 (blue) and D3 (red) are exposed to the outer surface.

Figure 2

Schematic representation of the LPS molecule of Gram-negative bacteria. The lipid-A moiety consists of two parts: a lipid fraction composed of fatty-acid chains, which anchors LPS in the bacterial outer membrane, and a sugar backbone that links the molecule to the core oligosaccharide (core OS). The O-antigen chain, located at the most external part of the LPS molecule, is built of repetitive saccharide units that vary in number (n) among different bacterial cells. Smooth (S)-LPS is comprised of all three components, while rough (R)-LPS lacks the O-antigen subunit. The number and chemical structure of the acyl chains and sugar moieties represented in the figure can vary. NAG, N-acetylglucosamine. Kdo, 3-deoxy-d-manno-oct-2-ulosonic acid.

Figure 3

Structure representation of the surface α-glucan described in A. piscicola AH-3. The polysaccharide chain is comprised of α-d-glucosyl units connected by α-1,4-linkages, and branched through α-1,6-glycosidic bonds (m, n, and p stand for different numbers of repeated d-glucose monomers).