Bacteria are covered by polysaccharides at the outer surface in the form of capsules, glycoproteins, or glycolipids. Such a bacterial sugar coat constitutes the principal antigens in most pathogenic bacteria and plays an important role in host-pathogen interactions. With more understanding of polysaccharide biosynthesis and its interplay with polymer modification and synthesis, scientists have recognized the research potential of polysaccharides in developing novel antibacterial strategies and novel applications like epidemiological markers.
Bacterial polysaccharides mainly are carbohydrates like capsular polysaccharides (CPS) and lipopolysaccharides (LPS). Capsular polysaccharides are highly-hydrated homo- or hetero-polymers that are composed of repeating sugar units joined by glycosidic linkages. They usually are inserted into the cell surface of bacteria by covalent attachments to either phospholipid or lipid-A molecules. Lipopolysaccharide, on the contrary, is a membrane component characteristic of Gram-negative bacteria consisting of lipid A, core-oligosaccharide, and O-polysaccharide (or O-antigen) joined by a covalent bond. With a series of advanced technologies, including chemical degradation techniques, nuclear magnetic resonance spectroscopy, and mass spectrometry techniques, researchers now can comprehensively carry out structural characterization and analysis of bacterial polysaccharides and understand their functions.
CPS usually constitutes the outermost layer of the cell and gets involved in mediating interactions between bacteria and the environment, for which polysaccharide capsules are implied as important virulence factors for several bacterial pathogens. Moreover, CPS can protect bacteria from phagocytosis if the pathogen is attacked by innate immune responses. Capsular polysaccharides prevent the activation of phagocytosis by decreasing antibody opsonization and masking ligands for phagocytic cell attachment. Therefore, polysaccharides antibodies as biomarkers have become the research hotspot in the field of disease diagnosis and treatment. Researchers at the Queensland University of Technology have successfully characterized the genomic loci in Acinetobacter baumannii that are responsible for cell-surface polysaccharide synthesis and have proven them to be effective epidemiological markers to track A. baumannii.
Lipopolysaccharides are large molecules localized in the outer layer of the bacterial membrane and populate much of the cell surface. LPS can establish a permeability barrier, protecting bacteria from toxic molecules such as antibiotics and bile salts. In addition to being a key component of the cell envelope, LPS also contributes to host-pathogen interactions with the innate immune system. Bacterial adaptive changes, including modulation of LPS synthesis during chronic infection, could protect disease by preventing phagocytosis and adhesion to epithelial cells with O-antigen lipopolysaccharide or enhancing host immune response evasion with a production of less immunogenic lipid A, etc. In some cases, the immune response against the bacterial polysaccharide could be too dramatic to be toxic to the host, which confers protection against the disease in some depth.
Polysaccharides could also be found in fungi and yeast in the form of glucans, chitin, and mannans, playing a major role in the cell wall structure. Current research on fungal polysaccharides mainly aims at identifying influential factors for their biological activity and elucidating their interactive role in various chemical medicine. The biological activities of fungal polysaccharides are shown to influence anti-tumor, anti-microbial, immune-stimulation or immunomodulatory activity, nutritional component, and hypoglycemic activity.
With more understanding of the structure of various polysaccharides, details about the mechanism of action of polysaccharides in different systems are being revealed. Polysaccharides of bacteria and fungi will share more extensive applications in diagnostics and therapeutics.
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