fungi such as, spore, hyphae and among the different species. For example, 12% chitin is found in spore while 40% in hyphae [42, 45–47].
2.5 Bacterial Cell Wall Polysaccharides
Bacterial cell wall have a mesh-like complex structure that is essential for the maintenance of structural integrity and shape of most of the bacteria. It is also responsible to provide shape, protection to the bacterial cell and medium of interaction with bacterial environment. The cell wall of Gram-positive bacteria is exclusively composed of polysaccharides that are merged in peptidoglycan that surround the plasma membrane of the cell. Further, the bacterial polysaccharides have been distributed into the extracellular and intracellular saccharides on the basis of their morphology. The intracellular polysaccharides placed inside the bacterial cell, the part of the plasma membrane, are part of the peptidoglycan, lipopolysaccharides, periplasmic glucans and capsular polysaccharides that are present in the structural part of the bacterial cell wall (Figure 2.4) [48, 49].
Extracellular polysaccharides have a branched structure which is made of sugars and their derivatives in repeated sequence and are known as exopolysaccharides. On the basis of sugar units or their derivatives, the exopolysaccharides have further two types like, homopolysaccharides that include pullulan curdlan, cellulose, etc and heteropolysaccharides such as xanthan and gellan. The bacterial polysaccharides provide evolutionary adaptation to the cell through helping in cellular attachment, prevent from desiccation and act as virulence factor. Bacterial exopolysaccharides have been exceedingly used in food, textile, papers and pharmaceutical industry such as in cosmetics, wound dressing and medicines [50–54].
2.5.1 Peptidoglycan
Peptidoglycan is also known as murein. They are polymers and are composed of sugars and proteins which form a web-like structure outside of the cell membrane of the bacterial except the archaea. Structurally, peptidoglycan is composed of alternating units of β (1–4) linked N-acetylglucosamine and N-acetylmuramic acid. The N-acetylmuramic acid is covalently linked to small polypeptides that consist of alanine and other unusual amino acids such as, D-amino acid and L-diaminopimelic acid to form a 3D network. Interestingly, these unusual amino acids are generally not found in other proteins, thus provide high protection from peptidase enzymes. The 3D network of the peptidoglycan provides elasticity, porosity and scaffolding to the bacterial cell wall. Other bacterial polysaccharides such as capsular polysaccharides and teichoic acid have been anchored in peptidoglycan, thus giving a covering layer of the glycan to the bacteria which directly exposed to their environment. So, peptidoglycan is most prominent in Gram positive bacteria with 99% dry molecular weight and 10% in gram negative bacteria [55, 56].
Figure 2.4 The given illustration depicts the bacterial cell wall that composed of peptidoglycan and LPS. Peptidoglycan is made of alternating units of β (1–4) linked N-acetylglucosamine and N-acetylmuramic acid. LPS has further three components such as, Core polysaccharides, O-antigen, and lipid-A which is notorious for their pathogenicity [66].
2.5.2 Lipopolysaccharides
Lipopolysaccharide (LPS) is a conjugated biomolecule that consists of polysaccharides covalently bound to lipid. It is also known as lipoglycan and is prominently located in the outer membrane of Gram-negative bacteria. Liposaccharides are more notorious due to their pathogenic nature. The toxicity of LPS was first time reported by Richard Friedrich Johannes Pfeiffer who coined the name of bacterial endotoxin. The endotoxin is secreted by live bacterial cells and as well as after their cell degradation. Endotoxin acts as strong antigen and can elicit body immune response. Despite the pathogenic nature of LPS, it helps the bacteria in adhesion and bacteriophage sensitivity, etc. [57, 58].
The LPS have three further components such as: O-antigen, Core polysaccharides and lipid-A. The O-antigen is composed of the long chain of the repetitive glycans subunits and is also known as O-polysaccharides. O-antigen has been attached to the very end of the LPS molecules and it may vary from one bacterial strain to another. The core polysaccharides itself is composed of oligosaccharide sugars such as heptose and 3-deoxy-damnnnoocutlsonics acid and being attached to lipid-A. The third part of the LPS is lipid-A, which has been considered the most conserved region of the LPS and consist of disaccharides like, phosphorylated glucosamine being attached to many fatty acids. Being the hydrophobic nature of the fatty acids lead the anchoring of the lipid-A into bacterial membrane. Lipid-A is highly antigenic in nature and responsible for the pathogenicity of Gram-negative bacteria. When the cell lysed then the lipid-A set out into blood and induce septic shock, fever and diarrhea [59–61].
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