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Cryptococcus neoformans is the only fungal species of unarguable pathogenicity that possesses capsular polysaccharides, whose most abundant components are glucurono-xylo-mannans. There also, the capsule may be a factor in virulence. Returning to bacteria: compromise of the underlying bacterial cell wall, for instance through interference of the biosynthesis of the wall's peptidoglycan (see below)

by penicillin, causes autolysis, as the wall's integrity arises mostly from a high molecular weight, cross-linked and 'net-like' peptidoglycan polymer. That substance is composed of two components. The first is a polysaccharide of alternating N-acetyl-D-glucosamine (d-GIcNAc), linked (3(1—>4) to N-acetyl muramic acid (NAM = d-G1cNAc carrying a lactic acid substituent at its 0-3). The carboxyl group of the lactic acid in NAM is attached to a short peptide chain, which, in Staphylococcus aureus consists of L-alanine, D-y-glutamic acid, L-lysine and D-alanine. The d-alanine's carboxyl end is connected by a pen-taglycine to the e-amino group of the lysine of another tetrapeptide. Further bridging is probably achieved to make this giant molecule three dimensional. The peptides vary in differing bacterial species, but the intricate, resulting network of cross-linked polysaccharide/polypeptide is very similar (V).

Teichoic acids, composed of sugar residues, some of which are linked by phosphodiester- rather than acetal-linkages, may be attached to some of the GlcNAc residues by a phosphate linkage. Peptido-glycans occur in both gram-positive and gram-negative organisms, where they have similar, albeit not identical, structures.

Gram-negative bacteria possess a lipopolysaccha-ride (LPS), also called endotoxin. It is made up of three distinct segments.

First, the lipid A, which consists in most cases of two residues of N-acyl D-glucosamine, linked 3(1—>6). This disaccharide fragment is phosphoryl-ated at positions 1 and 4', and each residue is extensively esterified, as well as N-acylated, with long-chain fatty acids. The lethal toxicity the LPS can exhibit in animals (and humans) resides in the lipid A segment.

Second, the so-called core region is covalently attached to the 6' position of lipid A. It is a fairly short oligosaccharidic segment (usually less than 15 sugars) consisting of one or more residues of keto-deoxy-octulosonic acid (forming the linkage with the lipid A), heptoses, various sugars, ethanolamine (EtAm) and phosphate (P) substituents. It is similarly but not identically structured in most Gram-negative bacteria.

Third, the so called O-polysaccharide (O-PS) can possess up to roughly 100 sugar residues, made up of distinct repeating units, linear or branched, that can involve (from one to) several sugars. It is covalently linked to the core region.

The lipid A segment is inserted - and anchored through the hydrophobic interaction of its fatty acid chains with the cell wall lipids - into the fluid lipid bilayer of the bacterium's wall. The entire LPS is oriented so that its O-polysaccharide projects out etc. I

L-Ala

D-y-Glu

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