Microbiology MCQ Answer 52

· THE CELL WALL of Gram-negative bacteria is a thinner structure with distinct layers.

· There is an outer layer which is more like a cytoplasmic membrane in composition with the typical trilaminar structure.

· THE MAIN COMPONENT of the Gram-negative cell wall is lipopolysaccharide.

· Additionally there is present phospholipid, protein, lipoprotein and a small amount of peptidoglycan.

· The lipopolysaccharide consists of a core region to which are attached repeating units of polysaccharide moieties.

· A component of the cell wall of most Gram-negative bacteria is associated with endotoxic activity, with which are associated the pyrogenic effects of Gram-negative infections.

· On the side-chains are carried the bases for the somatic antigen specifity of these organisms.

· The chemical composition of these side chains both with respect to components as well as arrangement of the different sugars determines the nature of the somatic or O antigen determinants, which are such important means of serologically classifying many Gram-negative species.

· In many cases it has been shown that the reason for certain organisms belonging to quite different species, giving strong serological cross-reactivity is due their having chemically similar carbohydrate moieties as part of their lipopolysaccharide side chains, which generally have about 30 repeating units.

THESE SIDE CHAINS have as part of their structure frequently quite common hexoses, but in many cases there are also present some unusual hexoses, such as 6-deoxyhexoses and 3,6-dideoxyhexoses. Also O -methyl sugars may be part of these side chain structures.

• WHILE THERE IS a great deal of variation among these side chains, the composition of the core region is much more conserved among many Gram-negative groups of bacteria.
• Characteristic of the core region of many of the Enterobacteriaceae is phophorylated 2-keto-3-deoxyoctonoate, phosphorylated L-glycero-D-mannoheptose, D-galactose, D-glucose and N -acetyl-glucosamine.
• The first two are by far the most widespread components of the core region of many species.
• The lipid component of the lipopolysaccharide, known as lipid A is predominantly associated with the endotoxic activity.
• It is the most conserved part of the Gram-negative cell wall and the basis of its structure is a beta-1,6-linked D-glucosamine disaccharide.
• In some Pseudomonas and Rhodopseudomonas strains this is replaced by 2,3-diamino-2,3-dideoxy-D-glucose.
• This has phosphate residues in positions 1 and 4. To both the amino and hydroxyl groups of the glucosamine moieties are linked long-chain fatty acids. Among the enterobacterial lipopolysaccharides the N -acyl residues are frequently fatty acids like beta-hydroxymyristic acid or similar 3-hydroxyalkanoic acids. Some Gram-negative bacteria lack phosphate in their lipid A and some types contain other carbohydrates 2,3-diamino-2,3-dideoxy-D-glucose.

Category: Microbiology MCQs