Cell Walls

Question 1

Bacterial cell wall types

Answer 1

Gram-positive: single type of molecule

Gram-negative: multiple layers and complex

Question 2

Peptidoglycan composition

Answer 2

Polysaccharide with a backbone composed made up of
two sugar derivatives and amino acids
Structures connected to form a repeating structure, the glycan tetrapeptide

Question 3

Peptidoglycan sugar derivatives

Answer 3

• N-acetylglucosamine (G)
• N-acetylmuramic acid (M)

Question 4

Peptidoglycan amino acids

Answer 4

D-/L-alanine
D-glutamic acid
Either Diaminopimelic acid or lysine
No branched, aromatic, sulfur containing amino acids, histidine, arginine and proline

Question 5

Peptidoglycan peptide crosslinks

Answer 5

• The chains of peptidoglycan are linked together through cross-links of amino acids
• Gram negative
– Direct link (3 – 4) between amino group of DAP and
-COOH group of the terminal D-alanine
• Gram positive
– Occurs by way of peptide interbridge (e.g. glycine)
– Greater diversity

Question 6

Gram positive cell wall

Answer 6

• Peptidoglycan: 90 %
• Most contain the acidic molecule called Teichoic acids (TA)
– Ribitol-phosphate or Glycerol-phosphate connected by phosphate esters
– D-alanine or other sugars attached
– TA bonded to muramic acid in peptidoglycan
– Responsible for negative charge of cell wall
– They also bind Ca2+ and Mg2+ for transport into the cell
• Lipoteichoic acids: TA linked to cytoplasmic membrane lipids

Question 7

Gram-negative cell wall

Answer 7

• Peptidoglycan:10%
• LPS:TheOutermembrane
– Second bilayer (phospholipids and proteins)
– Lipids and polysaccharides linked to form a complex: Lipopolysaccharides (LPS)
LipidA:the Lipid portion of the LPS:
– It renders strength to the cell
– Endotoxin: Toxic component (cause gas, diarrhoea, vomiting in humans)

Question 8

Archaeal cell wall

Answer 8

• Pseudomurein instead of peptidoglycan = penicillin resistant
– N-acetylglucosamine and N-acetyltalosaminuronic acid
– Sugar derivatives linked by β-1-3 glycosidic linkages instead of β-1-4 in peptidoglycan
• Other Archaea lack pseudomurein but contain other polysaccharides
• Most common cell wall is the paracrystalline S-layer

Question 9

Functions of S- layer

Answer 9

• Protect cells from osmotic lysis
• May act as selective sieve that allows passage of low mw solutes while excluding large molecules or structures
• Retains proteins near the cell surface