Topic 2-L4 - Bacterial Cell Walls

Question 1

Primary function of cell wall

Answer 1

prevent cells from bursting due to osmotic pressure (high concentration of solutes in cell)

• Also provides cell shape, rigidity

Question 2

Gram-positive bacteria:

Answer 2

thick cell wall, no outer membrane

Question 3

Gram-negative bacteria:

Answer 3

thin cell wall, have an outer membrane

Question 4

Peptidoglycan is a

Answer 4

lattice-like structure formed from chains of glycans linked together by peptide bridges

Question 5

Sugar backbone:

Answer 5

Alternating N-Acetylglucosamine

NAG) & N-Acetylmuramic Acid (NAM

Question 6

Peptide crosslinks:

Answer 6

Short peptide chain (attached to NAM) covalently linked to peptides from adjacent chains via peptide bond.

Question 7

Glycan chains

Answer 7

• N-acetylglucosamine (GlcNAc, NAG) & N-acetylmuramic acid (MurNAc, NAM) connected by β(1,4) linkage (glycosidic
bond)

Question 8

Peptides in cell wall:

Answer 8

• Sequence can vary between species • Note D-isomers of amino acids (proteins
made of only L-isomers)

Question 9

Crosslinks – peptide bonds -

Answer 9

primarily between position 3 (diaminopimelic acid “DAP” – can be a lysine) and position 4(D-alanine)

Question 10

Gram-negative bacteria

Answer 10

• Flexible, porous, but still quite strong
• Additional strength to Gram negative
  envelope provided by outer membrane

Question 11

Gram-positive becteria

Answer 11

• Much thicker peptidoglycan layer – can be 15+ layers (~20-35 nm)
• Interbridges (absent from Gram –ve walls)

Question 12

Interbridges (found in gram + not -)

Answer 12

help connect different peptidoglycan layers. These are also peptide crosslinks, extended
by a few amino acids. Sequence can vary.

Question 13

Gram positive cell walls: Teichoic acids

Answer 13

• Long polymers comprised of glycerol phosphate or ribitol phosphate with attached D-glucose and/or D-alanine
• Covalently attached to peptidoglycan (“teichoic acids” or “wall teichoic acids”) or cytoplasmic membrane (“lipoteichoic acids”)

Question 14

Teichoic acids priovide

Answer 14

cell strength (ionic interactions between neighboring strains bridged by metal ions)

Question 15

Teichoic acids

Answer 15

• Help trap divalent metal ions such
  as Mg2+ (negative charge)
• Barrier & attachment functions

Question 16

Gram positive cell walls: Wall- associated proteins

Answer 16

• Proteins on the cell surface of bacteria can serve many important functions, such as cell adhesion
• In Gram-positive bacteria, these proteins
  typically associate (covalently or non-
  covalently) with the cell wall
• Some also interact with teichoic acid

Question 17

Are cell walls stagnant ?

Answer 17

Cell walls are not stagnant. They are constantly being synthesized, degraded and remodeled under many conditions. This is particularly true during cell growth/division.

Question 18

Gram staining

Answer 18

1. Cells stained uniformly with insoluble crystal violet-iodine complex (purple)
2. Cells are “decolorized” with alcohol.
3. Cell are counterstained with safranin (pink)

Question 19

Gram positive (staining)

Answer 19

thick layer of peptidoglycan is dehydrated – pores close and prevent escape of crystal violet dye – cells are stained purple (dark, dominant over pink counterstain)

Question 20

Gram negative (staining)

Answer 20

decolorizing agent degrades outer membrane, thin/porous peptidoglycan layer does not retain purple stain. Cells appear pink due to safranin counterstain.

Question 21

Mycoplasma: Unusual bacteria that lack cell walls

Answer 21

• Mycoplasma pneumoniae is a human pathogen. It is an intracellular parasite that lives within host cell
• Minimal osmotic pressure within host cell – cells burst in low
  solute environments.

-Specialized/unusually strong cell membrane.

Question 22

In isotonic (high solute) conditions,
cell wall can be degraded & a

Answer 22

“protoplast” (Gram +ve bacterium

lacking cell wall) is released.

Question 23

In isotonic (high solute) conditions,
cell wall can be degraded & a

Answer 23

“Spheroplast” is term for Gram

negative