Topic 2-L4 - Bacterial Cell Walls Flashcards
Primary function of cell wall
prevent cells from bursting due to osmotic pressure (high concentration of solutes in cell)
- Also provides cell shape, rigidity
Gram-positive bacteria:
thick cell wall, no outer membrane
Gram-negative bacteria:
thin cell wall, have an outer membrane
Peptidoglycan is a
lattice-like structure formed from chains of glycans linked together by peptide bridges
Sugar backbone:
Alternating N-Acetylglucosamine
NAG) & N-Acetylmuramic Acid (NAM
Peptide crosslinks:
Short peptide chain (attached to NAM) covalently linked to peptides from adjacent chains via peptide bond.
Glycan chains
• N-acetylglucosamine (GlcNAc, NAG) & N-acetylmuramic acid (MurNAc, NAM) connected by β(1,4) linkage (glycosidic
bond)
Peptides in cell wall:
• Sequence can vary between species • Note D-isomers of amino acids (proteins
made of only L-isomers)
Crosslinks – peptide bonds -
primarily between position 3 (diaminopimelic acid “DAP” – can be a lysine) and position 4(D-alanine)
Gram-negative bacteria
- Flexible, porous, but still quite strong
- Additional strength to Gram negative
envelope provided by outer membrane
Gram-positive becteria
- Much thicker peptidoglycan layer – can be 15+ layers (~20-35 nm)
- Interbridges (absent from Gram –ve walls)
Interbridges (found in gram + not -)
help connect different peptidoglycan layers. These are also peptide crosslinks, extended
by a few amino acids. Sequence can vary.
Gram positive cell walls: Teichoic acids
- 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”)
Teichoic acids priovide
cell strength (ionic interactions between neighboring strains bridged by metal ions)
Teichoic acids
- Help trap divalent metal ions such
as Mg2+ (negative charge) - Barrier & attachment functions
Gram positive cell walls: Wall- associated proteins
- 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
Are cell walls stagnant ?
Cell walls are not stagnant. They are constantly being synthesized, degraded and remodeled under many conditions. This is particularly true during cell growth/division.
Gram staining
- Cells stained uniformly with insoluble crystal violet-iodine complex (purple)
- Cells are “decolorized” with alcohol.
- Cell are counterstained with safranin (pink)
Gram positive (staining)
thick layer of peptidoglycan is dehydrated – pores close and prevent escape of crystal violet dye – cells are stained purple (dark, dominant over pink counterstain)
Gram negative (staining)
decolorizing agent degrades outer membrane, thin/porous peptidoglycan layer does not retain purple stain. Cells appear pink due to safranin counterstain.
Mycoplasma: Unusual bacteria that lack cell walls
- 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.
In isotonic (high solute) conditions, cell wall can be degraded & a
“protoplast” (Gram +ve bacterium
lacking cell wall) is released.
In isotonic (high solute) conditions, cell wall can be degraded & a
“Spheroplast” is term for Gram
negative
