MBIO 1010 Lecture 8

• Gram negative structure

  • Inner membrane (cytoplasmic membrane)
  • peptidoglycan
  • Outer membrane
  • LPS

  Cell wall is the LPS + outer membrane and peptidoglycan

  Cell envelope is all of these together
• Gram positive structure

  • cytoplasmic membrane
  • peptidoglycan

  cell wall is the peptidoglycan

  cell envelop is all of this
• Backbone formed of NAM and NAG connected by glycosidic bonds
• crosslinks formed by peptides
• peptidoglycan strand is helical

  • allows 3-dimensional crosslinking
• E. coli has one layer, it is gram negative
• we need peptide bonds to provide the rigidity that are the light blue with the arrow heads
• some cell walls can be 50-100 layers thick

  bacillus species
• Prokaryotes that lack cell walls

  • mycoplasmas (pathogenic bacteria)
  • group of pathogenic bacteria
  • have sterols in cytoplasmic membrane - adds strength and rigidity to membrane
  • survive in the cytoplasm of another cell and so don't need their own cell wall
  • by not having peptidoglycan helps them with invading host defence mechanisms - defenses like targeting cell wall
• Not all prokaryotes have a cell wall
• peptidoglycan is a function specific to bacteria
• Thermoplasma

  • species of archaea
  • contain lipoglycans in membrane that have strengthening effect
  • other species of archaea have cell walls but not archaea have peptidoglycan
  • no cell wall and has lipoglycans in the cell wall that adds the strength
• Polysaccharide - many sugar
• lipo = lipid
• The way LPS is structures is that a lipid portion (Lipid A) is embedded in the membrane and the polysaccharide portion is outside the membrane

  the polysaccharide can be broken into two components

  • Core polysaccharide
  • O-polysaccharide

  Endotoxin (lipid A): the toxic component of LPS

  in the membrane we lipid A portion would be going into the membrane

  Only have LPS facing the external environment (outer leaflet)
• Along with LPS there is also phospholipids in the outer leaflet
• total cell wall contains ~10% peptidoglycan
• polysaccharides can differ in length
• Three channels through the membrane - porins
• Periplasm: space located between cytoplasmic and outer membranes

  • ~15nm wide
  • contents have gel-like consistency
  • house many proteins
• Porins: channels for movement of hydrophilic low-molecular-weight substances
• periplasm is only present in gram negatives

  not in gram positives because they have one membrane
• • In gram stain reaction, insoluble crystal violet-iodine complex forms inside cell
  • complex is extracted by alcohol from gram-negative, not gram positive bacteria
  • gram positive bacteria have thick cell walls consisting mainly of peptidoglycan
  • becomes dehydrated during alcohol step so pores in wall close
  • prevents crystal violet iodine complex from escaping
  • gram negative bacteria - alcohol penetrated outer membrane, crystal violet iodine complex is extracted from the cell and the cells appear nearly invisible until counterstained with second dye (safarin)
• Archaeal cell walls

  • No peptidoglycan
  • typically no outer membrane
  • pseudomurein
  • polysaccharide similar to peptidoglycan
  • composed of N-acetylglucosamine and N-acetyltalosaminuronic acid
  • found in the cell walls of certain methanogenic archaea
  • cell walls of some archaea lack pseudomurein
• peptidoglycan and pseudomurein are similar

  • both have N-acetylglucosamine
  • pseudomurein has N-acetyltalosaminuric acid
• Pseudomurein

  • pseudo: not quite
  • murein: wall
• The differences in Pseudomurein and peptidoglycan

  • beta 1-3 linkage instead of beta 1-4 linkages which is targeted by lysozyme to cut it up and pseudomurein (lysozyme doesn't affect archaea)
  • N-acetyltalosaminuronic acid
  • they are the L-amino acid and peptidoglycan are mostly D
  • D isomer is uncommon
• S-layers

  • most common cell wall type among archaea
  • consist of protein or glycoprotein
  • paracrystalline structure - can either be hexagonal, tetragonal, trimeric
  • some archaea have only S-layer (no other cell wall components)
  • most have additional cell wall elements
  • bacteria can also have S-layers
  • archaea can never peptidoglycan
  • no matter what will be the outer most layer (closest to the environment)
  • in archaea you can have thick S-layer that can on the cell wall role, confirming the cell shape, structural strength, protecting it from osmotic lysis
• summary of archaeal cell walls

  • variety of structures possible
  • some closely resemble peptidoglycan like pseudomurein
  • others lack polysaccharide completely
  • most archaea contain some type of cell wall structure - functions to prevent osmotic lysis and gives it shape
  • because they lack peptidoglycan, archaea are resistant to lysozyme and penicillin
  • no pathogenic archaea