Bacterial Cell Walls Lecture Notes PDF

2-3: Bacterial Cell Walls Lecture Overview: • The structure and function of the bacterial cell walls • Textbook: Chapter 2.3 Cell wall o Vast majority of bacterial cells contain a cell wall (very rare examples lack cell wall). o Primary function: prevent cells from bursting due to osmotic pressure...

2-3: Bacterial Cell Walls Lecture Overview: • The structure and function of the bacterial cell walls • Textbook: Chapter 2.3 Cell wall o Vast majority of bacterial cells contain a cell wall (very rare examples lack cell wall). o Primary function: prevent cells from bursting due to osmotic pressure (high concentration of solutes inside cell) o Also provides cell shape, rigidity o Bacterial cells classified as “Gram positive” or “Gram negative” o Gram-positive bacteria: thick cell wall, no outer membrane o Gram-negative bacteria: thin cell wall, have an outer membrane Cell wall - peptidoglycan o All bacterial cell walls comprised of peptidoglycan (AKA “murein”). Archaea & Eukarya lack peptidoglycan Unit repeated & crosslinked to form peptidoglycan o Peptidoglycan is a lattice-like structure formed from chains of glycans linked together by peptide bridges o Peptido (short peptides) glycan (sugars). o Sugar backbone: Alternating N-Acetylglucosamine (NAG) & N-Acetylmuramic Acid (NAM) o Peptide crosslinks: Short peptide chain (attached to NAM) covalently linked to peptides from adjacent chains via peptide bond. Textbook, Fig 2.8 Cell wall - peptidoglycan & familiarize Glycan chains • N-acetylglucosamine (GlcNAc, NAG) & N-acetylmuramic acid (MurNAc, NAM) connected by β(1,4) linkage (glycosidic bond) ↳) - a oftengetsattacke Wall cell attacked Peptide: • Sequence can vary between species, conserved within a single organism • Note D-isomers of amino acids (proteins made of only L-isomers) D Textbook, Fig 2.8 ↳ made by other isomers not Ribosomes • Crosslinks – peptide bonds - primarily between position 3 (diaminopimelic acid “DAP” – can be a lysine) and position 4 (D-alanine) Cell wall: Gram-negative bacteria o Typically ~1-3 peptidoglycan layers thick. ~2-7 nm thick. o Flexible, porous, but still quite strong o Additional strength to Gram negative envelope provided by outer membrane Textbook, Fig 2.9 Cell wall: Gram-positive bacteria o Much thicker peptidoglycan layer – can be 15+ layers (~20-35 nm) o Interbridges (absent from Gram –ve walls) help connect different peptidoglycan layers. These are also peptide crosslinks, extended by a few amino acids. Sequence can vary. ↳ longer sequence of a . Cr length adds by adding extra amino-acids Textbook, Figs 2.9/2.10 Peptidoglycan peptide sequence o Sequence of peptide in PG is conserved for a given bacterium o Not a random peptide machinery assembles this sequence o Over evolution, different bacteria have evolved slightly different machinery ~ evolving not to but to ↳how to reviment improve peptides stick the o Result is that you get different (specific) sequences in different organisms or are tweaks built chainstogether Gram positive cell walls: Teichoic acids o Long polymers comprised of glycerol phosphate or ribitol phosphate with attached D-glucose and/or D-alanine o Covalently attached to peptidoglycan (“teichoic acids” or “wall teichoic acids”) or cytoplasmic membrane (“lipoteichoic acids”) o Provide cell strength (ionic interactions between neighboring strains bridged by metal ions) o Help trap divalent metal ions such as Mg2+ (negative charge) o Barrier & attachment functions Textbook, Fig 2.10 Gram-positive cell walls: Wall-associated proteins o Proteins on the cell surface of bacteria can serve many important functions, such as cell adhesion o In Gram-positive bacteria, these proteins typically associate (covalently or noncovalently) with the cell wall o Some also interact with teichoic acid Textbook, Fig 2.10 Cell wall – porous and dynamic o The cell wall is porous and is not generally a permeability barrier, except to large molecules (e.g. large proteins/protein complexes) o 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 1. Cells stained uniformly with insoluble crystal violet-iodine complex (purple) 2. Cells are “decolorized” with alcohol. 3. Cell are counterstained with safranin (pink) Gram positive 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 – decolorizing agent degrades outer membrane, thin/porous peptidoglycan layer does not retain purple stain. Cells appear pink due to safranin counterstain. Gram staining - example From textbook image (Fig. 1.23) Mix of E. coli (Gram negative, rods) and Staphylococcus aureus (Gram positive, cocci) Mycoplasma pneumoniae: Unusual bacteria that lack cell walls o Mycoplasma pneumoniae is a human pathogen. It is an intracellular parasite that lives within host cell o Minimal osmotic pressure within host cell – cells burst in low solute environments o Specialized/unusually strong cell membrane Textbook, Fig 2.10