These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. Lecture 6 Outline of Last Lecture I. Prokaryotic Cell common features II. Biochemical composition of Bacteria III. Techniques to study Cell parts IV. Cell membrane (components, variations, and parts) V. Cell wall Outline of Current Lecture VI. Peptidoglycan Structure VII. Strands are Crosslinked VIII. Patterns of Cell Wall Formation IX. Action of Antibiotics X. Nucleoid XI. Gene expression XII. DNA transcription into RNA XIII. DNA transcription into RNA XIV. Protein Synthesis and Secretion XV. DNA replication XVI. The Bacterial Cytoskeleton Current Lecture - Peptidoglycan Structure: o Most bacterial cell walls are made up of peptidoglycan (or murein) Meshlike polymer of identical subunits forming long strands o Name comes from two sugar residues N-acetylglucosamine (NAG) β 1-4 linkage - Glucose amine N-acetylmuramic acid (NAM) - Know both names completely - Only present in bacteria (no other domains) - Where the name of urime comes from? o Alternating D- and L- amino acids o Order of peptidoglycan connections: L-alanine following the NAM and NAG - Bond between NAM and NAG is sensitive to “lysozyme” Then D-Glutamic acid BIO 326M 1stEdition Then meso-Diaminopimelic acid (DPI) - Forms the third peptide bond - Also only present in bacteria Then D-Alanine At this site, a bond may be formed to link a peptide to another chain, causing the terminal D-alanine to be lost - Isoniazid – mycolic a^ [to Be readdressed later in lecture] - Strands are Crosslinked o Peptidoglycan strands have a helical shape o Peptidoglycan chains are crosslinked by peptides for strength Inter-bridges may form peptidoglycan sacs interconnected networks various structures occur o Subunit synthesized in the cytoplasm Peptide cross-bridge forms with release of D-alanine. Blocked by penicillin Cross-bridge formation blocked by vancomycin, which binds D-Ala-D-Ala and prevents release of terminal D-alanine. - Peptidoglycan cell wall is outside and the subunits have to attach to the preexisting cell wall - L vs D form differences o Proteins are typically L and differ by the rotation of the light - D amino acids help the cell wall to protect from proteases simply due to the chirality during synthesis o Proteases search for L forms - How the cell wall modifies the size of the cell wall (Patterns of Cell Wall Formation) o Autolysins: Makes a cut but holds on until the next subunit comes and binds to prevent the external water from rushing in and lysing the cell Carries out limited digestion of peptidoglycan Activity allows new material to be added to wall and division to occur o Inhibition of peptidoglycan synthesis can weaken cell wall and lead to lysis o Many commonly used antibiotics inhibit cell wall formation o Mycoplasmas uses sterols to make reasonably stronger plasma membranes - Action of Antibiotics: o Cycloserine Interferes with peptidoglycan synthesis itself Inhibits racemase- L-ala to D-ala - D-ala-D-ala dipeptide Fairly toxic antibiotic - Not usually prescribed - Used for tuberculosis - Used in labs for interference with cell wall synthesis - Converts meta-alanine to D-alanine toxicity o Penicillin Inhibits transpeptidation o Vancomycin inhibits both transpeptidation and transglycosylation o Bacitracin Applied as an ointment and applied locally Does not change length or affect chain Does not interfere with peptidoglycan Hydrophilic and needs a chaperone to cross the plasma membrane - Uses large organic molecule (bactoprenolPP) Inhibits conversion of bactoprenolPP to bactoprenolP o Gram-Positive and Gram-Negative Bacteria additional envelope layers Fairly large layer of peptidoglycan - Gram positive have thick cell walls o Phylum Firmicutes o Contents from outer-most to inner-most: (outer-most) Capsule (not all species) - Polysaccharides S-Layer (not all species) - Protein Thick cell wall - Amino acid cross-links in peptidoglycan o ~80 nm of peptidoglycan layers - Teichoic acids for strength o responsible for negative charge pathogenicity or toxicity responsible may be linked to just the peptidoglycan units (inner-most) Plasma membrane o Gram negative have thin cell walls Phylum Proteobacteria Contents from outer-most to inner-most: - Outer membrane lies outside the thin peptidoglycan layer [outer leaflet] (consists of one or two sheets) o LPS [Lipopolysaccharides] [inner leaflet] Lipid A (Embedded in outer membrane) Core polysaccharide (O side chain extend out from the cell) O side chain (O antigen) (endotoxin, pathogenic) o Porin (outer membrane trimer proteins): Allow entry sugars and peptides Some allow antibiotics (ampicillin) to enter - Periplasm o Braun’s lipoproteins connect outer membrane to peptidoglycan o (Lipoproteins and Peptidoglycan) - Inner membrane o Membrane proteins - *Importance of LPS: o Contributes to negative charge on cell surface o Helps stabilize outer membrane structure o May contribute to attachment to surfaces and biofilm formation o Creates a permeability barrier o Protection from host defenses (O antigen) o Can act as an endotoxin (lipid A) o Mcyobacterial Cell Envelope Very different from the Gram positive and significantly different from the Gram negative Unusual membrane lipids are present: - Mycolic acidso Large lipids that make the envelope waxy, difficult to stain, and to dehydrate Unusual membrane sugars present: - Arabinogalactans Contents from outer-most to inner-most: - Capsule o Phenolic glycolipid o Phospholipid o Sugar mycolate - Cell wall core o Mycolic acids (attached to arabinan) o Arabinan Ethambutol blocks synthesis o Galactan o Peptidoglycan - Cell membrane - Isoniazid – mycolic a^ (specific against mycolic acid infection) *Takes about 2 weeks for colonies to show up on the agar plates - *Due to multiple layers the cell has difficulty accumulating the nutrients - *The layer does not have an exact purpose as of yet o Eukaryotic Microbes NOT BEING COVERED - Nucleoid: o An
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