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Procaryote anatomy: cell envelope, motility, endospores

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Last revised: Wednesday, February 2, 2000
Ch. 3 (p. 51-69) in Prescott et al, Microbiology, 4th Ed. Note: These notes are provided as a guide to topics the instructor hopes to cover during lecture. Actual coverage will always differ somewhat from what is printed here. These notes are not a substitute for the actual lecture! Copyright 2000. Thomas M. Terry

Cell wall

chemical structure of peptidoglycan (symbols: G = NAG, M = NAM)

Unusual properties of PG:

Mechanism of synthesis of Peptidoglycan

Structural Basis for the Gram Stain

Osmotic effects

Effects of lysozyme and pencillin

Challenge Question

Spheroplasts, Protoplasts L-forms, and Mycoplasmas

Archaea: different cell walls

What are the Archaea?

Structural Differences between Archaea and Bacteria

  1. Cell Wall Architecture
    • Bacterial walls are made of peptidoglycan, a polymer of N-acetyl glucosamine and N-acetyl muramic acid (glycan chain) with short peptides containing both D- and L-amino acids.
    • Archaeal walls differ widely, and are made from different materials. Walls of methanogens often contain pseudopeptidoglycan, similar to peptidoglycan, but with slightly different sugars and archictecture. Other archaea use polysaccharides, proteins, or glycoproteins as wall materials.
    • Many eukaryotes have cell walls. These are built of a variety of materials, but never peptidoglycan.
  2. Fatty Acid Linkages
    • In Bacteria and Eukaryotes, membrane fatty acids are linked to glycerol by ester bonds.
    • In Archaea, membranes are built from different types of lipids, polymers of the highly unsaturated molecule isoprene. These lipids are linked by ether bonds, not ester bonds.
  3. Structure of RNA Polymerase
    • RNA polymerase is a crucial enzyme required for the synthesis of new RNA molecules. In bacteria, there is a single type of this enzyme, and it is built of four subunits.
    • In eukaryotes, there are three different enzymes, and they each possess 8-12 subunits.
    • Archaea have intermediate properties; they have only a single enzyme, like bacteria, but it is made of 8-12 subunits, like eukaryotes.
  4. Initiation Codon
    • Proteins are synthesized on ribosomes, with the precise sequence of amino acids dictated by the genetic code.
    • Ribosomes recognize a unique codon, called the initiation codon (AUG), as the correct location to begin synthesizing a protein.
    • In eukaryotes and archaea, the AUG codon always specifies the amino acid methionine
    • In Bacteria, the AUG codon specifies N-formylmethionine, a modified form of methionine.

Table summarizing differences between Bacteria and Archaea

Property Bacteria Archaea Eukarya
Cell wall Made of peptidoglyan Made of various materials, not peptidoglyan (If present) cellulose, others
Lipids Fatty acids present, linked by ester bonds Isoprenes present, linked by ether bonds Fatty acids present, linked by ester bonds
RNA polymerase enzyme Single small enzyme; 4 subunits Single large enzyme; many subunits Three large enzymes; many subunits
Protein synthesis 1st amino acid = formylmethionine 1st amino acid = methionine 1st amino acid = methionine

Outside the envelope

  1. glycocalyx (also called slime layer, capsule). Not found in all bacteria.
    • varies in thickness, rigidity.
    • important in adhesion, ability to avoid phagocytosis
    • some suggestion that many bacteria lose layer when cultured in laboratory.
    • May be much more prominent in nature than thought.
    • Bacterial adhesion promotes formation of biofilms, masses of bacteria encased in large aggregates of extracellular matrix. Biofilms are not well-understood, but incredibly important. Most bacteria may live largely in biofilms rather than as free organisms (the dispersal stage). Biofilms are harder to get rid of, more resistant to antibiotics.
  2. fimbriae & pili
    • short, rigid protein rods, similar in size to flagella, but not involved in motility.
    • function in adhesion, formation of pellicles at liquid surfaces. Function not entirely clear. Pili sometimes involved in pathogenic adhesion (e.g. gonnorhaea)
    • View electron micrograph of E. coli with frimbriae
  3. Flagella
    • curved filament made of flagellin protein: travels through hollow tube, assembles at external end.
    • can be arranged in two ways:
      flagella types
      1. polar flagellation: flagella attached at one (monopolar) or both (bipolar) ends. Ex: Pseudomonas aeruginosa
      2. peritrichous flagellation: flagella attached at many sites around cell periphery. Ex: E. coli
    • attached to cell via basal region
    • flagellar rotor can rotate in either direction: CW or CCW. Signals from cell control direction of rotation. See Motility below for application
    • flagellar rotor is the only circular rotor found in nature, aside from human artifact


  1. Flagellar Motility & Chemotaxis
    • View Bacterial motility from "Cells Alive"
    • Flagella can rotate clockwise (CW) -- in peritrichous cells, flagella then become limp, cell TUMBLES or TWIDDLES -- or countercloskwise (CCW) -- flagellar bundle then becomes rigid, cell RUNS. Rotor is always spinning one direction or other.
    • Energy for rotation comes from Proton gradient.
    • flagellated bacteria move through gradients, TOWARD ATTRACTANTS, AWAY from REPELLANTS.
    • How? detect temporal gradient. If moving towards attractant, suppress tumbles. If moving away, increase frequ. of tumbles.
    • complex mechanism in cell membrane: (1) protein receptors bind chemical; (2) membrane proteins (Methyl-accepting chemotaxis proteins) transmit signal.
  2. Other forms of motility
    • some bacteria are motile w/o flagella. GLIDING MOTILITY. move slowly across surfaces, involves sulfur-containing lipids.

Structural adaptations to inhospitable environments

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