Bacterial Cell Structures and Treatments 1

Bacterial Cell Structures and Treatments 1

Bacterial Cell Structures and Treatments 1

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Bacterial Cell Envelope

  • The cell envelope usually consists of two components:
    • an outer cell wall or peptidoglycan (gram-positive and gram-negative)
    • a cytoplasmic or plasma membrane (gram-positive and gram-negative)
    • an outer membrane (gram-negative only)
  • The cell wall (peptidoglycan) is chemically inert (unreactive). It is composed of sub-units found nowhere else in nature.
  • The cell envelope of gram-negative bacteria can produce symptoms of disease (endotoxins).
  • The site of action of some of the most effective chemotherapeutic antibiotics is the cell wall peptidoglycan.
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Bacterial Cell Envelope

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Gram-Staining

  • Distinguishing between the different cell envelope
  • The gram-stain played a major role in classifying bacteria
  • Two major groups
    • gram-positive
    • gram-negative
  • Gram refers to staining procedures (Christian Gram)
    • based on the cell wall structure of bacteria
    • important for susceptibility to osmotic pressure, antibiotics, etc.
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Gram-Staining

  • Involves three different chemical reagents
    • the primary stain (crystal violet)
    • Gram's iodine (mordant) forms an insoluble complex by binding to the primary stain
      • The decolourising agent (ethanol 95%) which acts as a lipid solvent as well as protein-dehydrating agent
    • The counter stain (safranin)
  • In gram-positive, the low lipid concentration is important for the retention of the complex iodine-crystal violet: cell remains blue
  • In gram-negative, the high lipid concentration found in the outer layers of the cell wall is dissolved which facilitates the release of the iodine-crystal violet complex leaving the cell colourless
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Gram-Positive and Gram-Negative

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Cell Wall Peptidoglycan

  • The cell wall peptidoglycan is a rigid structure which gives shape to the bacterium
  • The wall also prevents osmotic rupture of the protoplast in dilute solutions
  • When no cell wall remains the whole structure is called a protoplast - protoplasts always assume a spherical form even when derived from rod-shaped organisms
  • The protoplast accounts for 60-80% of the dry weight of the whole cell
  • When part of the cell wall remains attached to the protoplast the whole structure is called a spheroplast
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Cell Wall Peptidoglycan

  • The peptidoglycan, which is the backbone of the cell wall, is a macromolecule consisting of two major sub-units (amino sugars) and amino acids
    • N-acetyl-muramic acid and N-acetyl-glucosamine that alternate to form a high molecular weight polymer
    • a chain of several amino acids is attached to each of the N-acetyl-muramic acid molecules
  • The bacterial cell wall is a three-dimensional rigid, multilayered network around the organism
  • Within this open meshwork are found a number of matrix materials, chiefly polysaccharides
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Cell Wall Peptidoglycan

  • They include the immunologically active substances which determine the antigenic specificity of certain organisms (e.g. the C polysaccharide and M protein of species of Streptococcus)
  • One group of these matrix is the techoic acids: highly acidic substances present only in the cell walls of gram-positive bacteria
  • The cell wall peptidoglycan is the target of the B-lactams, glycopeptides, bacitracin, D-cycloserine and fosfomycin
    • They all inhibit cell wall peptidoglycan synthesis
    • The different B-lactams target different enzymes (penicillin-binding protein)
    • B-lactams and glycopeptides interfere with the late stage of cell maturation (cross-linking)
    • Bacitracin, D-cycloserine and fosfomycin interfere with early stages of cell synthesis
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Cytoplasmic Membrane

  • The cytoplasm is bounded by a plasma membrane which is distinct from the enveloping cell wall
  • The chemical composition of the plasma membrane is similar to that of the mammalian cell membranes and includes 40% lipid (much in the form of phospholipids), 60% protein and small amounts of carbohydrates. Sterols are absent

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Cytoplasmic Membrane

  • The plasma membrane is the effective permeability barrier of the cell regulating the inflow and outflow of metabolites to and from the protoplast
  • The membrane is semi-permeable. Generally, only low moleculae weight materials can penetrate to the inside of the cell
  • The membrane contains many proteins/enzymes essential to the bacterium survival; e.g the proton motive force that generates adenosine triphosphate (ATP; energy)
  • The plasma membrane is the target of very few antibiotics and is also the target of antimicrobial peptides
    • Polymyxins (polymyxin B), colistin and daptomycin. Polymyxin B and colistin affects membrane permeability. Daptomycin affects membrane function
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Efflux Proteins

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Gram-Negative Bacteria

  • Periplasmic Space
    • The periplasmic space of gram-nagative bacteria contains exoenzymes called periplasmic enzymes, e.g. penicillinase hydrolyses and thereby inactivates penicillin - antibiotics
  • Outer membrane
    • The outer membrane is composed of a layer of lipopolysaccharides (LPS) and one layer of phospholipids
      • LPS component (lipid A) is an endotoxin (septic shock syndrome)
      • LPS component (carbohydrate) important for identification/immunology
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Cytoplasm

  • Complicated three-dimensional network of fine fibrils stretching from the plasma membrane to the nuclear mass
  • It lacks of elaborate endoplasmic reticulum characteristic of the eukaryotic cell
  • In the bacterial cell the structures devoted to protein synthesis are the ribosomes 
    • Protein constitutes more than 50% of the dry weight of a typical bacterial cell wall
    • Almost 90% of all energy the cell utilises in synthesising cell components is used for the manufacture of proteins
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Cytoplasm: Ribosomes

  • Ribosomes are characterised by their sedimentation properties when centrifuged at very high speeds in an ultracentrifuge
  • Bacterial ribosomes are termed 70s ribosomes, which distinguish them from the 80s ribosomes of eukaryotic cells
  • Each ribosome is made of 2 parts, and each part in turn is made of two different kinds of macromolecules
    • ribosomal protein
    • ribosomes RNA (rRNA)
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Cytoplasm: Ribosomes

  • Many antibiotics inhibit protein synthesis by interacting with the bacterial ribosome:
    • Tetracylines
    • Macrolides
    • Lincosamides
    • Streptogramins
    • Aminoglycosides/aminocyclitols
    • Chloramphenicol
    • Fusidic acid
    • Mupirocin
    • Linezolid
  • These target sites may be close together leading the problems such as bacterial cross-resistance
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Cytoplasm: Inclusion Bodies

  • Many bacteria form and store granules in their cytoplasm in the form of high molecular weight polymers
    • glycogen: a storage form of both carbon and energy
    • polymer of B-hydroxybutyric acid (storage form of both carbon and energy)
  • Phosphate can be stored as granular inclusions of polymeric phosphate volutin
  • Protein crystals are sometimes found. Bacillus thuringiensis crystals are used as insecticide 
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Cytoplasm: Nuclear Apparatus

  • Bacteria lack an organised nucleus but they possess DNA
  • The chromosome exists as a closed circle in all bacteria and is not surrounded by a nuclear membrane
  • In addition some bacterial cells possess a short length of extrachromosomal DNA in the form of a plasmid
  • Plasmids range in size from approximately 0.1 to 10% of the chromosome
  • Plasmids often play an important role in the transfer of genetic material between bacteria
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Cytoplasm: Nuclear Apparatus

  • Many antibiotics inhibit the synthesis of nucleic acid, often indirectly
    • Quinolones (DNA gyrase - DNA replication and transcription inhibition)
    • Rifampicin (RNA polymerase - transcription)
    • Sulphonamides and trimethoprim (folic acid synthesis)
    • Nitrofurantoin, mitrodinazole (DNA interactions)
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