Microorganisms and Bacterial Cell Structure

• Microorganisms


  All organisms most likely originated from a universal ancestor due to our genetics being so similar
• Groups of organisms


  • Eukaryotes
  • Prokaryotes
• Eukaryotes


  Include eukarya, which include humans, animals and all other types of microorganisms
• Prokaryotes


  Include bacteria and archaea
• Prokaryote


  Derived from the work Eubacter, which, in Greek, means 'true' bacteria; they include human pathogens and can be clinical or environmental
• Archaea


  Environmental organisms designed to live in very extreme conditions, e.g. sulfur springs, volcanoes, at the bottom of the ocean, thus they are rarely, if ever, encountered
• Prokaryotes vs. Eukaryotes


  • Eukaryotes possess a true cell nucleus, where the chromosomes are separated from the cytoplasm by a nuclear membrane
  • Prokaryotes have no true nucleus, normally possessing just a single chromosome that's not separated from the other cell contents by a membrane
• Prokaryotes


  • Are not compartmentalised, meaning they don't have membrane-bound organelles
  • Do have a cell membrane, but they lack sterols (e.g. cholesterol)
  • Have a single circular chromosome
  • Have ribosomes (needed for protein production), which are 70S subunits and smaller than in eukaryotes (which have 80S ribosomes)
• Differences between Prokaryotes and Eukaryotes

  • Nucleus
  • Nucleosomes
  • Operons/polycistronic mRNAs
  • Introns
  • TATA box binding protein
  • Organelles
  • Chromosomes
  • RNA polymerase
  • Protein initiator amino acid
  • Peptidoglycan
• Eukaryotes tend to have a much larger DNA than prokaryotes/bacteria and because of this, they tend to have features like introns, TATA box (which help with the reading of genetics) etc.
• Bacterial cells, however, have much simpler DNA which is much shorter compared to eukaryotes, so they have features like operons, which aren't found in eukaryotes.
• In bacteria, chromosomes are circular and not in a double helix. They also have peptidoglycan, which makes up the cell wall of bacterial cells (found only in bacterial cells).
• Morphology (Shape) of Bacteria

  • Rod-shaped —known as a bacillus
  • Round/spherical — known as a coccus
  • Spiral shape, known as spirochaetes
  • Egg shape, known as an ovoid
  • Curved rods, known as vibrios
• Grouping of bacteria


  • Diplo - if they like to go in pairs
  • Strepto - if they like to form a chain
  • Staphylo - if they like to cluster together
  • No prefix - if they like to live individually
• Bacterial cell structure


  • One chromosome that codes for everything essential for its survival
  • Plasmids - extra-chromosomal DNA that can code for things that help it survive but aren't essential for growth
• Plasmids


  Extra stores of DNA that allow bacteria to replicate their chromosomes quickly to produce more bacteria
• Pathogenesis


  Ability to cause an infection, e.g. could code for something that helps bacteria attach to the host
• Antibiotic resistance factors


  If a bacterium encounters an antibiotic, it can switch on antibiotic resistance factors to help it break down the antibiotic
• Cell envelope


  • Gram positive and Gram negative, based on a differential staining technique called the Gram stain
• Peptidoglycan cell wall


  • A polysaccharide made up of N-acetylmuramic acid and N-acetylglucosamine, a unique feature of bacterial cells
  • Highly cross-linked in three dimensions with peptide links, which holds it together and forms a thick polymeric mesh, providing rigidity and stopping cells from bursting
  • Made up of amino acids, which could be of the D or L shapes
• Gram Positive Bacteria


  • Have a thick cell wall consisting of more peptidoglycan (between 60% and 80% compared to Gram-negative bacteria)
  • Cell walls also frequently contain acidic polysaccharides called teichoic acids, which are either ribitol phosphate or glycerol phosphate molecules that are connected by phosphodiester bridges
  • Lipoteichoic acids are found in some Gram-positive bacteria and they are glycerol-teichoic acids that are bound to membrane lipids
• Gram Negative Bacteria


  • Have a cell wall containing less peptidoglycan, but they have a second membrane structure found outside the peptidoglycan layer, called the outer membrane
  • The periplasm is the area between the outer surface of the cytoplasmic membrane and inner surface of the outer membrane and is gel-like in consistency, containing sugars and an abundance of proteins, e.g. hydrolytic enzymes and transport proteins, in addition to the peptidoglycan
• Outer Membrane of Gram Negative Bacteria


  • Not symmetrical, being composed of proteins, lipoproteins, phospholipids and a component unique to Gram-negative bacteria, lipopolysaccharide LPS
  • Attached to the peptidoglycan by a lipoprotein, with one of its ends covalently attached to peptidoglycan and the other end embedded in the outer membrane
  • A major permeability barrier, like with all cell membranes, meaning it's good at keeping substances out
• Because Gram negative bacteria have an outer membrane, it makes Gram negative infections harder to treat than Gram positive infections.
• Periplasm


  The area between the outer surface of the cytoplasmic membrane and inner surface of the outer membrane, gel-like in consistency and containing the cell wall
• Contents of the periplasm

  • Sugars
  • Hydrolytic enzymes
  • Transport proteins
  • Peptidoglycan
• Gram-negative bacteria outer membrane

  • Not symmetrical, composed of proteins, lipoproteins, phospholipids and lipopolysaccharide (LPS)
  • Attached to peptidoglycan by a lipoprotein
  • Major permeability barrier, good at keeping substances out
  • Makes gram-negative infections harder to treat than gram-positive
• Periplasmic space


  Space between the cell (inner) membrane and outer membrane that holds the cell wall
• The outer membrane is not a phospholipid bilayer, even though it contains phospholipids in the inner leaf and LPS in the outer layer
• Proteins in the outer membrane

  • Some form trimers that traverse the whole membrane, creating water-filled channels or porins
  • Others found at either the inner or outer face of the membrane
• Functions of the outer membrane

  • Not permeable to enzymes or large molecules, keeping certain enzymes from diffusing away
  • Not permeable to hydrophobic compounds, resistant to dissolution by detergents
• Lipopolysaccharide (LPS)


  • Determines the antigenicity of the gram-negative cell and is extremely toxic to animal cells
  • Consists of lipid A, core polysaccharide and O-specific polysaccharide
• Lipid A


  Responsible for the toxic and pyrogenic properties of gram-negative bacteria, made of a disaccharide of glucosamine phosphate bound to fatty acids
• Gram-positive bacteria


  • Have no outer membrane, therefore no periplasmic space
  • Degradative enzymes are released to the external environment
• Flagella


  Long helical-shaped structures that project from the cell surface and move the bacterial cell in a propeller-like action
• Flagella structure


  • Filament made from multiple copies of the protein flagellin
  • Has a hook where the filament enters the cell surface, attached by a series of complex proteins called the flagellar motor
• Flagella arrangement


  • Single flagellum
  • Multiple flagella over entire surface (peritrichous)
• Pili/Fimbriae


  Hair-like projections used for attachment, structurally and chemically similar but with different functions
• Pili


  Used for attachment between bacterial cells during conjugation, a process of genetic exchange
• Fimbriae


  Used for attachment to host epithelium to cause infection, act as adhesins