Bacteriology

• Bacterial Structure


  • Bacterial cells reveal various component structures
  • Some are external to the cell wall, others are internal to the cell wall
• Flagella


  Hairlike, helical appendages that protrude through the cell wall and are responsible for swimming motility

  
• Types of flagella arrangements

  • Monotrichous (polar)
  • Aphitrichous
  • Lophotrichous
  • Petritrichous

  
• Flagella


  • Composed of three parts: basal body, short hook, helical filament
  • Some gram negative bacteria have a sheath surrounding the flagellum
  • Basal body composition is unknown, hook and filament are composed of protein subunits (monomers) arranged in a helical fashion
  • Protein of the filament is known as flagellin

  
• Motility


  Bacterial and Archaea have directed movement
• Flagella movement


  1. Bacterial flagella rotates to move or push the cell

  2. Flagella motor is driven by the proton motive force

  3. Bacteria having polar flagella swim in a back and forth fashion by reversing the direction of flagella rotation

  4. Bacteria having lateral flagella swim in a more complicated manner

  
• Spirochete motility


  • Lack external flagella
  • Multiple flagella form axial fibril which winds around the cell
  • Flagella remain in periplasmic space inside outer sheath
  • Corkscrew shape exhibits flexing and spinning movements

  
• Twitching motility


  A form of surface-associated bacterial movement involving Type IV pili
• Gliding motility


  The ability of certain rod-shaped bacteria to translocate on surfaces without the aid of external appendages such as flagella, cilia, or pili
• Chemotaxis


  Movement towards or away from chemical attractants or repellants
• Chemotaxis


  • Movement is in response to a temporal gradient sensed by chemoreceptors on the cytoplasmic membrane
  • Not limited to chemical gradients, can also be in response to light, temperature, oxygen, osmotic pressure, and gravity
• Pili/Fimbriae


  Short, thin appendages on the surface of bacterial cells that help cells adhere to surfaces and play a role in pathogenicity
• Pili


  • Structurally similar to fimbriae but longer, thicker, and less numerous
  • Sex pili involved in the process of conjugation
• Capsules and Slime Layer (Glycocalyx)

  Viscous substance (gelatinous polysaccharide) surrounding a bacterial cell forming a covering layer or envelope
• Capsules and Slime Layer

  • Capsular material is visible in light microscope, not highly water soluble, and does not readily diffuse away
  • Slime material is highly water soluble and dissolves in the medium
  • May aid in motility
• Functions of Capsules


  • Provide protection against drying, block attachment of bacteriophages, exclude detergents, protect from phagocytosis, aid in attachment to surfaces, provide nutrients
• 1. Layer


  Regularly structured layers of protein or glycoprotein that self-assemble on the cell surface
• Functions of S-Layer


  • Protect from ion and pH fluctuations, osmotic stress, enzymes, and predation
  • Maintain shape and rigidity
  • Promote adhesion to surfaces
  • Protect from host defenses
• Cell Wall


  Rigid structure beneath external structures that gives shape to the cell and prevents bursting
• Types of cell walls based on Gram stain

  • Gram-positive: thick peptidoglycan
  • Gram-negative: thin peptidoglycan and outer membrane
• Peptidoglycan


  • Rigid layer composed of NAM, NAG, and amino acids that provides strength and rigidity to the cell wall
  • Gram-positive have more peptidoglycan than Gram-negative
  • Gram-negative have an outer membrane in addition to peptidoglycan
• Cells whose walls have been completely removed (protoplasts) are incapable of normal growth and division
• Portions of the peptidoglycan must continually be degraded by wall associated hydrolytic enzymes so that new polymers can be added
• Gram-positive cell wall


  • Contains a much greater amount of peptidoglycan (90% of cell wall)
  • Also contains teichoic acid which protects from thermal injury and imparts a negative charge
• Gram-negative cell wall


  • More complex, contains other substances in addition to peptidoglycan
  • Has an outer membrane rich in lipids that serves as an impermeable barrier
• Teichoic acid


  Usually present in small amounts and protects bacteria from thermal (heat) injury and imparts a negative charge
• Gram positive cell wall structure

  • More complex than Gram negative cell wall structure
  • Contains other substances in addition to peptidoglycan e.g. polysaccharides covalently linked to peptidoglycan
• Gram negative cell wall structure

  • Outer membrane rich in lipids
  • Outer membrane serves as an impermeable barrier to prevent the escape of important enzymes and to protect from external chemicals and enzymes
  • Outer membrane is anchored in the peptidoglycan by means of lipoprotein
  • Outer membrane is a bilayer consisting of mainly phospholipids, proteins and lipopolysaccharides (LPS)
  • No teichoic acid
• Lipopolysaccharides (LPS)


  • Have toxic properties and are also known as endotoxin
  • Impart a negative charge
  • Help stabilize outer membrane structure
  • May contribute to attachment to surfaces and biofilm formation
  • Create a permeability barrier
  • Provide protection from host defenses (O antigen)
• Atypical cell walls


  • Mycoplasma lacks cell wall
  • Archaea have pseudomurein, they lack peptidoglycan
  • L-forms are mutant bacteria with defective cell walls
• Cytoplasmic membrane


  • Absolute requirement for all living organisms
  • Some bacteria also have internal membrane systems
  • Encloses the cytoplasm
  • Is selectively permeable
  • Is a phospholipid bilayer with peripheral and integral proteins
• Peripheral proteins


  Loosely connected to membrane and easily removed
• Integral proteins


  Embedded within membrane and carry out important functions
• Cytoplasmic membrane functions


  • Interacts with external environment
  • Contains receptors for detection of and response to chemicals in surroundings
  • Contains transport systems
  • Carries enzymes for metabolic reactions, such as nutrient breakdown, energy production and photosynthesis
• Protoplasts


  Bacterial cells consisting of the cytoplasmic membrane and lacking cell wall
• Spheroplasts


  Consist of the cytoplasmic membrane plus outer membrane, lacking cell wall
• Mesosomes


  Irregular unfolding of the plasma membrane
• Passive processes for movement of materials across membranes

  1. Materials move from areas of higher to lower concentration and no energy is expended by the cell

  2. H2O, O2 , and CO2 often move across membranes this way

  3. In simple diffusion, molecules and ions move until equilibrium is reached
• Facilitated diffusion


  1. Substances are transported by transporter proteins across the membranes from areas of higher to lower concentration across a selectively permeable membrane until equilibrium is reached

  2. Uses membrane bound carrier molecules (permeases)

  3. Smaller concentration gradient is required for significant uptake of molecules

  4. Effectively transports glycerol, sugars, and amino acids

  5. More prominent in eukaryotic cells than in bacteria or archaea
• Osmosis


  Solvent (water) molecules move across the membranes from areas of higher to lower concentration across a selectively permeable membrane until equilibrium is reached