3. Microbial Cell Structure and Functions

• Microbial Morphology


  The study of the size, shape, and arrangement of a living organism

  
• Cellular morphology


  The study of the size and shape of the cell
• Typical bacteria


  • Less than 3 μm in size
• Cocci


  • 0.5 to 3 μm in diameter
• Bacilli


  • 0.15 to 2 μm in width and 0.5 to 20 μm in length
• Bacterial Shapes


  • Coccus - spherical and ovoid
  • Bacillus - cylindrical
  • Spiral - curve or loose spiral

  
• Prokaryotic Cell

  Exhibited by organisms falling under Domain Bacteria and Domain Archaea
• Cell Membrane


  • Gatekeeper for the entrance and exit of dissolved substances
  • Selective permeability
  • Anchorage
  • Energy conservation and consumption
• Bacterial Membrane


  • Phospholipid bilayer containing embedded proteins
  • Composed of hydrophobic tails (fatty acids) and hydrophilic head (glycerol molecule containing phosphate)
  • Strengthened by hopanoids and sterol-like molecules
  • Archaeal membrane

  
• Cell Wall


  • Gives shape and rigidity on the cell
  • Confers structural strength on the cell to keep it from bursting due to osmotic pressure
  • Contributes to pathogenicity
  • Site of action of several antibiotics
• Lysozyme

  • Weakens the peptidoglycan and cause cell lysis
  • Acts as a major line of defense against bacterial infection which are present in human secretions including tears, saliva, and other bodily fluids
  • Destroys pre-existing peptidoglycan, penicillin blocks a key step in its biosynthesis
• Peptidoglycan


  • Made up of rigid polysaccharide
  • Not present in Archaea and Eukarya
  • Composed of alternating repeats of N-acetylglucosamine and N-acetylmuramic acid
  • 90% of cell wall in a Gram positive bacteria consist of peptidoglycan and form many layers

  
• Teichoic acid


  Embedded in the cell wall and function to bind divalent metal ions, such as Ca2+ and Mg2+, prior to their transport into the cell
• Lipoteichoic acid


  Covalently bonded to membrane lipids rather than to peptidoglycan
• LPS or lipopolysaccharide

  • Small amount of peptidoglycan in cell wall of Gram negative
  • Has two components: core polysaccharide and O-specific polysaccharide
  • Outer membrane is found in second lipid bilayer

  
• Periplasm


  • The space located between the outer surface of the cytoplasmic membrane and the inner surface of the outer membrane
  • Spans about 15 nm
  • Porins - Channels for the entrance and exit of solutes (Nonspecific porins and Specific porins)
• Cell Wall Differences

  • Gram-positive bacteria
  • Gram-negative bacteria

  
• Archaeal Cell Wall

  • Methanogens have pseudomurein and other polysaccharide cell walls
  • Pseudomurein = peptidoglycan with N-acetylglucosamine and N-acetyltalosaminuronic acid
  • S-Layer - Paracrystalline surface layer, most common type of cell wall in Archaea

  
• Cell Surface Structures


  • Glycocalyx
  • Capsule
  • Slime
  • Fimbriae
  • Pili
  • Hami

  
• Glycocalyx


  Organized in a tight matrix that excludes small particles and is tightly attached
• Slime or slime layer

  • More easily deformed and loosely attached
  • It will not exclude particles
  • More difficult to see microscopically
• Fimbriae


  • Thin filamentous structures made of protein that extend from the surface of a cell
  • 2-10 nm in diameter
  • Enable cells to stick to surfaces
  • Form pellicles or biofilms on solid surfaces

  
• Pili


  • Typically longer and only one or a few pili are present on the surface of a cell
  • All Gram negative produce pili while many Gram positive contain pili
  • Can be receptors for certain types of viruses, they can be easily seen under the electron microscope when they become coated with virus particles
• Functions of the Pili


  • Adhesion
  • Conjugation
• Hamus or Hami


  • Unique attachment structure resembles a tiny grappling hook
  • Affix cells to a surface to form a networked biofilm
  • Prevent cells from being washed away in groundwater flowage
• Gas Vesicles


  • Conical-shaped structures made of protein
  • Length: 300-1000 nm, Width: 45-120 nm
  • Appears as irregular bright inclusions seen in light microscopy or transmission electron microscope
  • Primitive form of vacuoles

  
• Gas Vesicle Proteins


  • GvpA (major protein) - Forms the watertight vesicle shell and is a small, hydrophobic, and very rigid protein
  • GvpC (minor protein) - Strengthens the shell of the gas vesicle by cross-linking
• Endospores

  • Highly differentiated cells that are extremely resistant to heat, harsh chemicals, and radiation
  • Serve as survival structures and enable the organism to endure unfavorable growth conditions
  • Visible by light microscopy as strongly refractile structures and through electron microscopy
  • Visualized through special staining techniques such as Schaeffer-Fulton spore staining

  
• Parts of Endospore


  • Exosporium - outermost layer providing outer covering
  • Spore coats - innermost layer where spore-specific protein is located
  • Cortex - consists of loosely cross-linked peptidoglycan
  • Core - contains the core wall, cytoplasmic membrane, cytoplasm, nucleoid, ribosomes, and other cellular essentials
  • Dipicolinic acid - found in endospores but not in vegetative cells

  
• Endospore Formation

  • Dormant stage of a bacterial life cycle
  • Vegetative cell > endospore > vegetative cell
  • Bacillus spp. and Clostridium spp.

  
• Steps in Vegetative Cell Process

  1. Activation - Occurs when endospores are heated for several minutes at an elevated but sub-lethal temperature

  2. Germination - Rapid process (occurring in a matter of minutes), is signaled by the loss of refractility of the endospore and loss of resistance to heat and

  
• Schaeffer-Fulton spore staining


  Staining technique used to visualize endospores

  
• Malachite green


  Stain used in Schaeffer-Fulton spore staining, infused into the spore with steam

  
• Endospore


  Dormant stage of a bacterial life cycle
• Parts of endospore


  • Exosporium - outermost layer providing outer covering
  • Spore coats - innermost layer where spore-specific protein is located
  • Cortex - consists of loosely cross-linked peptidoglycan
  • Core - contains the core wall, cytoplasmic membrane, cytoplasm, nucleoid, ribosomes, and other cellular essentials

  
• Dipicolinic acid


  Found in endospores but not in vegetative cells
• Endospore formation


  Vegetative cell > endospore > vegetative cell
• Bacteria that form endospores

  • Bacillus spp.
  • Clostridium spp.

  
• Vegetative cell process

  1. Activation

  2. Germination

  3. Outgrowth
• Activation


  Occurs when endospores are heated for several minutes at an elevated but sub-lethal temperature