Lesson 2: Cell Surface Structures

Cell Surface Structures

• Capsules and Slime Layers

  • Composed of polysaccharides; facilitate attachment, protection, and environmental alteration.
  • Biofilms form on solid surfaces from thick layers of slime.
  • Capsules are organized, visible via light and electron microscopy, and stain with India ink.
  • Slime layers are loosely attached, deformed easily, detectable in colonies, and enriched in a common matrix.

• Fimbriae, Pili, and Hami

  • Pili are thin protein filaments, aiding in surface attachment, biofilm formation, and liquid pellicles; all gram-negative bacteria produce them.
  • Types of Pili:
    • Conjugative pili enable genetic exchange.
    • Electrically conductive pili assist in metabolic processes by conducting electrons.
    • Type IV pili facilitate adhesion and motility, important for pathogens like cholera and gonorrhea.
    • F pilus (sex pilus) serves in genetic material transfer during mating.
  • Fimbriae are short pili primarily for attachment.
  • Hami resemble grappling hooks, aiding Archaea in forming networked biofilms.

Cell Inclusions

• Often visible under light microscopy, serve as storage and structural components.

• Carbon Storage Polymers

  • Poly-β-hydroxybutyric acid (PHB) is the main prokaryotic inclusion, synthesized from excess carbon.
  • Glycogen, a glucose polymer, acts as a carbon and energy reservoir.

• Polyphosphate, Sulfur, and Carbonate Minerals

  • Polyphosphate may store inorganic phosphate for nucleic acid and phospholipid biosynthesis.
  • Sulfur bacteria oxidize sulfur compounds, crucial for energy metabolism.
  • Carbonate materials are formed by cyanobacteria, with Gloeomargarita identified for producing specific granules.

• Gas Vesicles

  • Confer buoyancy, allowing microbes to float and position optimally in water.
  • Composed of conically shaped vesicles that are impermeable to water, allowing gas permeability.
  • Bloom is a mass accumulation of gas-vesiculate microbes, typically observed in sunlight-rich environments.

• Magnetosomes

  • Contain magnetic iron oxides, aiding bacteria in orientation along magnetic fields (magnetotaxis).
  • Exhibit various shapes, primarily spiked or rectangular.

Endospores

• Unique spores produced by gram-positive Bacillales and Clostridiales, enabling survival in harsh conditions.
• Cycle involves: vegetative cell → endospore → vegetative cell; triggered by nutrient limitation for sporulation.
• Structure includes multiple protective layers and SASPs for DNA protection against UV and desiccation.
• Germination occurs upon nutrient availability and involves activation, germination, and outgrowth phases.

Cell Locomotion

• Cells exhibit two primary movement types: swimming and gliding, with taxis directing movement towards or away from stimuli.

• Flagella provide swimming motility, varying in structure and movement based on their location and number on the cell.

  • Speed reaches up to 60 cell-lengths/sec, rotating in a clockwise (CW) or counterclockwise (CCW) manner for directional change.
  • Types include polar (located at one/both ends) and peritrichous (surrounding the entire cell).

• Flagellum Structure

  • Comprises filament, hook, and motor components that rotate to propel the cell.

• Archaella

  • Similar to flagella but found in archaea, rotate using ATP hydrolysis, and possess different structural proteins.

Cilia

• Short hair-like structures facilitate motility and food acquisition, powered by ATP.
• Display four types of movement: pendulus, unciform, infundibuliform, and undulant.

Cytoskeletal Components

• Intermediate Filaments support nuclear and plasma membranes.
• Microtubules are made from tubulin proteins, forming hollow structures essential for cell shape and transport.

Microbial Locomotion

• Internal movement, via cytoplasmic streaming, is guided by the cytoskeleton.

• External motility features pseudopodia, cilia, and flagella for movement.

• Surface Motility

  • Twitching involves type IV pili, moving cells forward through attachment and retraction, occurring in groups.
  • Gliding offers smooth, continuous movement along the cell's length.

Chemotaxis

• Directed movement in response to chemical stimuli, observed in bacterial swimming.

• Two types of rotation: runs (smooth swimming) and tumbles (random jiggling), enabling biased random walks towards attractants.

• Additional Types of Taxis

  • Phototaxis (light response), osmotaxis (ionic strength), hydrotaxis (water availability), aerotaxis (response to O2), and magnetotaxis (interaction with magnetic fields) aid microbial navigation in their environments.