Prokaryotic Cells: Shapes, Arrangements, and Flagella

Prokaryotic Cells Overview

• Prokaryotic cells lack a nucleus.
• Bacteria and Archaea are examples of prokaryotic cells.
• Prokaryotic cells often work in groups rather than behaving as single units.

Basic Shapes of Bacteria

• Bacillus: Rod-shaped bacteria.
• Coccus: Spherical bacteria.
• Spiral: Bacteria with a spiral shape, including Spirillum (associated with Lyme disease), Vibrio (associated with diarrheal diseases), and Spirochete (associated with syphilis and Lyme disease).
• Approximately 80% of bacteria have one of these basic shapes.

Arrangements of Bacteria

• Pairs: Diplococci or diplobacilli.
• Clusters: Staphylococci.
• Chains: Streptococci or streptobacilli.
• Staph is skin bacteria that looks like clusters.
• Strep looks like chains of bacteria.

Flagella

• Flagella are protein structures that facilitate bacterial movement.
• Flagella are located outside the cell wall and made of chains of flagellin.
• Chemotaxis: Movement toward nutrition (positive chemotaxis) or away from toxins (negative chemotaxis).
• Phototaxis: Movement toward light (positive phototaxis) or away from light (negative phototaxis).

Arrangements of Bacterial Flagella

• Peritrichous: Flagella/cilia spread evenly over the organism.
• Monotrichous and polar: Single flagellum located at one end ("polar") of the organism for movement in one direction.
• Lophotrichous and polar: Multiple flagella are located at the same spot on the bacteria's surface, moving in one direction; they are "polar" flagella at the polar end.
• Amphitrichous and polar: Flagella at both ends, going through the bacteria from outside to inside.

Axial Filaments (Endoflagella)

• Axial filaments, also known as endoflagella, are inside the bacterial cell.
• Spirochetes have a twisted appearance because of them.
• Anchored at one end, allowing for rotation and cell movement.

Fimbriae

• Fimbriae specifically bind to receptors on a host cell's surface.
• They are composed of the protein "pilin," with a few to hundreds per cell.
• Fimbriae enable bacteria to cause disease.
• Attachment:
  • Sticks to the surface, creating a hairlike structure.
  • Can have several hundred sex pili.
• Conjugation: One or two sex pili that go from inside to outside, facilitating DNA transfer between cells.

Pili

• Pili structures are similar to fimbriae.
• There is only 1-2 pili per cell, but are longer than fimbriae.
• Pili facilitate DNA transfer between cells.
• Pili maintain gut health.
• Pili can cause drug resistance.

Glycocalyx

• Glycocalyx helps bacteria escape phagocytosis.
• It is located outside the cell wall providing a sticky texture.
• Capsule: neatly organized for preventing phagocytosis.
• Slime layer & biofilm: unorganized and loose, highly resistant to scraping.
• Extracellular polysaccharide allows cells to attach.

Cell Walls and Membranes

• Prevents osmotic lysis and maintains cell shape.
• Cell walls are made of peptidoglycan (linked by polypeptides), which is a polymer of disaccharides (N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)).

Gram-Positive Bacteria

• Thick cell wall (90% peptidoglycan).
• Up to 40 layers thick in some bacteria.
• NAG-NAM chains (sugar chains) are joined by peptide cross bridges (covalent bonds), forming a polymer.
• Includes teichoic acids and lipoteichoic acids which extend from the cell membrane through the cell wall.
• Wall teichoic acids link to the peptidoglycan layers only.

Gram-Negative Bacteria

• Thin cell wall (up to 20% peptidoglycan).
• NAG/NAM chains are joined by peptide bonds directly between the tetrapeptide side chains (infrequent) -> polypeptides.
• They do not have teichoic acids.
• Outer membrane surrounds it.
• Located between cell wall and plasma membrane.
• The Lipid polysaccharide “LPS” (lipid A + polysaccharide).
• Difficult to treat.

Outer membrane of Gram-Negative Bacteria

• Lipid A (endotoxin, immunogenic) confuses the immune system, which could increase blood pressure & vasodilation and is toxic when found in blood or the GI tract.
• PS (O polysaccharide) provides antigenic properties.
• Hard to treat because they reject drugs
• Strong negative charge helps evade phagocytosis.
• Provides a barrier for hydrophobic molecules and those above ~1000 MW (including antibiotics and detergents).
• Helps stabilize the inner cell membrane to withstand osmotic stress.

Atypical Cell Walls

• Acid-fast cell walls: Similar to gram-positive, waxy lipid (mycolic acid) bound to peptidoglycan (i.e. Mycobacterium which causes TB).
• Mycoplasmas do not have cell walls.
• Archaea: Walls not made of peptidoglycans, made of pseudomurein (lacks NAM & D-amino acids).

The Plasma Membrane

• Composed of a phospholipid bilayer, peripheral proteins, integral proteins, transmembrane proteins.
• Functions for selective permeability through:
  • Passive transport (high to low without assistance)
  • Facilitated diffusion (solute combines with transport protein in membrane)
  • Osmosis
  • Active transport uses ATP (e.g., Na+ and K+ pump)

Osmosis

• Isotonic solution: No net movement of H2O.
• Hypotonic solution: H2O moves into the cell, causing it to burst.
• Hypertonic solution: H2O moves out of the cell, causing plasmolysis.

Inside the Membrane

• Cytoplasm
• Nucleoid: bacterial chromosome.
• Usually a single chromosome of no specific shape.
• Chromosome attached to the cell membrane and occupies 20% of the cell volume.
• Ribosome (protein factory):
  • Actively growing = a lot of ribosomes.
  • Thousands per cell giving cytoplasm granular appearance.
  • Humans don't have same size ribosome.
  • Subunits made up of rRNA including: a small subunit = 30s, a large subunit = 50s and the complete ribosome = 70s (50 on top of 30).

Plasmid DNA

• Plasmid DNA is antibiotic resistant and can survive in high temperature
• It is a specialized form of genetic material in bacteria that is separate from the chromosome.
• It replicates independently.
• Plasmids are importnat because they can be antibiotic resistant genes

Spores

• Spores are made by certain Gram-positive bacteria in response to environmental stress.
• Endospores:
  • Are resting cells that are resistant to desiccation, heat, and chemicals.
  • Bacillus and Clostridium are examples.
  • Sporulation: endospore formation.
  • Germination: return to vegetative state.

Explore prokaryotic cells, their basic shapes (bacillus, coccus, spiral), arrangements (pairs, clusters, chains), and the function of flagella in bacterial movement. Learn about chemotaxis and the structure of flagella, which are made of flagellin.