Characteristics of Prokaryotic and Eukaryotic Microorganisms

Overview

A comprehensive understanding of microbial cell structure and organization is essential in microbiology. This includes the size, shape, and arrangement of cells, as well as the composition and function of cellular components such as membranes, walls, and external appendages.

Size, Shape, and Arrangement of Bacterial Cells

Cell Size

• Bacteria typically range from 0.2–2.0 μm in diameter and 2–8 μm in length.

• Size varies with bacterial type, age, and environmental conditions.

Cell Shapes

• Coccus (plural: cocci): Spherical shape.

• Bacillus (plural: bacilli): Rod-shaped.

• Spiral: Includes vibrio (comma-shaped), spirillum (rigid spiral), and spirochete (flexible spiral).

Cell Arrangements

• Cocci: Can be found singly, in pairs (diplococci), chains (streptococci), clusters (staphylococci), or tetrads.

• Bacilli: Can be single, in pairs (diplobacilli), chains (streptobacilli), or palisades.

• Spirals: Usually single, but can form chains.

Prokaryotic Cell Structure

External Structures

• Flagella: Used for motility.

• Pili: Involved in genetic exchange and motility.

• Fimbriae: Aid in adherence to surfaces and biofilm formation.

• Glycocalyx: Capsule or slime layer for protection and adherence.

Internal Structures

• Nucleoid: Region containing the circular, double-stranded DNA genome.

• Ribosomes: Sites of protein synthesis (70S in prokaryotes).

• Inclusions: Reserve deposits of nutrients.

• Plasmids: Small, circular DNA molecules with supplemental genetic information.

Glycocalyx: Capsule and Slime Layer

Definition and Composition

• Glycocalyx: Thick outer covering of the plasma membrane, composed of glycoprotein and glycolipid.

• Capsule: Well-organized, not easily washed off; made of polysaccharide or polypeptide.

• Slime Layer: Diffuse, unorganized, easily removed; made of glycoproteins.

Functions

• Adherence to solid surfaces.

• Prevents dehydration.

• Linked to pathogenesis in some cases.

• Prevents attack by other organisms (e.g., phagocytosis).

• Traps nutrients and forms biofilms.

Examples of Capsules in Microorganisms

Pili, Fimbriae, and Flagella

Pili

• Made of pilin protein; involved in genetic exchange (conjugation).

• Motility: twitching and gliding.

Fimbriae

• Composed of proteins; used for adherence to surfaces (especially in Gram-negative bacteria).

• Facilitate biofilm formation.

Flagella

• Helical structures made of flagellin protein; used for movement.

• Antigenic; can induce antibody production.

• Structure includes basal body, hook, and filament.

Types of Flagella Arrangements

Flagellar Movement

• Clockwise rotation: tumbling motion.

• Counter-clockwise rotation: running (straight movement).

Cell Envelope

Definition

• Includes the plasma membrane, cell wall, and any additional layers (capsule or slime layer).

Cell Membrane/Plasma Membrane

• Boundary separating the cell from its environment.

• Functions:

  • Selectively permeable: controls movement of ions and molecules.

  • Site of metabolic processes: respiration, photosynthesis, lipid and cell wall synthesis.

  • Provides structural support and maintains cell shape.

  • Important in cell signaling and communication.

Structure

• Composed of phospholipids (glycerol, fatty acids, phosphate).

• Eubacteria: ester linkages; Archaea: ether linkages.

• Eukaryotic membranes contain sterols; bacterial membranes contain hopanoids (sterol-like molecules).

Archaeal Membranes

• Ether glycerol bonds, not ester bonds.

• Terpene-derived lipids; some have monolayer membranes (tetra-ether glycerol).

• Cyclopentane rings increase stability.

Proteins in Membranes

• Peripheral and integral proteins.

• Eubacteria have more membrane proteins than eukaryotes.

Transport Across the Membrane

Passive Transport

• Diffusion: Movement from higher to lower concentration (O2, N2, CO2, ethanol, H2O).

• Osmosis: Water movement due to solute concentration differences.

• Facilitated Diffusion: Uses channel or carrier proteins; no energy required.

Active Transport

• Requires cellular energy (usually ATP).

• Types: Uniporter, Symporter, Antiporter, ATP-binding cassette (ABC) transporters.

• Group translocation: Substance is chemically altered during transport (e.g., PEP phosphotransferase system).

Osmotic Effects

• Plasmolysis: Water leaves cell, causing shrinkage.

• Plasmoptysis: Water enters cell, causing swelling and lysis.

Bacterial Cell Walls: Gram Negative and Gram Positive

Peptidoglycan Structure

• Composed of two glucose derivatives: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

• Linked by β-1,4 glycosidic bonds.

• Tetrapeptide chains: L-alanine, D-glutamate, meso-diaminopimelic acid (Gram-negative), L-lysine (Gram-positive), D-alanine.

Functions of Cell Wall

• Determines cell shape.

• Provides structural support (prevents osmotic lysis).

• Serves as antigenic determinant.

• Target for antibiotics and disinfectants.

• Lysozyme (in saliva and tears) breaks down cell walls.

• Contributes to virulence.

Gram Negative Cell Envelope

• Complex structure: outer membrane, thin peptidoglycan layer (6–20% of cell wall).

• Outer membrane contains lipopolysaccharide (LPS), lipoproteins, and porins.

• Bayer's junction connects outer membrane to plasma membrane.

Lipopolysaccharide (LPS) Structure

• Lipid A: N-acetyl glucosamine disaccharide with toxic fatty acids.

• Core polysaccharide: Oligosaccharide (4–12 sugars).

• O-side chain: Repeating units (3–5 sugars).

• LPS (especially Lipid A) is known as endotoxin; can cause organ failure in humans.

Summary Table: Gram-Positive vs Gram-Negative Cell Walls

Additional info:

• Mycoplasma are bacteria that lack peptidoglycan, conferring resistance to antibiotics targeting cell wall synthesis.

• Hopanoids and sterols increase membrane stability in microbial cells.

• Flagellar movement is essential for chemotaxis, phototaxis, and other microbial taxes.