Prokaryotic Cell

Prokaryotic Cell Structure

  • Colony Morphology: Characteristics such as texture, margin, elevation, and shape which help identify bacterial colonies.

    • Shape: Circular, irregular, filamentous, rhizoid.

    • Margin: Entire, undulate, lobate, curied.

    • Elevation: Flat, raised, convex, pulvinate, umbonate.

    • Size: Measured in millimeters (mm), described as pinpoint, small, medium, large.

    • Texture: Dry, moist, mucoid, brittle, viscid, viscous, butyrous.

    • Opacity: Transparent, opaque, translucent, iridescent.

    • Odour: Specific smells aiding in identification, e.g., earthy odour from actinomycetes, fruity smell from some fungi, faecal odour from E. coli.

    • Colour: Pigmented colonies seen in culturing.

Prokaryotic Genetic Material

  • Structure: Prokaryotic cells typically contain a single circular DNA molecule known as the prokaryotic chromosome.

  • Properties:

    • Circular or linear.

    • Contains extrachromosomal DNA (plasmids).

    • Single copy per gene, generally less repetitive DNA.

Ribosomes

  • Tiny particles composed of protein and RNA.

  • Functions as the site for protein synthesis, with numbers varying by growth stage (up to 20,000 in rapidly growing E. coli).

  • Prokaryotic ribosomes are classified as 70S, smaller than eukaryotic (80S) ribosomes.

Structures Surrounding the Bacterial Cell

  • Capsule: Protects against phagocytosis, enhances adherence.

  • Slime Layer: Less structured than capsules, provides similar functions.

  • Pili: Hairlike projections for attachment to surfaces, some for genetic transfer (sex pili).

  • Flagella: Motility structures made of flagellin, aiding in movement.

  • Mesosome: Extensions of the cell membrane involved in various cellular functions.

Cell Wall and Peptidoglycan Structure

  • Function: Provides structural support and shape, protects against osmotic lysis, and can contribute to pathogenicity.

  • Peptidoglycan Composition: Made of repeating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) units, linked by peptide chains.

  • Gram-Positive vs. Gram-Negative:

    • Gram-positive: Thick peptidoglycan layer (90%), teichoic acids present.

    • Gram-negative: Thin peptidoglycan layer (5-10%), outer membrane with lipopolysaccharide (LPS) and periplasmic space.

Staining Techniques

  • Types:

    • Negative Staining: To observe shape.

    • Simple Stain: Uses a single dye for overall visualization.

    • Differential Stains: E.g., Gram and acid-fast staining, allow distinguishing between types of bacteria based on cell wall composition.

    • Special Stains: For specific structures such as flags, capsules, and spores.

    • Nuclear Stain: Acridine orange helps visualize DNA.

Procedures for Specific Staining Techniques

  • Gram Staining Protocol:

    1. Primary stain (crystal violet) applied.

    2. Mordant (iodine).

    3. Decolorization with alcohol.

    4. Counterstain (safranin).

  • Acid-Fast Staining: Identify mycobacteria, involves carbol fuchsin and acid alcohol.

  • Capsule Staining: Uses nigrosin and safranin to visualize capsules as clear halos against a dark background.

  • Spore Staining Protocol: Malachite green used for spores, water for decolorizing, and safranin for vegetative cells.

Summary of Key Structures and Functions in Prokaryotic Cells

  • Common Structures: Plasma membrane, cytoplasm, ribosomes, nucleoid, cell wall, capsule, and others such as flagella and pili for attachment.

  • Functions: Protect against external factors, aid in nutrient transport, enable motility, and facilitate reproduction and genetic diversity through plasmids and conjugation.