Microbiology Chapter 4: Prokaryotic Cells and Microorganisms

General Characteristics of Cells and Life

• Universal Cell Characteristics: All living organisms, whether unicellular or multicellular, are comprised of cells that possess a basic shape (spherical, cubical, or cylindrical), an internal content called cytoplasm enclosed by a cell membrane, and genetic material (DNA chromosomes), ribosomes, and metabolic capabilities.

• Basic Cell Types:

  • Eukaryotic Cells: Found in animals, plants, fungi, and protists. These contain a double-membrane-bound nucleus containing DNA and membrane-bound organelles that compartmentalize the cytoplasm for specific biochemical functions.

  • Prokaryotic Cells: Found in bacteria and archaea. These lack a nucleus and other membrane-bound organelles.

• Fundamental Biological Processes:

  • Reproduction and Heredity: Involves generating offspring through sexual or asexual means, with a genome composed of DNA organized into chromosomes.

  • Growth and Development: In microbiology, growth refers to both the increase in population size through reproduction and the enlargement of an individual organism during maturation.

  • Metabolism: Thousands of chemical and physical reactions required for cell function, including nutrient processing and waste discharge.

  • Responsiveness: The ability to interact with external stimuli (light, chemicals, other cells) via signaling and self-propulsion (motility).

  • Support and Transport: Includes mechanisms for protection (cell walls), storage (vacuoles, granules), and the movement of materials into and out of the cell.

External Structures of Bacterial Cells

• Appendages for Motility:

  • Flagella: A specialized appendage consisting of three parts: the filament (helical structure made of the protein flagellin), the hook (curved sheath), and the basal body (rings anchored in the cell wall). It rotates $360^{\circ}$ to push the cell forward.

  • Flagellar Arrangements:

    • Monotrichous: Single flagellum at one end.

    • Lophotrichous: Small bunches emerging from a single site.

    • Amphitrichous: Flagella at both ends of the cell.

    • Peritrichous: Flagella dispersed over the entire cell surface.

  • Flagellar Responses: Movement is guided by external stimuli such as chemicals (chemotaxis) or light (phototaxis). Signals trigger either counterclockwise rotation (resulting in a smooth linear run) or clockwise rotation (resulting in a tumble).

  • Periplasmic Flagella (Axial Filaments): Internal flagella enclosed between the outer sheath and peptidoglycan layer, typically found in spirochetes; they produce motility through a twisting or flexing motion.

• Appendages for Attachment and Channeling:

  • Fimbriae: Fine, hairlike protein bristles used strictly for adhering to surfaces and other cells.

  • Pili: Flexible tubular structures made of the protein pilin, found primarily in gram-negative bacteria. They facilitate conjugation, which is the partial transfer of DNA between cells.

• Glycocalyx: A surface coating made of sugars and/or proteins.

  • Slime Layer: Loosely organized and attached, protecting against dehydration and nutrient loss.

  • Capsule: Highly organized and tightly attached; it inhibits phagocytosis by white blood cells, increasing pathogenicity, and aids in the formation of biofilms.

The Cell Envelope and Gram Stain Differentiation

• Structure: The cell envelope consists of the cell wall and the cell membrane, maintaining the integrity of the cell.

• Gram-Positive Bacteria:

  • Cell Wall: A thick layer of peptidoglycan ($20-80\,\text{nm}$ thick).

  • Components: Includes teichoic acid and lipoteichoic acid, which assist in cell wall maintenance, enlargement during division, and immune response stimulation.

  • Periplasmic Space: Narrow if present.

• Gram-Negative Bacteria:

  • Cell Wall: A thin layer of peptidoglycan ($8-11\,\text{nm}$ thick).

  • Outer Membrane: Provides an additional layer containing lipopolysaccharides (LPS) and porin proteins. The lipid portion of LPS acts as an endotoxin.

  • Periplasmic Space: Extensive, located between the inner and outer membranes.

• Peptidoglycan Structure: A macromolecule consisting of long glycan chains made of alternating $N\text{-acetyl glucosamine}$ (NAG) and $N\text{-acetyl muramic acid}$ (NAM). These chains are cross-linked by short peptide fragments (tetrapeptide chains) to provide rigid but flexible support.

• The Gram Stain Technique: A differential stain used for classification. Gram-positive cells retain crystal violet and appear purple; gram-negative cells lose crystal violet and take up safranin, appearing red.

• Atypical Cell Walls:

  • Acid-Fast Bacteria: Such as Mycobacterium and Nocardia, which have a gram-positive wall structure with lipid mycolic acid (cord factor), providing resistance to dyes.

  • Wall-less Bacteria: Mycoplasma species lack a cell wall entirely and are stabilized by sterols; they are highly pleomorphic.

Internal Bacterial Anatomy

• Cell Membrane: A phospholipid bilayer with embedded proteins (fluid mosaic model). It is selectively permeable and serves as a site for energy reactions and nutrient processing.

• Cytoplasm: A gelatinous solution (70%–80% water) acting as a solvent for nutrients, sugars, amino acids, and salts.

• Genetic Material:

  • Nucleoid/Chromosome: A single, circular, double-stranded DNA molecule containing the cell's essential genetic information.

  • Plasmids: Small, circular, non-essential DNA molecules that can be transferred between cells and are used in genetic engineering.

• Ribosomes: Sites of protein synthesis composed of 60% rRNA and 40% protein. Prokaryotic ribosomes are $70S$, consisting of a $50S$ large subunit and a $30S$ small subunit.

• Inclusions and Granules: Intracellular storage bodies used by the cell when environmental nutrients are depleted.

• Cytoskeleton: An internal network of protein polymers (such as actin filaments) that contribute to cell shape.

• Endospores: Inert, resting cells produced by genera like Bacillus and Clostridium. They undergo a two-phase life cycle: the vegetative cell (active) and the endospore (dormant).

  • Sporulation: The formation process triggered by adverse conditions. Endospores are resistant to heat, freezing, radiation, and chemicals.

  • Longevity: Can remain viable for up to $250\,\text{million}$ years.

  • Destruction: Requires pressurized steam at $120^{\circ}\text{C}$ for $20-30$ minutes.

Bacterial Morphology and Classification

• Common Shapes:

  • Coccus: Spherical (e.g., Staphylococcus aureus).

  • Bacillus: Rod-shaped (e.g., Legionella pneumophilia).

  • Coccobacillus: Short and plump rod.

  • Vibrio: Gently curved (e.g., Vibrio cholerae).

  • Spirillum: Helical, rigid twisted rod (e.g., Aquaspirillum).

  • Spirochete: Flexible, spring-like (e.g., Borrelia burgdorferi).

• Arrangements:

  • Cocci: Diplococci (pairs), Tetrads (fours), Streptococci (chains), Staphylococci (irregular clusters), Sarcina (cubical packets).

  • Bacilli: Diplobacilli (pairs), Streptobacilli (chains), Palisades.

• Classification Systems:

  • Bergey’s Manual of Systematic Bacteriology: Classification based on genetic (phylogenetic) information. It divides prokaryotes into Domain Archaea and Domain Bacteria.

  • Major Bacterial Phyla:

    1. Pseudomonadota: Varied gram-negative bacteria, including E. coli and Salmonella.

    2. Bacillota: Gram-positive with low G+C content (e.g., Bacillus, Clostridium, Staphylococcus).

    3. Actinomycetota: Gram-positive with high G+C content (e.g., Mycobacterium, Streptomyces).

  • Diagnostic Scheme: Uses phenotypic traits (cell wall, shape, physiology) to identify medical pathogens.

Archaea: The Extremophiles

• Distinct Domain: Archaea are more closely related to Eukarya than to Bacteria in their genetic sequences and protein synthesis mechanisms.

• Unique Features: They possess unique ribosomal RNA signatures, unique membrane lipids (long-chain branched hydrocarbons with ether linkages), and lack peptidoglycan in their cell walls.

• Habitats: Often live in extreme environments (extremophiles). Types include:

  • Methanogens: Methane producers.

  • Hyperthermophiles: Adapted to very high temperatures.

  • Extreme Halophiles: Adapted to high salt concentrations.

  • Sulfur Reducers: Inhabit volcanic vents.

Questions & Discussion

• What is the difference between a slime layer and a capsule? A slime layer is loosely attached and unorganized, whereas a capsule is a dense, highly organized protectant that aids in pathogenicity.

• How do prokaryotic ribosomes differ from eukaryotic ones? Prokaryotic ribosomes are $70S$ while eukaryotic ribosomes are typically $80S$.

• Is sporulation a form of reproduction? No, sporulation is a survival mechanism; one vegetative cell produces one endospore, which later germinates back into one vegetative cell.

• How does the G+C content affect classification? The percentage of guanine and cytosine in DNA is an indicator of genetic relatedness; significant differences suggest organisms are less likely to be closely related.

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