Chapter 4/5 - Bacteria Parts and Pieces Summarized

Chapter 4/5: Cell Structure and Function

Overview of Topics

  • Cell Structure and Function

    • External Structures

    • Cell Envelope

    • Internal Structures

    • Cell Shapes, Arrangement, and Sizes

    • Classification

Section: Infectious Exam Case Study

  • Patient with Tuberculosis

    • Diagnosis using Ziehl Neelsen stain

    • Mycobacterium tuberculosis

    • Function is closely tied to its structure

    • Features:

      • Thick waxy outer covering makes it difficult to stain

      • Grows slowly and resides within macrophages

      • Drug sensitivity is a factor to consider

Section: Prokaryotic Cells

  • Distinct Features from Eukaryotic Cells

    • Lack of membrane-bound organelles

    • Unbound DNA instead of a membrane-bound nucleus

    • Composition of cell wall made of peptidoglycan

    • Size comparatively smaller than eukaryotic cells

    • Impacts nutrient entrance rate and surface to volume ratio

Section: External Structures/Appendages of Prokaryotic Cells

  • Appendages include:

    • Flagella

    • Pili

    • Fimbriae

    • Glycocalyx

    • Comprised of capsule and slime layer

    • Cell Envelope consisting of:

    • Cell wall

    • Cell membrane

Section: Internal Structures of Prokaryotic Cells

  • Internal Composition:

    • Cytoplasm

    • Main component is water and proteins, site for metabolism

    • Ribosomes

    • Serve as protein factories, differing from eukaryotic ribosomes

    • Inclusions

    • Include storage granules and vacuoles

    • Nucleoid/Chromosome

    • Actin cytoskeleton

    • Endospore

    • Allows survival in harsh environments

Section: The Cell Envelope

  • Function: Serves as a cell boundary and gateway.

  • Layers: May consist of up to three layers depending on cell type:

    1. Outer membrane

    2. Cell wall

    3. Cytoplasmic membrane

Section: Cell Wall Composition

  • Gram Positive Cell Wall Features:

    • Thick peptidoglycan (PG) layer

    • Contains acidic polysaccharides, teichoic acid, and lipoteichoic acid

  • Gram-negative Cell Wall Features:

    • Thin PG layer

    • Outer membrane with lipopolysaccharide (LPS)

    • Contains porins for transport

Section: Differences Between Gram-positive and Gram-negative Cells

  • Gram-negative

    • Contains all three layers

    • Outer membrane is a bilayer (lipopolysaccharide layer outside and a phospholipid layer inside)

    • The outer membrane is attached to the cell wall via lipoproteins

    • Serves protective functions but also acts as a toxin to mammals

  • Gram-positive

    • High integrity due to the presence of teichoic acids in the thick cell wall

    • Lacks LPS layer and periplasmic space

  • Mycoplasma:

    • Categorized as lacking a cell wall, which creates varied shapes

Section: Functions of Cell Wall

  • Characteristics:
    a. Composed of peptidoglycan (murein)
    b. Target for certain antibiotics (e.g., Penicillin, which inhibits cell wall formation)
    c. Rigid and shapes the cell
    d. Porous for molecule passage while maintaining strength
    e. Prevents bursting under turgor pressure
    f. Influences differential staining reactions in laboratory settings

Section: Peptidoglycan Structure

  • Composition:

    • Glycan portion consists of alternating sugars:

    • N-acetylglucosamine (NAG)

    • N-acetylmuramic acid (NAM)

    • They form long chains across the cell wall

    • Linked via peptide portions (proteins), connected specifically to NAM

Section: Cell Membrane Structure

  • Phospholipid Bilayer Composition:

    • Comprised of a glycerol, two fatty acids (hydrophobic), and a phosphate group (hydrophilic)

    • Contains integral proteins that serve several roles (receptor sites, transport proteins)

    • Note: Cholesterol is absent in prokaryotic membranes

    • Site for chemiosmosis for ATP production

Section: Bacterial Cytoplasmic Membranes

  • Functions:

    • Passive Processes:

    • Includes diffusion, facilitated diffusion, and osmosis

  • Active Processes:

    • Active transport and Group translocation, involving chemical modification of substances during transport

Section: Appendages - Pili and Fimbriae

  • Fimbriae

    • Hair-like appendages that provide attachment capabilities

    • Used in anchoring to surfaces like teeth and mucosal membranes

  • Pili

    • Specialized type of fimbria, longer but fewer (1-2 per cell)

    • Facilitate DNA transfer between bacterial cells (conjugation)

Section: Flagella

  • Functionality:

    • Differ from eukaryotic flagella; made of flagellin protein

    • Movement mechanism is propeller-like rather than wave-like

    • Distinct structural arrangements (

      • E.g.,

        • Monotrichous (one flagellum)

        • Lophotrichous (clustered flagella)

        • Amphitrichous (flagella at both ends)

        • Peritrichous (flagella surrounding the cell)

Section: Glycocalyx

  • Types:

    • Capsule: Thick, rigid, protects from immune detection

    • Slime Layer: Thinner, less rigid; aids in attachment

  • Functions:

    • Protect bacteria from desiccation, phagocytosis, and assists in biofilm formation

Section: Endospores

  • Survival Structures:

    • Formed under adverse conditions

    • Contains critical cellular components, not reproduction

    • Only produced by certain bacteria (e.g. Bacillus and Clostridium)

    • Autoclaving is necessary to destroy these spores at 121°C, 15-20 psi, for 15-20 minutes

    • Descriptive stages of endospore formation from a vegetative cell are provided, showcasing the development process from DNA replication to maturation

Section: Bacterial Cell Shapes and Arrangements

  • Cell Shapes:

    • Coccus: spherical

    • Rod (Bacillus): cylindrical

    • Curved or Spiral: includes spirilla and spirochetes

  • Cell Arrangements:

    • Single: isolated cells

    • Diplo: in pairs (e.g., Diplococcus, Diplobacillus)

    • Strepto: in chains (e.g., Streptococcus, Streptobacillus)

    • Staphylo: clustered configurations (e.g., Staphylococcus)

Section: Classification of Bacteria

  • Classification Methods:

    • Phenotypic Methods: Based on morphology and biochemical tests

    • Molecular Methods: Use DNA and protein sequencing (e.g., 16S RNA)

  • Implications: Classification aids in understanding medical importance and bacterial taxonomy

Note:

  • Furthermore, the classification methods have led to the identification of major taxonomic groups of bacteria and medically significant families/genera, aiding clinical decision-making in microbiology.