HO - 04 Prokaryotes

Topic 4: Prokaryotic Cells

Introduction to Bacteria

  • Hans Gram: Developed the Gram stain technique, which distinguishes between two kinds of bacteria.

    • Gram-positive:

      • Stain purple due to a thick cell wall that retains crystal violet.

      • No outer membrane present.

    • Gram-negative:

      • Stain pink due to an outer membrane that prevents retention of the violet stain; minimal cell wall.

      • Loses violet stain and reveals a counterstain.

Cell Wall Structure

Gram-positive Bacteria

  • Cell Wall Composition:

    • Thick peptidoglycan layer (approx. 80 nm thick) with multiple layers.

    • No outer membrane.

  • Structure:

    • Cell Wall and Plasma Membrane are closely positioned - this layer is ~4 nm thick.

Gram-negative Bacteria

  • Cell Wall Composition:

    • Thin peptidoglycan layer (typically a single layer, approx. 20 nm thick) and outer phospholipid membrane.

    • Contains an outer membrane made of lipopolysaccharides (LPS).

  • Periplasmic Space: The space between the inner and outer membranes.

Peptidoglycan Layer

  • Composition:

    • Alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

    • Cross-linked via tetrapeptide sidechains and peptide cross-bridges.

  • Structure: Linear chains formed by β (1→4) links between NAG and NAM.

Antibiotics and Cell Wall Synthesis

Penicillin

  • Mechanism:

    • Inhibits bacterial cell wall formation by binding to enzymes that facilitate cross-linking in the peptidoglycan layer.

    • Result: Bacteria undergo lysis due to compromised wall synthesis.

Lysozyme

  • Discovery: Discovered by Alexander Fleming, breaks down bonds in the peptidoglycan layer, compromising the structural integrity of the wall.

  • Location: Found in human secretions like tears, saliva, mucus, and milk.

  • Function: Breaks β(1→4) links between NAG and NAM, making bacteria vulnerable to lysis.

Structures of Escherichia coli

Features of Gram-negative E. coli

  • Outer Structure:

    • Contains lipopolysaccharides, fimbriae, and flagella.

  • Lipopolysaccharides:

    • Provides protection for the cell and plays a role in pathogenicity.

    • O antigen: Used in nomenclature of bacterial strains (e.g., O104:H4 linked to the 2011 Germany outbreak).

    • Lipid A: Functions as an endotoxin, triggering immune responses.

Fimbriae and Pili

  • Fimbriae:

    • Short tubular proteins that facilitate attachment to host cells.

    • Not used for locomotion, unlike flagella.

  • Pili:

    • Long protein tubes that assist in genetic material transfer during conjugation.

E. coli Flagellum

  • Functionality: Functions as a molecular motor powered by a proton gradient (gradient generated by proton movement).

Protective Layers

Capsules and Slime Layers

  • Capsule:

    • Rigid polysaccharide layer providing protection and aiding adhesion.

  • Slime Layer:

    • Semi-rigid layer that helps in attachment and biofilm formation.

Biofilms

  • Formation: Result from adhesion of the polysaccharide layer to surfaces and other bacterial cells.

  • Example: Dental plaque is a common biofilm.

Unique Bacterial Groups

Mollicutes

  • Characteristics: Lack cell walls, making them resistant to β-lactam antibiotics.

  • Pathogenic Species: Some cause diseases in humans, such as atypical pneumonia and various infections.

Archaea

  • Differences from Bacteria:

    • Membrane phospholipids contain ether bonds instead of ester bonds.

    • Most have surface-layer proteins; some possess pseudopeptidoglycans.

Genetic Material in Prokaryotes

Cytoplasm and Chromosomes

  • Cytoplasm: Includes all contents inside the plasma membrane, excluding the nucleus.

  • Chromosome:

    • Circular double-stranded DNA molecule (0.5 – 7.0 Mb long).

    • Contains genetic information necessary for function and reproduction, replicated during cell division.

    • Typically single, forming a nucleoid without a surrounding membrane.

Plasmids

  • Characteristics: Small circular dsDNA separate from the chromosome, can vary in size and copy number.

  • Types of Plasmids:

    • R-plasmids: confer antibiotic resistance; can also encode the formation of sex pili.

    • F-plasmids: enable plasmid transfer between cells via sex pili.

Ribosomes

  • Functionality: Sites of protein synthesis, translating mRNA into proteins.

  • Structure: Composed of two subunits (large 50S, small 30S = total 70S), containing ribosomal RNA and associated proteins.

Compartmentalization in Prokaryotes

Membrane Infolds

  • Types:

    • Membrane infolds serve for respiration and may include thylakoid membranes for photosynthesis.

  • Bacterial Microcompartments: Such as carboxysomes, provide enzymatic compartmentalization for various functions.

Magnetotactic Bacteria

  • Characteristics: Gram-negative with magnetosomes, facilitating movement along magnetic fields.

  • Pathogenic vs. Beneficial:

    • Pathogenic E. coli (O157:H7): Produces Shiga toxin.

    • Beneficial E. coli (K-12): Resident in the gut microbiome.

    • Lab Strain (DH5α): Not naturally occurring, designed for laboratory use.

Shiga Toxin

  • Mechanism: Interferes with protein synthesis by targeting ribosomes and is classified as an endotoxin.

  • Comparative Analysis: Examines differences between cytosol vs. cytoplasm, pathogenic vs. beneficial strains, and types of plasmids.

Review Topics

Structures and Functions to Study

  • Gram-positive and negative cell walls

  • Archaea cell walls

  • Peptidoglycan and lipopolysaccharide layers

  • Fimbriae and sex pili

  • Overview of E. coli strains and their characteristics.