BIO104 Cellular and Microbial Biology - Lecture Notes

BIO104 Cellular and Microbial Biology

Lecture 7: The Microbial Cell (1)

Instructor: Dr. Eva Sonnenschein

Contact: e.c.sonnenschein@swansea.ac.uk

Overview of Prokaryotic Cells

Escherichia coli

  • Focus on shapes of prokaryotic cells

  • Discussion on the prokaryotic cell envelope, which includes:

    1. Cell membrane

    2. Cell wall

    3. Outer membrane (specific to Gram-negative bacteria)

Bacterial Cell Types

  • Classification based on cell envelope:
    I. Gram-positive bacteria
    II. Gram-negative bacteria
    III. Acid-fast bacteria
    IV. Mycoplasmas

Comparison of Prokaryotic and Eukaryotic Cells

Characteristics of Eukaryotic Cells

  • True nucleus present

  • Membrane-bound organelles of bacterial ancestry (e.g., mitochondria, plastids).

Characteristics of Prokaryotic Cells

  • Free-floating genetic material

  • Presence of mesosome, an organelle involved in either DNA replication and cell division or excretion of exoenzymes.

Shapes of Prokaryotic Cells

Major Shapes of Bacteria

  • Bacteria primarily exhibit three major shapes with sub-categorizations:

    1. Cocci (spherical)

    • Subtypes include:

      • Diplococci

      • Tetrads

      • Streptococci

      • Sarcinae

      • Micrococci

    1. Bacillus (rod-shaped)

    • Subtypes include:

      • Bacillus

      • Coccobacilli

      • Myco- rods

      • Spore forming bacteria

      • Corynebacterium

      • Streptomyces

    1. Spirillum/Spirochete (spiral-shaped)

    • Subtypes include:

      • Vibrios

      • Spirilla

      • Spirochetes

Shapes of Archaea Cells

  • Archaea can exhibit four diverse shapes:

    1. Bacillus (rod)

    2. Branched

    3. Coccus

    4. Square shape

Prokaryotic Cell Envelope

Functions of the Prokaryotic Cell Envelope

  • Maintains Shape: Rigid structure

  • Protection: Shields against environmental hazards

  • Prevents Bursting: Essential in a hypotonic environment

Structural Composition

  • Comprises:

    1. Cell (plasma) membrane

    2. Cell wall

    3. Outer membrane (specific to Gram-negative bacteria)

The Cell Membrane

Structure of the Bacterial Cell Membrane

  • Composed of a phospholipid bilayer:

    • Glycerol-phosphate head attached to a fatty acid, forming hydrophilic heads and hydrophobic tails.

Vulnerability

  • The lipid nature of the membrane makes it susceptible to chemical agents.

Additional Layer: The Cell Wall

  1. Most prokaryotes possess a tough outer layer, the cell wall.

  2. Gram-negative bacteria additionally have an outer membrane.

The Cell Wall

Composition and Function

  • The cell wall is primarily made of murein (peptidoglycan), a sugar polymer with amino acid side chains.

    • Glycan chains are constructed from alternating units of N-acetylglucosamine and N-acetyl muramic acid connected via a β-(1,4)-glycosidic bond.

    • Cross-linking of glycan chains occurs via short peptides (4-5 amino acids long) linked to each N-acetyl muramic acid unit.

  • Function: Maintains shape

Impact of Lysozyme

  • Lysozyme: An enzyme that hydrolyzes the bacterial cell wall.

  • For example, when Bacillus megaterium is treated with lysozyme, the rod-shaped bacteria transform into spherical forms, demonstrating the importance of the cell wall in shape maintenance.

    • Lysozyme disrupts the β-(1,4) linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine.

Gram Classification of Bacteria

Gram-Positive vs. Gram-Negative

  • Gram-positive Bacteria:

    • Characterized by a thick peptidoglycan cell wall.

    • Contains teichoic acids (e.g., teichoic and lipoteichoic acid) which increase rigidity and help in adhesion.

  • Gram-negative Bacteria:

    • Thin peptidoglycan layer with an additional outer membrane structure.

    • The outer membrane grants resistance to harmful chemicals.

Gram Staining Procedure

  • Gram Staining: Divides bacteria into Gram-positive (staining purple) and Gram-negative (staining red) based on cell wall structure.

    • Method established by Hans Christian Gram to enhance visibility of bacteria in tissue sections.

    • The staining process includes the use of crystal violet and iodine to form a complex that is retained by Gram-positive bacteria.

    • The dehydration of the peptidoglycan layer during the staining process traps the dye in Gram-positive bacteria but not in Gram-negative bacteria, which requires a counterstaining step with methylene blue.

Outer Membrane of Gram-Negative Bacteria

Composition and Characteristics

  • The outer membrane has an asymmetric bilayer composed of lipopolysaccharides (LPS) on the outer layer and phospholipids on the inner layer.

    1. Lipid A: Glycolipid component

    2. Polysaccharide Core: LPS core structure

    3. O Antigen: Long carbohydrate chains (up to 40 repeated sugars), which confer hydrophobic qualities to the membrane.

Immunogenic Response

  • LPS is highly immunogenic, eliciting strong antigenic reactions in vertebrates.

Nutrient Uptake Challenges

  • The hydrophobic nature of the outer membrane can impede nutrient absorption.

Porins

  • Porins: Protein channels intrinsic to the outer membrane of Gram-negative bacteria, facilitating the diffusion of hydrophilic compounds such as sugars, amino acids, and ions.

The Periplasm

  • Periplasm is the gel-like space located between the inner cell membrane and the outer membrane, containing murein and housing:

    1. Degradative enzymes that break down larger molecules for transport across the inner membrane.

    2. Proteins that specifically bind sugars and amino acids to enhance nutrient absorption.

    3. β-lactamases, which inactivate certain antibiotics like penicillins and cephalosporins.

Alternative Outer Membrane: Acid-Fast Bacteria

Mycobacterial Structure

  • Some bacteria (e.g., Mycobacteria) lack an LPS outer membrane; instead, they possess mycolic acids (60-90 carbon long fatty acids) forming their outer layer.

  • Mycolic acids assemble into lipid bilayers, with porins embedded, connecting to the peptidoglycan (murein) cell wall through complex sugars.

Characteristics of Acid-Fast Bacteria

  • Acid-fast bacteria are resistant to harsh chemicals and desiccation due to their hydrophobic outer membrane.

  • Cannot be identified by Gram staining techniques.

Acid-Fast Staining Process

  • Steps for identifying acid-fast bacteria:

    1. Stain with carbol fuchsin (red) using heat to loosen the waxy layer.

    2. Wash with acid-alcohol mixture to decolorize non-acid-fast cells.

    3. Counterstain with methylene blue; acid-fast bacteria retain the red color, whereas non-acid-fast bacteria turn clear and are subsequently stained blue.

Mycoplasmas

Characteristics

  • Mycoplasmas do not possess a cell wall, only a cell membrane.

  • They contain sterols within their cell membranes to enhance rigidity.

  • In clinical contexts, they are resistant to β-lactam antibiotics due to the absence of peptidoglycan.

Additional Exterior Layers and Appendages

Purpose and Functionality

  • Some bacteria and archaea can produce additional layers or appendages:

    • Capsules and slime layers

    • Flagella

    • Pili

Capsular Structure

  • Capsules and slime layers:

    • Composed of high-molecular polysaccharides or amino acid polymers. A tightly attached coat is termed a capsule; a loosely attached coat is termed a slime layer.

    • Conditions for production are often environmental cues that prompt these protective layers.

Function of Capsules

  • Functions include water and nutrient retention, resistance to chemical diffusion, adherence to surfaces, and the formation of biofilms.

  • Capsules also provide a defense mechanism against phagocytosis by white blood cells.

  • Example organisms:

    • Streptococcus mutans (involved in tooth decay)

    • Porphyromonas gingivalis (contributing to periodontal disease)

    • Neisseria meningitidis and Haemophilus influenzae (associated with meningitis).

Flagella

Structure and Function

  • Flagella propel bacteria through liquids and are important in biofilm formation and virulence.

  • Structure includes:

    1. Long helical filament (often several times longer than the bacterial cell).

    2. Connecting hook.

    3. Basal body equipped with a rotor that facilitates rotation.

  • Components of the Flagellum:

    • The filament is rigid and composed of the protein flagellin, which is highly antigenic.

    • The hook acts as a joint between the filament and the basal body, made of hook protein.

  • Basal Body: A rod with rings that anchors the flagellum and allows it to rotate, consisting of different proteins depending on Gram classification (Gram-negative: 4 rings; Gram-positive: 2 rings).

Pili

Structure and Purpose

  • Pili are hair-like structures that mediate attachment to hosts and surfaces, facilitate protein and nucleic acid transfer, and enable motility.

  • Types of Pili:

    • Commonly mediate attachment through adhesins to mucosal surfaces.

    • Assists in evading phagocytosis and is highly antigenic.

    • Primarily found in Gram-negative bacteria, less prevalent in Gram-positive bacteria.

Twitching Motility

  • Involves the extension and retraction of pili through the processes of polymerization and depolymerization.

    1. The tip of the pilus adheres to a surface.

    2. The pilus shortens, dragging the entire bacterium forward like a grappling hook.

Revision Session

Details

  • Date: Friday, 8th November, 12 PM

  • Instructions: Students should send their questions regarding lecture materials by Friday, 1st November.

Summary

  • Review the shapes of prokaryotic cells.

  • Understand the structure and function of the prokaryotic cell envelope, including:

    • Gram-positive bacteria

    • Gram-negative bacteria

    • Acid-fast bacteria

  • Familiarize with the specifics of Mycoplasmas.

  • Describe the various types of prokaryotic appendages.