Bacteriology IV

Cell Membrane/Wall & Cellular Locomotion

Learning Outcomes

  • Understand the general structure of the cell membrane and wall.

  • Describe the functions of both structures.

  • Discuss the different types of bacterial motility and locomotion.

  • Understand different arrays of flagella and the basics of flagella structure.

  • Consider chemotaxis behavior.

  • Consider the role of fimbriae and pili.

  • Understand the differences between gliding, twitching, and swarming motility.

Cell Membrane and Wall

Cytoplasmic Membrane

  • Roles: Acts as a 'gatekeeper' for diffusion into and out of the cell.

  • Structure:

    • Composed of a phospholipid bilayer with embedded proteins.

    • Hydrophobic (water-repelling) and hydrophilic (water-attracting) components exist within this bilayer.

    • Thickness: 6-8 nm.

  • Components:

    • Integral membrane proteins: Span the membrane and are involved in various functions.

    • Peripheral proteins: Located on the inner or outer surface of the membrane.

Functions of the Cytoplasmic Membrane

  1. Selective permeability: Acts as a barrier to the diffusion of polar and charged molecules, ensuring only certain substances enter or exit the cell.

  2. Transport:

    • Involves transport proteins that allow accumulation of solutes against their concentration gradients, requiring energy (ATP).

  3. Energy conservation:

    • Site of generation and dissipation of the proton motive force, comparable to potential energy in a charged battery.

Additional Functions

  • Permeability Barrier: Prevents leakage while allowing nutrient transport.

  • Protein Anchor: Hosts various proteins facilitating transport and chemotaxis.

  • Energy Generation: Responsible for generating the proton motive force essential for cellular functions.

Cell Wall

Peptidoglycan

  • Structure:

    • Rigid polysaccharide providing structural strength and resistance to osmotic pressure.

    • Composed of strands forming a sheet around the cell, interconnected by cross-links to create a polymer structure.

    • Constitutes approximately 90% of the Gram-positive cell wall.

Gram-Negative Bacteria

  • Have an outer membrane that includes the lipopolysaccharide (LPS) layer, which serves as an effective barrier but has a minor structural role.

  • Understanding the structural differences is crucial for antibiotic targeting; antibiotics effective against Gram-positive bacteria often show weak efficacy against Gram-negative species.

Bacterial Locomotion

  • Most bacteria exhibit motility often facilitated by flagella.

  • Other mechanisms include gas vesicles for aquatic species and gliding, twitching, or swarming motility, where:

    • Flagella: Thin appendages crucial for movement.

    • Length: Approximately 20 nm in thickness and primarily composed of the protein flagellin.

    • Structure: Helically shaped, with different structural arrangements between Gram-positive and Gram-negative bacteria.

Types of Flagellar Arrangement

  1. Monotrichous: A single flagellum.

  2. Lophotrichous: A tuft of flagella at one location.

  3. Amphitrichous: One flagellum at each end.

  4. Peritrichous: Flagella distributed around the entire cell.

  5. Atrichous: No flagella.

Chemotaxis

  • Describes how organisms move in response to chemical gradients.

    • E. coli's behavior changes when encountering attractants, switching from direct swimming to more frequent tumbling in their absence; moving towards attractants and away from repellents alters run and tumble frequency.

Other Forms of Locomotion

  • Gliding motility: Independent of flagella, pili, or fimbriae.

  • Twitching motility: Involves Type IV pili, significant for pathogenicity and biofilm formation (e.g. Pseudomonas aeruginosa).

  • Swarming motility: Characterized by rapid, coordinated movement.

Fimbriae and Pili

  • Fimbriae: Structures sometimes termed attachment pili; crucial for adherence to surfaces.

  • Pili Types:

    1. Conjugation (sex) pili: Involved in genetic transfer between cells.

    2. Type IV pili: Associated with adhesion and twitching motility.

Bacterial Conjugation

  • Process of genetic transfer involving pili, where a donor produces a pilus to attach to a recipient, facilitating DNA transfer.

Biofilms

Biofilms Overview

  • Definition: A biofilm is a microbial, sessile community characterized by:

    1. Irreversible attachment to surfaces or to each other.

    2. Embedding in a self-produced matrix of extracellular polymeric substances (EPS).

    3. Altered phenotype compared to planktonic cells affecting growth rate and gene transcription (Donlan and Costerton, 2002).

Characteristics of Biofilms

  • Social Life of Microbes:

    • Composed of aggregates sharing common traits like high biodiversity, retention of nutrients, and exoenzymes, leading to physiological differences from planktonic cells.

Stages of Biofilm Formation

  1. Adhesion: Both reversible and irreversible phases.

  2. Maturation 1: Formation of microcolonies surrounded by EPS.

  3. Maturation 2: Development of a continuous biofilm.

  4. Dispersion: Involves programmed cell death and lytic phage expression.

Role of EPS in Biofilms

  • Comprising biopolymers (polysaccharides, proteins, nucleic acids) that define the microenvironment of embedded cells.

Impact of Biofilms

  • Significant barriers to infection control, healing, and treatment due to structured populations presenting high resistance to antimicrobial agents.

BIOL2038/2044 Microbiology Study Notes

Bacteriology IV: Cell Membrane/Wall & Cellular Locomotion

Learning Outcomes

  • Understand the biochemical and physical structure of the bacterial cytoplasmic membrane and cell wall.

  • Describe the specific functions including permeability, transport, and energy conservation.

  • Discuss the mechanisms and types of bacterial motility (flagellar, gliding, twitching, swarming).

  • Detail the structural components of flagella and the mechanics of the flagellar motor.

  • Analyze chemotaxis behavior and the "run and tumble" model.

  • Evaluate the role of surface appendages like fimbriae and pili in adhesion and conjugation.

  • Explain the developmental stages and ecological significance of biofilms.

Cell Membrane and Wall

Cytoplasmic Membrane

  • Roles: Acts as the primary selective barrier ('gatekeeper') controlling the flux of nutrients and wastes.

  • Fluid Mosaic Model:

    • Composed of a phospholipid bilayer (typically $6-8$ nm thick) with proteins floating within or associated with the bilayer.

    • Lipid Composition: Phosphoglycerides consisting of a glycerol backbone, two fatty acid chains (hydrophobic tail), and a phosphorylated head group (hydrophilic head).

    • Structural Reinforcement: Unlike eukaryotes (which use sterols), many bacteria use sterol-like molecules called hopanoids to stabilize the membrane and regulate fluidity.

  • Membrane Proteins:

    • Integral Proteins: Deeply embedded, often transmembrane spanning multiple times; involved in transport and signal transduction.

    • Peripheral Proteins: Lipoproteins or membrane-associated proteins often anchored to the membrane, participating in metabolic processes.

Functions of the Cytoplasmic Membrane

  1. Permeability Barrier: Prevents the leakage of cytoplasmic constituents. Large, polar, or charged molecules require specific transport systems.

  2. Protein Anchor: Site of many proteins involved in transport, bioenergetics (ATP synthesis), and chemotaxis.

  3. Energy Conservation:

    • The membrane separates charge, creating the Proton Motive Force (PMF).

    • Movement of protons (H+H^+) across the membrane generates potential energy used for ATP synthesis (via ATP synthase), flagellar rotation, and active transport.

Nutrient Transport Systems

  • Simple Transport: Driven by the energy in the proton motive force (e.g., symporters and antiporters).

  • Group Translocation: Chemical modification of the transported substance driven by phosphoenolpyruvate (e.g., the Phosphotransferase System/PTS for sugar uptake).

  • ABC System: Periplasmic binding proteins and ATP-hydrolyzing proteins involved in high-affinity transport.

Cell Wall

Peptidoglycan (Murein)

  • Chemical Structure:

    • A polymer of alternating sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by β−1,4\beta-1,4 glycosidic bonds.

    • Subunits are cross-linked by short peptides (typically 4 amino acids) attached to NAM. In Gram-negatives, this is often a direct cross-link; in Gram-positives, it involves a peptide interbridge (e.g., glycine interbridge).

  • Function: Protects against osmotic lysis. The internal osmotic pressure of a bacterium can reach $20$ atmospheres.

Gram-Positive vs. Gram-Negative

  • Gram-Positive: Thick layer of peptidoglycan (20−8020-80 nm). Contains Teichoic acids and Lipoteichoic acids, which provide a negative charge and attract cations.

  • Gram-Negative: Thin peptidoglycan layer (2−72-7 nm) located within the periplasm (the space between the inner and outer membranes).

    • Outer Membrane: Contains the Lipopolysaccharide (LPS) layer.

    • LPS Components: Lipid A (endotoxin), Core polysaccharide, and O-specific polysaccharide (antigenic determinant).

    • Porins: Transmembrane protein channels that allow the diffusion of small hydrophilic molecules across the outer membrane.

Bacterial Locomotion

Flagella Structure and Function

  • Composition: Primarily the protein flagellin (FliCFliC).

  • The Flagellar Motor:

    • Driven by the proton motive force (H+H^+ gradient), not ATP.

    • Rotor: The Central shaft and the L, P, MS, and C rings.

    • Stator: Mot proteins that surround the rotor and generate torque by conducting protons.

  • Synthesis: Flagellin molecules are passed through a hollow core and added to the tip, not the base.

Types of Flagellar Arrangement

  1. Monotrichous: Single polar flagellum.

  2. Lophotrichous: Tuft of flagella at one pole.

  3. Amphitrichous: Flagella at both poles.

  4. Peritrichous: Flagella inserted at many locations around the cell surface.

Chemotaxis Mechanism

  • Run: Counter-clockwise (CCW) rotation leads to smooth forward movement.

  • Tumble: Clockwise (CW) rotation causes the cell to stop and reorient randomly.

  • Biased Random Walk: In the presence of an attractant, 'runs' become longer and 'tumbles' are suppressed as long as the concentration gradient is increasing.

Other Locomotion Types

  • Gliding: Smooth movement along a surface without flagella; involves specialized proteins or polysaccharide slime.

  • Twitching: Mediated by Type IV Pili; the pilus extends, binds to a surface, and retracts, pulling the cell forward (ATP-dependent).

  • Swarming: Coordinated, multicellular spreading across a surface, usually involving peritrichous flagella and surfactant production.

Biofilms

Developmental Stages

  1. Reversible Attachment: Initial docking via van der Waals forces or pili/fimbriae.

  2. Irreversible Attachment: Stronger anchoring via Extracellular Polymeric Substances (EPS).

  3. Microcolony Formation: Cell division and recruitment of other microbes.

  4. Biofilm Maturation: Development of complex architecture with water channels for nutrient delivery.

  5. Dispersion: Release of planktonic cells to colonize new environments, often triggered by environmental stress or nutrient depletion.

Significance of Biofilms

  • Resistance: Cells in biofilms are up to $1000 \times$ more resistant to antibiotics and disinfectants due to the physical barrier of the EPS and the presence of slow-growing 'persister' cells.

  • Quorum Sensing: Bacterial communication via chemical signal molecules (autoinducers) that regulate gene expression based on cell density.