Bacterial Cell Structure & Function

Bacterial Cell Structure & Function

Structure & Chemical Composition

• Flagellum: Composed of protein, serves a key role in bacterial motility.
• Pilus: Made of glycoprotein, facilitates adherence to host cell surfaces, enhancing colonization.
• Fimbria: Contributes to adherence; often shorter than pili.
• Spore: Contains a keratin-like coat, dipicolinic acid, peptidoglycan, and DNA; enables survival against dehydration, chemicals, and heat.
• Capsule: Typically a polysaccharide layer, protects bacteria from phagocytosis, mediates surface adherence, and plays a vital role in biofilm formation, relevant in indwelling catheters.
• Slime (S) layer: A loose polysaccharide network, aids in surface adherence and protection.
• Outer membrane:
  • Comprised of an outer leaflet with endotoxin (LPS/LOS) and embedded proteins such as porins.
  • Functions uniquely in Gram-negative bacteria, allowing transport and acting as a barrier.
• Periplasm: The space between the cytoplasmic membrane and outer membrane in Gram-negative bacteria, accumulates components leaving cells, including hydrolytic enzymes like β-lactamases.
• Cell wall: Features a peptidoglycan structure providing rigid support and protection against osmotic pressure damage due to its cross-linked peptide side chains.
• Cytoplasmic membrane: A phospholipid bilayer with embedded proteins, serves as the site for oxidative and transport enzymes, and involves penicillin-binding proteins (PBPs) in cell wall synthesis.

Bacterial Cell Envelope

• Unique to Gram-positive bacteria: Presence of lipoteichoic acid, which can induce inflammatory responses.
• Common features in both Gram-positive and Gram-negative bacteria:
  • Flagellum, Pilus, Capsule, Cell wall (peptidoglycan), Cytoplasmic membrane.
• Unique to Gram-negative bacteria:
  • Endotoxin/LPS, which contributes to toxicity and immune responses.
  • Porin proteins that facilitate nutrient transport.
  • An outer membrane providing an additional protective barrier.
  • A wider periplasmic space containing enzymes like β-lactamase, which confer antibiotic resistance.

Bacterial Cell Structure Overview

• Bacterial cells possess various components that define their structure and function, each playing a unique role in survival and interaction.

Flagellum

• Composed of protein, enabling motility.
• Propels bacteria through liquid environments, aiding in movement.

Pilus

• Made of glycoprotein, facilitates adherence to surfaces.
• Allows bacteria to attach to cells, other bacteria, and inanimate objects.

Fimbria

• Similar composition to pili, also glycoprotein-based.
• Functions in adherence to surfaces, playing a crucial role in colonization.

Spore

• Consists of a keratinlike coat, dipicolinic acid, peptidoglycan, and DNA.
• Acts as a dormant and resistant stage, providing survival in harsh conditions.
• Thick coat enhances resistance to dehydration, chemicals, and heat.

Capsule

• Typically composed of polysaccharides, occasionally proteins.
• Protects bacteria from phagocytosis, making it harder for immune cells to engulf them.

Slime Layer

• Consists of a loose network of polysaccharides.
• Facilitates adherence to surfaces and enables the formation of biofilms, important for bacterial communities in places like catheters.

Outer Membrane (Gram-Negative Only)

• Contains endotoxins (LPS/LOS) and embedded proteins, including porins.
• Lipid A component induces immune responses (TNF and IL-1).
• Antigenic properties of most outer membrane proteins help with immune evasion.

Periplasm

• Located between the cytoplasmic membrane and outer membrane in gram-negative bacteria.
• Accumulates components that exit the bacterial cell, including hydrolytic enzymes like β-lactamases.

Cell Wall

• Comprised of peptidoglycan, a sugar backbone with cross-linked peptide side chains.
• Provides rigid support and protects against osmotic pressure-related damage.

Cytoplasmic Membrane

• Formed from a phospholipid bilayer with embedded proteins and lipoteichoic acids in gram-positive bacteria.
• Serves as the site for oxidative phosphorylation and various enzymatic reactions.

Bacterial Cell Structure

• Flagellum: Composed of protein; essential for bacterial motility, enabling movement toward favorable environments.
• Pilus: Made of glycoprotein; facilitates adherence to host cell surfaces, playing a crucial role in colonization and infection.
• Fimbria: Hair-like appendages that aid in attachment to surfaces; not detailed in structural composition.
• Spore: Contains a keratin-like coat, dipicolinic acid, peptidoglycan, and DNA; predominantly found in gram-positive bacteria. Provides survival advantages by resisting dehydration, chemicals, and heat.
• Capsule: A discrete layer primarily composed of polysaccharides (occasionally proteins); functions to protect bacteria from phagocytosis by immune cells, enhancing virulence.
• Slime Layer: A loose network of polysaccharides; assists with adherence to various surfaces, contributing to biofilm formation.
• Outer Membrane: Comprises an outer leaflet rich in endotoxin (LPS/LOS) and embedded proteins like porins; plays a vital role in biofilm formation (e.g., in indwelling catheters).
• Periplasm: The space between the cytoplasmic and outer membranes in gram-negative bacteria, containing peptidoglycan; acts as a conduit for various molecules and enzymes.
• Cell Wall: Consists primarily of peptidoglycan, a sugar backbone with peptide side chains cross-linked by transpeptidase; provides structural support and protection against osmotic pressure.
• Cytoplasmic Membrane: A phospholipid bilayer enriched with embedded proteins, including penicillin-binding proteins (PBPs); crucial for nutrient transport and maintaining cellular integrity.
• Endotoxin (in Gram-negative bacteria): Comprising lipid A which induces inflammatory responses (TNF and IL-1) and O polysaccharide (antigenic); triggers immune reactions.
• Hydrolytic Enzymes: Enzymes such as β-lactamases that break down antibiotics, providing a survival mechanism in hostile environments.
• Oxidative and Transport Enzymes: Present in the periplasm, where PBPs also participate in cell wall synthesis; contribute to cellular metabolism and nutrient uptake.

Structure and Chemical Composition

• Flagellum: Composed of protein, enabling bacterial motility.
• Pilus and Fimbria: Made of glycoproteins, facilitating adherence to surfaces.
• Spore Composition: Contains keratinlike coat, dipicolinic acid, peptidoglycan, and DNA, contributing to bacterial survival under adverse conditions.
• Capsule: Typically a discrete layer constructed from polysaccharides, serving protective functions.
• Slime Layer: Consists of a loose network of polysaccharides, aiding in adhesion.
• Outer Membrane (Gram-negative):
  • Outer leaflet includes endotoxin (LPS/LOS) and embedded proteins such as porins and other outer membrane proteins (OMPs).
  • Inner leaflet primarily composed of phospholipids.
• Periplasm: Region between the cytoplasmic and outer membranes in gram-negative bacteria, containing peptidoglycan.
• Cell Wall: Structured of peptidoglycan, consisting of a sugar backbone cross-linked by peptide side chains through transpeptidase.
• Cytoplasmic Membrane: A phospholipid bilayer with embedded proteins like penicillin-binding proteins (PBPs) and enzymes, crucial for cell integrity.

Appendages

• Motility Functions: Structure aids in adherence of bacteria to various surfaces.

Specialized Structures

• Gram Positive Bacteria:
  • Structures enhance survival, resist dehydration, chemical damage, and heat exposure.

Cell Envelope

• General Functions:
  • Protects bacteria against phagocytosis.
  • Facilitates adherence to surfaces and plays a significant role in biofilm formation, such as on indwelling catheters.

Gram Negative Specifics

• Endotoxin (Lipid A): Triggers immune responses like TNF and IL-1; features an antigenic O polysaccharide component.
• Embedded Proteins: OMPs often exhibit antigenic properties.
• Porins: Allow nutrient transportation across the outer membrane.

Function

• Component Accumulation: Manages components exiting gram-negative cells, including hydrolytic enzymes like β-lactamases.
• Structural Support: Netlike structure offers rigidity and protection against osmotic pressure.
• Enzyme Activity: Site for oxidative and transport enzymes; PBPs are essential for cell wall synthesis.
• Immune Induction: Lipoteichoic acids stimulate TNF-α and IL-1 production.

Cell Envelope Diagram Components

• Common to Both Gram Types: Flagellum, pilus, capsule, cell wall, peptidoglycan, and cytoplasmic membrane.
• Unique to Gram Positive: Presence of lipoteichoic acid.
• Unique to Gram Negative: Includes endotoxin/LPS, porins, outer membrane, and a wider periplasmic space harboring β-lactamases.
• Distinctions: Gram-positive bacteria are characterized as Gram +, while Gram-negative are indicated as Gram -.