Microbial Cell Structure and Function: Study Notes for Microbiology 300

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Microbial Cell Structure and Function

Introduction

Understanding the structure and function of microbial cells is fundamental in microbiology. This topic covers the diversity of cell envelopes, surface structures, inclusions, and mechanisms of motility in bacteria and archaea, with reference to the textbook Brock Biology of Microorganisms.

Cell Envelopes and Cell Walls

Bacterial Cell Walls

Bacterial cell walls provide structural support and protection. They are essential for maintaining cell shape and preventing osmotic lysis.

Gram-positive bacteria: Thick peptidoglycan layer; sensitive to penicillin and lysozyme.
Gram-negative bacteria: Thin peptidoglycan layer and an outer membrane containing lipopolysaccharide (LPS).
Protoplasts: Bacterial cells without cell walls, survive only in isotonic solutions.
Mycoplasma: Prokaryotes lacking cell walls; have sterols in their plasma membrane for stability.
Archaea: Cell walls may contain pseudomurein (similar to peptidoglycan but resistant to penicillin and lysozyme) or other polymers; some have phospholipids with ether bonds.

Example: Mycoplasma pneumoniae causes walking pneumonia and lacks a cell wall.

Archaeal Cell Walls

Archaeal cell walls are diverse and may contain pseudomurein, glycan polymers, or protein layers. They are often resistant to antibiotics targeting peptidoglycan.

Pseudomurein: A polysaccharide similar to peptidoglycan, found in some methanogenic archaea.
Protein S-layers: Crystalline protein arrays providing structural support.
Ether-linked phospholipids: Unique to archaea, contributing to membrane stability.

Cell Surface Structures

Capsules and Slime Layers

Capsules and slime layers are gelatinous external layers composed of polysaccharides or proteins. They play roles in protection, adhesion, and immune evasion.

Capsule: Well-organized, tightly attached to the cell wall; protects against desiccation and host immune responses.
Slime layer: Loosely organized, easily washed off; aids in surface attachment.
Function: Important in pathogenesis and vaccine development (e.g., pneumococcal vaccine).

Example: Leuconostoc mesenteroides produces a slime layer.

Fimbriae and Pili

Fimbriae and pili are proteinaceous appendages on the surface of many bacteria, involved in adhesion, motility, and genetic exchange.

Fimbriae: Short, numerous; mediate attachment to surfaces and biofilm formation.
Pili: Longer, fewer; involved in attachment, conjugation (genetic exchange), and twitching motility.
Comparison Table:

Feature	Fimbriae	Pili
Length	Short	Long
Number per cell	Hundreds	Few (1-10)
Function	Adhesion to surfaces	Genetic exchange, motility, adhesion

Cell Inclusions

Storage Granules

Cell inclusions are intracellular deposits of nutrients or minerals, allowing microorganisms to store resources for later use.

Polyhydroxyalkanoates (PHA): Lipid storage granules; used in biodegradable plastic production.
Glycogen: Glucose polymer for energy storage.
Polyphosphates: Inorganic phosphate storage.
Sulfur globules: Elemental sulfur storage, especially in sulfur bacteria.
Magnetosomes: Magnetic iron mineral inclusions; allow orientation in magnetic fields.
Gas vesicles: Protein-bound structures that confer buoyancy to planktonic cells.

Example: Cupriavidus necator accumulates PHA for bioplastic production.

Microbial Locomotion

Flagella and Swimming Motility

Flagella are whip-like appendages that enable swimming motility in many bacteria and archaea.

Structure: Composed of filament, hook, and basal body.
Types of flagellation:
- Polar: Flagella at one or both ends of the cell.
- Peritrichous: Flagella distributed over the entire cell surface.
Movement: Rotation of flagella driven by proton motive force.
Speed: Up to 60 cell lengths per second.

Example: Salmonella enterica uses peritrichous flagella for motility.

Gliding Motility

Some bacteria move over surfaces without flagella, using gliding motility mechanisms involving slime secretion or surface proteins.

Mechanisms: Polysaccharide slime extrusion, surface protein movement.
Example: Myxococcus xanthus exhibits gliding motility.

Chemotaxis and Other Taxes

Bacteria and archaea can move in response to environmental stimuli, a behavior known as taxis.

Chemotaxis: Movement toward or away from chemical stimuli.
Phototaxis: Response to light.
Aerotaxis: Response to oxygen.
Osmotaxis: Response to ionic strength.
Hydrotaxis: Response to water.
Run and tumble behavior: Alternating straight runs and random tumbles to navigate gradients.

Example: Capillary tube assays are used to study chemotaxis.

Endospores

Endospore Formation and Function

Endospores are highly resistant, dormant structures formed by some Gram-positive bacteria in response to adverse conditions.

Resistant to: Heat, chemicals, radiation.
Role: Survival and dispersal in harsh environments.
Formation: Triggered by nutrient limitation; involves a complex differentiation process.
Germination: Return to vegetative growth when conditions improve.

Example: Bacillus anthracis, Clostridium botulinum, and Clostridium tetani produce endospores.

Stages of Sporulation

Vegetative cell
Sporulating cell
Mature endospore
Germination

Summary Table: Key Cell Structures and Functions

Structure	Composition	Function	Example Organism
Cell wall	Peptidoglycan, pseudomurein, protein	Shape, protection	Bacillus subtilis, Archaea
Capsule	Polysaccharide/protein	Protection, adhesion	Streptococcus pneumoniae
Fimbriae	Protein	Attachment	Salmonella enterica
Pili	Protein	Conjugation, motility	Escherichia coli
Flagella	Protein	Motility	Bacillus subtilis
Endospore	Multiple layers	Survival	Bacillus anthracis
Inclusion bodies	Lipid, phosphate, sulfur, iron	Storage	Cupriavidus necator

Key Equations

Proton Motive Force (PMF) for Flagellar Rotation:

Run and Tumble Frequency (Chemotaxis):

Additional info: Some context and examples were inferred to clarify fragmented notes and provide a self-contained study guide.