Microbial Cell Structure and Function: Study Notes for Microbiology

Microbial Cell Structure and Function

Introduction to Microbial Cell Biology

Microbiology is the study of microorganisms, which are organisms too small to be seen with the naked eye. Understanding the structure and function of microbial cells is fundamental to the field, as it provides insight into how these organisms grow, reproduce, and interact with their environments.

• Microorganisms include bacteria, archaea, viruses, fungi, and protozoa.

• Cell structure determines nutrient uptake, growth rate, and environmental adaptation.

• Reference: Brock Biology of Microorganisms, 15th Edition, Chapters 1 and 2.

Cell Size and Its Biological Implications

Size Range of Microbial Cells

Microbial cells vary greatly in size, which affects their physiology and ecological roles. Smaller cells have a higher surface area-to-volume ratio, enhancing nutrient uptake and growth rates.

• Bacteria: Typically 0.3–10 μm in diameter.

• Viruses: Much smaller, e.g., Influenza virus (~0.085 μm), T2 bacteriophage (~0.065 x 0.095 μm).

• Red blood cells: ~7 μm (for comparison).

• Ultra-small cells: Ultramicrobacteria can be even smaller than typical bacteria.

Key Point: The small size of microbial cells allows for rapid nutrient exchange and faster growth compared to larger cells.

The Cell Envelope: Structure and Components

Cytoplasmic (Cell) Membrane

The cytoplasmic membrane is a semi-permeable barrier that separates the cell's interior from its environment. It is essential for maintaining homeostasis and mediating transport.

• Phospholipid bilayer: Composed of hydrophilic heads and hydrophobic tails.

• Functions:

  • Permeability barrier: Controls entry and exit of substances.

  • Protein anchor: Holds transport and signaling proteins.

  • Energy conservation: Site of proton motive force generation.

Archaeal Membranes

Archaeal membranes differ from those of Bacteria and Eukarya in their chemical composition and structure.

• Ether linkages in phospholipids (vs. ester linkages in Bacteria/Eukarya).

• Isoprene units instead of fatty acids.

• Can form lipid monolayers (more heat resistant, found in thermophilic Archaea) or bilayers.

Transport Across the Cytoplasmic Membrane

Types of Transport

Cells use various mechanisms to move substances across the membrane, depending on the molecule's size and polarity.

• Simple/Passive diffusion: Small, nonpolar molecules (e.g., O2, CO2) pass freely.

• Facilitated diffusion: Uses membrane proteins (channels/carriers) for larger or polar molecules (e.g., glucose).

• Active transport: Moves solutes against their concentration gradient using energy (often ATP).

Types of Active Transporters:

• Uniporter: Transports one type of molecule.

• Symporter: Transports two molecules in the same direction.

• Antiporter: Transports two molecules in opposite directions.

Equation for Active Transport:

$\text{Rate} = k[\text{Solute}]$

Bacterial Cell Walls: Peptidoglycan Structure

Peptidoglycan Composition and Function

Peptidoglycan is a unique structural polymer found in most bacterial cell walls, providing rigidity and protection against osmotic pressure.

• Structure: Repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), cross-linked by short peptides.

• Function: Maintains cell shape and prevents lysis.

• Susceptibility: Degraded by lysozyme (found in tears, saliva) and beta-lactam antibiotics (e.g., penicillin).

Equation for Peptidoglycan Cross-linking:

$\text{Peptidoglycan} = (\text{NAG-NAM})_n + \text{Peptide Cross-links}$

Gram-Positive vs. Gram-Negative Cell Walls

Bacteria are classified based on their cell wall structure, which is revealed by the Gram stain.

• Gram-positive:

  • Thick peptidoglycan layer (up to 90%).

  • Contains teichoic acids (WTA) and lipoteichoic acids (LTA).

  • Stains purple in Gram stain.

• Gram-negative:

  • Thin peptidoglycan layer (~10%).

  • Outer membrane contains lipopolysaccharide (LPS).

  • Stains pink in Gram stain.

Gram Stain Procedure

• Crystal violet stains all cells.

• Iodine forms a complex with the dye.

• Alcohol decolorizes Gram-negative cells.

• Safranin counterstains Gram-negative cells pink.

Table: Comparison of Gram-Positive and Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Thickness	Thick (up to 90%)	Thin (~10%)
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present (contains LPS)
Gram Stain Color	Purple	Pink

Special Cell Wall Types and Cell Wall-Less Microbes

Archaeal Cell Walls

Archaea lack peptidoglycan and instead may have pseudomurein, protein, or glycoprotein-based cell walls. These structures confer resistance to certain antibiotics.

• Pseudomurein: Similar to peptidoglycan but not degraded by lysozyme or penicillin.

• Protein/glycoprotein walls: Common among methanogens.

Cell Wall-Less Prokaryotes

Some bacteria, such as Mycoplasma, lack cell walls entirely. These organisms are typically small and can survive in isotonic environments.

• Mycoplasma: Causes walking pneumonia; resistant to beta-lactam antibiotics.

• Protoplasts: Bacterial cells without cell walls, can survive in isotonic solutions.

Clinical Relevance: Pathogenic Bacteria and Cell Wall Components

Example: Escherichia coli O157:H7

Escherichia coli O157:H7 is a toxin-producing, pathogenic strain that can cause severe illness, including hemorrhagic colitis and hemolytic uremic syndrome (HUS).

• Associated with contaminated raw meat and milk.

• Identified by O (somatic) antigen 157 and H (flagellar) antigen 7.

• LPS (endotoxin) is a key virulence factor.

Summary Table: Key Cell Envelope Features

Cell Type	Main Wall Component	Susceptibility to Lysozyme/Penicillin	Special Features
Gram-Positive Bacteria	Peptidoglycan	Susceptible	Teichoic acids
Gram-Negative Bacteria	Peptidoglycan + Outer Membrane (LPS)	Less susceptible	Porins, periplasmic space
Archaea	Pseudomurein/Protein/Glycoprotein	Resistant	Lipid monolayer (in some)
Mycoplasma	None	Resistant	Smallest free-living cells

Additional info: Some details about the Colorado groundwater study and Caulobacter crescentus envelope were referenced but not fully explained in the notes. For more information, consult the textbook chapters listed above.