Pulsed-Field Gel Electrophoresis

Pulsed-Field Gel Electrophoresis - DNA's Electric Zigzag

• Definition: A technique for separating very large DNA molecules, ranging from >25 kb up to 10 Mb.
• Principle: Uses periodically changing directions of an electrical field. DNA molecules must reorient themselves to migrate through the agarose gel, and larger DNA molecules take longer to change direction, leading to separation by size.
• Advantages over Standard Gel Electrophoresis:
  • Separates much larger DNA fragments effectively.
  • Higher resolution for large DNA.

⭐ PFGE is considered a 'gold standard' for epidemiological typing of many bacterial pathogens due to its high discriminatory power for large DNA fragments.

Pulsed-Field Gel Electrophoresis - Gentle DNA Handling

⭐ A critical step in PFGE is the preparation of intact genomic DNA by embedding cells in agarose plugs before lysis and enzyme digestion to prevent mechanical shearing of large DNA molecules.

• 📌 Mnemonic: Clean Plugs Get Great Bands (Cells in Plugs, Lysis, Enzyme digestion, Gel, Bands/electrophoresis).
• Sample Prep: Cells in agarose plugs; Lysis; Proteinase K; Washing. Prevents DNA shearing.
• Restriction Digestion: Rare-cutting enzymes (e.g., NotI, SfiI) for large DNA fragments.
• Gel Casting: High-grade agarose (0.8-1.2%); TBE or TAE buffer.
• Electrophoresis:
  • Systems: CHEF (Contour-Clamped Homogeneous Electric Field - common), FIGE, TAFE.
  • Parameters: Switch times, field angles (e.g., 120°), voltage, temperature (4-14°C).
• Detection: Staining (Ethidium Bromide/EtBr, SYBR Safe); Documentation.

Pulsed-Field Gel Electrophoresis - Tracking Superbugs

Pulsed-Field Gel Electrophoresis (PFGE) is a DNA fingerprinting technique for large DNA molecules. It separates DNA by periodically changing electric field direction.

• Principle: Alternating electrical fields allow separation of very large DNA fragments (up to 10 Mb).
• Key Applications:
  • Epidemiological typing of bacterial pathogens:
    • E. coli O157:H7
    • Salmonella spp.
    • Listeria monocytogenes
    • MRSA (Methicillin-resistant Staphylococcus aureus)
  • Outbreak investigations: Identifying sources, transmission routes.
  • Molecular surveillance (e.g., PulseNet): Assessing genetic relatedness/clonality.
  • DNA fingerprinting.

⭐ PFGE is the core technology behind PulseNet, an international molecular subtyping network for foodborne disease surveillance, enabling rapid detection and investigation of outbreaks.

Pulsed-Field Gel Electrophoresis - Patterns & Pitfalls

• Interpretation of Patterns (DNA Fingerprints):
  • Compare banding patterns to reference strains.
  • Tenover Criteria for Relatedness:
    • 0 band difference: Indistinguishable
    • 1-3 differences: Closely related
    • 4-6 differences: Possibly related
    • ≥7 differences: Different

⭐ The Tenover criteria are widely used to interpret PFGE results, defining isolates as indistinguishable, closely related, possibly related, or different based on the number of DNA band differences after restriction digestion.

• Advantages vs. Disadvantages:

High‑Yield Points - ⚡ Biggest Takeaways

• PFGE separates very large DNA molecules, often megabase-sized, unlike standard gel electrophoresis.
• It employs periodically changing directions of the electric field to facilitate migration of large DNA.
• Crucial for molecular epidemiology and strain typing of bacteria in outbreak investigations.
• Requires intact, high-molecular-weight DNA often prepared in agarose plugs.
• DNA is typically digested with rare-cutting restriction enzymes prior to separation.
• Used for genome size estimation and creating physical maps of genomes.