Molecular Diagnostic Methods

Molecular Diagnostic Methods - PCR Powerhouse

• Polymerase Chain Reaction (PCR): In vitro exponential amplification of specific DNA/RNA.
  • Core Principle: Enzymatic replication of a target nucleic acid sequence.
  • Key Components: Template (DNA/cDNA), primers, Taq polymerase (heat-stable), dNTPs, Mg²⁺.
• PCR Cycle: Repeated 25-35 times. Results in $2^n$ fold amplification (n=cycles).

• Variants & Uses:
  • RT-PCR (Reverse Transcriptase PCR): RNA detection (converts RNA to cDNA first). E.g., HIV, Influenza.
  • qPCR (Real-Time PCR): Quantifies DNA/RNA during amplification using fluorescence.
  • Applications: Rapid pathogen ID (TB, Chlamydia), genetic testing, oncology.

⭐ Taq polymerase, isolated from Thermus aquaticus, is thermostable, withstanding high denaturation temperatures, making PCR automation possible.

Molecular Diagnostic Methods - Target Finders

• Nucleic Acid Probes: Labelled (e.g., $P^{32}$, biotin, fluorophores) ssDNA/RNA fragments; bind specific complementary target sequences.
• Hybridization: Process of probe annealing to target nucleic acid.
  • Solid-phase: Target immobilized on a membrane.
  • In-situ: Target within intact cells or tissues.
• Blotting Techniques: For detecting specific DNA/RNA sequences.
  • Southern Blot: DNA detection.

-   **Northern Blot:** RNA detection (no restriction digestion needed).
-   **Dot/Slot Blot:** Rapid detection without size separation.
> ⭐ 📌 SNoW DRoP Mnemonic: **S**outhern Blot = **D**NA; **N**orthern Blot = **R**NA; **W**estern Blot = **P**rotein.

• In Situ Hybridization (ISH): Localizes nucleic acids within cells/tissues.
  • FISH (Fluorescent ISH): Uses fluorescent probes. Applications: Gene mapping, detecting aneuploidies (e.g., Trisomy 21), pathogen identification. technique showing probes targeting specific DNA sequences in chromosomes)
• Microarrays (Gene Chips): Miniaturized arrays with thousands of probes; for high-throughput gene expression analysis, SNP genotyping.

Molecular Diagnostic Methods - Code Crackers

• Principle: "Crack the code" of microbial nucleic acids (DNA/RNA).
• Uses: ID, resistance, virulence, typing, epidemiology.

• 1. Hybridization (Known Sequences):
  • Probes detect specific DNA/RNA.
  • Techniques: Southern/Northern blots, FISH, DNA microarrays.
• 2. Amplification (Signal Boost):
  • PCR: Exponential DNA/RNA increase. Key types: RT-PCR (RNA), qPCR (quantifies).
  • Alternatives: LCR, TMA, NASBA.
• 3. Sequencing (Unknown Sequences):
  • Sanger: Dideoxy chain termination. Gold standard for single genes.
  • NGS: High-throughput, for genomes/metagenomes.
• 4. Genotyping/Typing (Strain Variation):
  • RFLP, PFGE: Restriction enzyme patterns.
  • MLST: Allelic variation in housekeeping genes. 📌 "Many Loci Sequenced Together".

  ⭐ PFGE is considered the gold standard for epidemiological typing of many bacterial pathogens.

Molecular Diagnostic Methods - Pathogen Hunt

Rapidly identifies pathogens by detecting their unique DNA/RNA, crucial for timely and targeted therapy.

• Core Principle: Detect pathogen-specific nucleic acids.
• Key Methods:
  • PCR (Polymerase Chain Reaction): Highly sensitive & specific.
    • RT-PCR: For RNA viruses (e.g., HIV, Influenza, SARS-CoV-2).
    • qPCR (Real-Time PCR): Quantifies pathogen load (viral load monitoring).
  • Sequencing (NGS, Sanger): Definitive ID, tracks outbreaks, detects resistance mutations.
  • LAMP (Loop-mediated Isothermal Amplification): Rapid, point-of-care testing.
• Advantages: Speed, detects non-culturable/fastidious organisms, high throughput.
• Applications: Diagnosis of sepsis, meningitis, TB, viral infections; antimicrobial resistance profiling.

⭐ PCR can detect as few as 1-10 copies of microbial DNA/RNA per sample, vital for early diagnosis even before seroconversion or in paucibacillary disease.

High‑Yield Points - ⚡ Biggest Takeaways

• PCR: Core for amplifying DNA/RNA; offers high sensitivity in detecting pathogens.
• Real-time PCR (qPCR): Allows quantification of microbial load, crucial for monitoring treatment response.
• Multiplex PCR: Detects multiple pathogens simultaneously, useful for syndromic panels.
• Nucleic Acid Hybridization Probes (e.g., FISH): Identify specific microbial sequences directly in samples.
• Gene Sequencing (Sanger/NGS): Essential for genotyping, resistance mutation detection, and epidemiological tracing.
• Molecular Typing (PFGE, MLST, Ribotyping): Crucial for strain differentiation during outbreak investigations.