Bacterial Genetic Recombination

Bacterial Recombination: Overview - Gene Swap Shop

• Process of genetic material transfer between bacterial cells (Horizontal Gene Transfer - HGT), distinct from vertical inheritance.
• Crucial for bacterial evolution, promoting genetic diversity, acquisition of new traits like antibiotic resistance and virulence factors.
• Main mechanisms:
  • Transformation: Uptake of exogenous naked DNA from environment.
  • Transduction: DNA transfer mediated by bacteriophages.
  • Conjugation: Direct transfer of DNA between cells via physical contact (e.g., sex pilus).

⭐ Most clinically significant: Conjugation often transfers plasmids carrying multiple antibiotic resistance genes (R-plasmids).

Transformation: Mechanism & Significance - Naked DNA Pickup

• Uptake of free, extracellular "naked" DNA (e.g., from lysed cells) by competent bacteria.
• Mechanism:
  • Donor dsDNA binds to recipient cell surface receptors.
  • One strand degraded by nucleases; complementary ssDNA enters cell.
  • Integrated into host genome by RecA-dependent homologous recombination.
• Competence:
  • Natural: E.g., S. pneumoniae, H. influenzae, Neisseria, Bacillus.
  • Artificial: Lab-induced (e.g., E. coli via CaCl₂, heat shock, electroporation).
• Significance: Genetic diversity, acquisition/spread of antibiotic resistance & virulence factors.

> ⭐ Griffith's experiment (1928) with *S. pneumoniae* first demonstrated transformation, proving DNA as genetic material.

Transduction: Phage-Mediated Transfer - Viral Gene Taxi

• Bacteriophage-mediated DNA transfer between bacteria; contributes to genetic diversity and antibiotic resistance spread.
• Two main types:
  • Generalized Transduction:
    • Lytic phages accidentally package any random bacterial DNA fragment.
    • Transfers any gene; low frequency.
  • Specialized Transduction:
    • Temperate phages transfer specific bacterial genes adjacent to prophage integration site.
    • Results from faulty excision of lysogenic prophage. High frequency for specific genes.

⭐ Lysogenic conversion by specialized transduction is crucial for toxin production in bacteria like Corynebacterium diphtheriae (Diphtheria toxin). (📌 ABCD'S Toxin)

Conjugation: Bacterial 'Mating' - Bacterial Mating Dance

• Direct DNA transfer between bacteria requiring physical cell-to-cell contact. 📌 Conjugation = Cell-to-Cell Contact.
• Mediated by F-plasmid (Fertility factor), encoding genes for sex pilus formation & DNA transfer (e.g., tra genes).
  • Donor cell: F+ (possesses F-plasmid).
  • Recipient cell: F- (lacks F-plasmid).
• Process (F+ x F-):
  • F+ extends pilus to F-, forms mating bridge.
  • Single strand of F-plasmid DNA transferred.
  • Both cells synthesize complementary strand; F- becomes F+.
• Hfr (High-frequency recombination) cells:
  • F-plasmid integrated into host chromosome.
  • Transfers chromosomal DNA; recipient usually remains F-.
• F' (F-prime) factor:
  • Imprecisely excised F-plasmid carrying some chromosomal genes. Creates merodiploids.

⭐ Conjugation is a key mechanism for horizontal gene transfer, notably spreading antibiotic resistance genes carried on R-plasmids.

Mobile Genetic Elements: Plasmids & Transposons - Mobile Gene Shufflers

• Plasmids: Extrachromosomal, circular, self-replicating DNA.
  • Carry genes for: antibiotic resistance (R-plasmids), toxins, virulence.
  • Replicate independently of chromosome.
• Transposons ("Jumping Genes"): DNA segments that move ("jump").
  • Move via replicative or non-replicative transposition.
  • Insert into plasmids/chromosome, transfer genes (e.g., antibiotic resistance).
  • Types: IS elements, composite transposons (carry other genes).

⭐ Key drivers of HGT & rapid evolution, esp. antibiotic resistance spread.

High‑Yield Points - ⚡ Biggest Takeaways

• Transformation: Uptake of naked DNA from environment; requires competence.
• Transduction: Bacteriophage-mediated gene transfer; generalized or specialized.
• Conjugation: Direct cell contact via pilus; F-plasmid mediated; Hfr cells show high recombination.
• Plasmids: Extrachromosomal DNA; carry antibiotic resistance (R-factors) or virulence genes.
• Transposons: "Jumping genes"; mobile DNA causing insertional inactivation or spreading resistance.
• Homologous Recombination: RecA-dependent exchange between similar DNA.
• Lysogenic Conversion: Phage genes alter host phenotype (e.g., toxin production).