Horizontal gene transfer mechanisms

Transformation - Naked DNA Pickup

• Direct uptake and incorporation of exogenous naked DNA from the environment.
• Requires recipient cells to be in a state of competence.
• Key naturally competent bacteria: 📌 Streptococcus pneumoniae, Haemophilus influenzae, Neisseria (SHiN).
• Many other bacteria (E. coli) can be made competent artificially in the lab (e.g., via heat shock or electroporation).

⭐ Griffith's experiment (1928) with S. pneumoniae was the classic demonstration of transformation, turning avirulent strains virulent.

Conjugation - Bacterial Sex Show

• Direct transfer of plasmid DNA from one bacterium to another; requires cell-to-cell contact.
• Donor cell (F+): Carries the fertility (F) factor plasmid and forms a sex pilus.
• Recipient cell (F-): Lacks the F factor.
• High-frequency recombination (Hfr): Occurs when the F factor integrates into the bacterial chromosome, allowing for the transfer of chromosomal genes.

⭐ In an Hfr × F- cross, the recipient cell typically remains F- because the entire integrated F factor is rarely transferred. This is a key distinction from a standard F+ × F- cross.

Transduction - Phage-Taxi Service

• Bacterial DNA is transferred from a donor to a recipient cell via a bacteriophage.

• Generalized Transduction
  • A "packaging error" during the lytic cycle.
  • Random segments of bacterial DNA are packaged into phage heads.
  • Any bacterial gene can be transferred.
• Specialized Transduction
  • An "excision error" during the lysogenic cycle.
  • A prophage excises imprecisely, taking adjacent bacterial genes.
  • Only specific genes near the prophage site are transferred.

⭐ Several key virulence factors are acquired this way. 📌 Mnemonic for toxins transferred via specialized transduction: "ABCD'S" - Shiga-like toxin, Botulinum toxin, Cholera toxin, Diphtheria toxin, Strep pyogenes Erythrogenic toxin.

Transposons - Jumping Gene Jamboree

• Mobile genetic elements (“jumping genes”) that move within or between DNA molecules (e.g., from chromosome to plasmid).
• Encode transposase, an enzyme that facilitates movement, flanked by inverted repeat sequences.
• Two main mechanisms:
  • Non-replicative (cut-and-paste): Moves from one site to another without leaving a copy.
  • Replicative (copy-and-paste): Leaves a copy behind at the original site.
• Complex transposons carry extra genes, like those for antibiotic resistance.

⭐ The VanA gene cluster, conferring vancomycin resistance in Enterococcus, is often carried on a transposon.

High‑Yield Points - ⚡ Biggest Takeaways

• Transformation: Uptake of naked environmental DNA by competent bacteria (S. pneumoniae, H. influenzae, Neisseria).
• Conjugation: Direct cell-to-cell transfer of plasmids (e.g., F-factor) via a sex pilus; key for antibiotic resistance spread.
• Generalized Transduction: A lytic phage mistakenly packages and transfers any bacterial DNA.
• Specialized Transduction: A lysogenic phage incorrectly excises, transferring only adjacent bacterial genes.
• Transposons: "Jumping genes" that move within or between genomes, often carrying resistance or virulence factors.