Horizontal gene transfer is the main mechanism by which bacteria exchange genetic material, including antibiotic resistance genes. The major processes include transformation, conjugation, transduction, and transposition.
Transformation
- Uptake of naked DNA from the environment (from lysed bacteria).
- Leads to stable integration and expression of new genes.
- Blocked by DNase (degrades naked DNA).
- Classically seen in: SHiN bacteria (Streptococcus pneumoniae, Haemophilus influenzae type b, Neisseria).
- Mnemonic: “SHiN organisms are transformed.”
Conjugation
F+ × F−:
- F+ cell carries the fertility plasmid (F factor) with sex pilus genes.
- Transfers a copy of plasmid DNA only (no chromosomal DNA).
- F− cell becomes F+.
Hfr × F−:
- When the F plasmid integrates into bacterial chromosome = Hfr cell (high-frequency recombination).
- During conjugation, leading portion of plasmid plus some adjacent chromosomal genes are transferred.
- Recipient remains F− but may acquire new bacterial genes.
Transduction
Generalized:
- Lytic phage mistakenly packages bacterial DNA into its capsid.
- Transfers random bacterial genes to another bacterium.
Specialized:
- Lysogenic phage integrates into the bacterial genome.
- Upon excision, adjacent bacterial genes are carried into the phage and transferred.
- Important toxins carried by lysogenic phage:
- A: Group A strep erythrogenic toxin
- B: Botulinum toxin
- C: Cholera toxin
- D: Diphtheria toxin
- S: Shiga toxin
-
Mnemonic: ABCD’S toxins.
Transposition
- “Jumping genes.”
- DNA segments (transposons) excise and reinsert at different sites.
- It can move between plasmids and chromosomes.
- Important in antibiotic resistance transfer across species (eg, vanA gene from Enterococcus to Staphylococcus aureus).
Comparison Table
| Mechanism | Key Feature | DNA Transferred | Example / Importance |
|---|---|---|---|
| Transformation | Uptake of naked DNA | Chromosomal fragments | SHiN organisms; blocked by DNase |
| Conjugation (F+ × F−) | Plasmid transfer via sex pilus | Plasmid DNA only | Antibiotic resistance plasmids |
| Conjugation (Hfr × F−) | Plasmid is integrated into the chromosome. | Plasmid + flanking chromosomal DNA | High-frequency recombination |
| Transduction (generalized) | Lytic phage packaging error | Random bacterial DNA | Any bacterial genes |
| Transduction (specialized) | Lysogenic phage excision | Specific bacterial genes + phage DNA | ABCD’S toxins |
| Transposition | Jumping DNA segments | Plasmid ↔ Chromosome | Multi-drug resistance plasmids |
Learning Objective (USMLE Step 1): Understand the mechanisms of horizontal gene transfer in bacteria, recognize their role in the spread of antibiotic resistance, and identify which bacterial toxins are encoded by lysogenic phages.