🧬 Bacterial genetic material
Bacteria harbour three types of DNA: the chromosomal genome, plasmids, and prophage DNA (integrated bacteriophage).
Chromosome
- Single, circular, covalently closed molecule (most species).
- Organised in loops around a protein core; ~2000 genes.
- Carries essential genes for survival.
Plasmids
- Extrachromosomal, small circular DNA (1.5–400 kb).
- Autonomous replication; often carry antibiotic resistance, exotoxin genes, or conjugation machinery (tra operon).
- Episomes can integrate into the chromosome via site-specific recombination.
Bacteriophage (prophage): temperate phage DNA stably integrated into the chromosome. Repressor proteins maintain lysogeny. Prophage genes can enhance virulence — lysogenic conversion (e.g. diphtheria toxin, cholera toxin, Shiga toxin).
🔄 Gene transfer mechanisms
Bacteria exchange genetic material via three principal routes: transformation, conjugation, and transduction. New DNA must be stabilised by recombination or exist as a plasmid.
Transformation
•
Conjugation
•
Transduction
Key principle: Linear DNA (exogenote) is degraded unless integrated by homologous recombination (requires recA and homology). Plasmid DNA can persist extrachromosomally.
🧪 Transformation
Uptake of naked, free DNA from the environment by competent bacteria.
- Naturally competent species: Streptococcus pneumoniae, Haemophilus influenzae, Neisseria spp., Bacillus spp.
- Competence is transient and environmentally regulated.
- Imported DNA is usually linear and homologous; stabilised by homologous recombination.
High yield Transformation does not require cell–cell contact or a phage vector.
🤝 Conjugation
Direct cell-to-cell transfer of DNA via a sex pilus. Donor cells harbour fertility (F) factors.
F⁺ × F⁻
- Plasmid-encoded F factor (free plasmid).
- oriT is transferred first, then plasmid genes.
- Recipient becomes F⁺; no bacterial chromosomal genes are transferred.
Hfr × F⁻
- F factor integrated into chromosome (episome).
- Transfer starts at oriT and proceeds linearly; chromosomal genes adjacent to the integrated F are mobilised.
- Mating is usually interrupted; recipient receives chromosomal genes but does not become Hfr.
Remember Conjugation requires cell–cell contact and a donor with F factor. Hfr crosses are used for bacterial gene mapping.
🦠 Transduction
Bacterial DNA is transferred by a bacteriophage vector.
| Feature | Generalised | Specialised |
|---|---|---|
| Mechanism | Accidental packaging of host DNA during lytic cycle | Excision error of a temperate prophage |
| Genes transferred | Any bacterial gene (random) | Only genes adjacent to the prophage integration site (e.g. gal or bio in λ phage) |
| Requires lysogeny | No | Yes (prophage must be integrated) |
Lysogenic conversion: prophage genes alter bacterial phenotype — e.g. Corynebacterium diphtheriae (diphtheria toxin), Vibrio cholerae (cholera toxin), Shiga toxin, botulinum toxin.
🧩 Recombination & integration
Homologous recombination exchanges DNA between a linear exogenote and the circular chromosome; requires recA and homology. Site-specific recombination integrates DNA at specific attachment sites (e.g. F factor integration, prophage insertion, transposons).
Key Transposons are mobile genetic elements that move via site-specific recombination, often carrying resistance genes.
💊 Antimicrobial resistance
Types
- Intrinsic – lack of target or inherent impermeability (e.g. Mycoplasma lacks peptidoglycan → resistant to penicillin).
- Chromosome-mediated – mutation in target (e.g. altered penicillin-binding proteins in MRSA).
- Plasmid-mediated – R plasmids carry resistance genes; often via enzymes (β-lactamases, acetyltransferases, efflux pumps).
Common plasmid-mediated mechanisms
| Antibiotic class | Resistance mechanism |
|---|---|
| Penicillins / cephalosporins | β-lactamase (hydrolysis of β-lactam ring) |
| Aminoglycosides | Acetyltransferase, phosphotransferase, adenyltransferase |
| Chloramphenicol | Acetyltransferase |
| Tetracyclines | Efflux pumps |
| Vancomycin | Altered cell wall precursor (D-Ala-D-Lac) — vanA/vanB |
ESBL Extended-spectrum β-lactamases (e.g. TEM-1) inactivate multiple cephalosporins; often plasmid-borne.
🔬 Susceptibility testing
Kirby–Bauer (disk diffusion)
- Agar plate inoculated with bacteria; antibiotic disks placed.
- Zone of inhibition measured → classify as S, I, or R.
- Advantages: cheap, easy, multiple drugs. Limitations: qualitative.
MIC & MBC
- MIC – lowest antibiotic concentration that inhibits visible growth (broth dilution).
- MBC – lowest concentration that kills ≥99.9% of inoculum (subculture to drug-free media).
- MBC is essential for immunocompromised patients.
Pearl MIC is the minimum inhibitory concentration; MBC is the minimum bactericidal concentration. MBC ≥ 32× MIC suggests tolerance.
⚡ High-yield USMLE pearls
▶ Clinical correlations
- Lysogenic conversion → toxins: diphtheria, cholera, botulinum, Shiga, erythrogenic toxin of S. pyogenes.
- R plasmids often carry multiple resistance genes via transposons (integrons).
- Vancomycin-resistant S. aureus (VRSA) acquired vanA from enterococci on a plasmid.
- Macrolide resistance due to methylation of 23S rRNA (MLS phenotype).
- Fluoroquinolone resistance via altered topoisomerase or efflux.
▶ Comparison of gene transfer
| Feature | Transformation | Conjugation | Transduction |
|---|---|---|---|
| Cell contact | No | Yes | No |
| Phage required | No | No | Yes |
| Free DNA | Yes | No | No |
| Recombination often needed | Yes | F⁺×F⁻ no; Hfr×F⁻ yes | Yes |