Bacterial Genetics

Horizontal Gene Transfer

• Vertical transmission passes genes parent to offspring; horizontal transfer moves genes between cells, even across species — the engine of resistance spread.
• Transformation: a competent cell takes up naked environmental DNA (the Griffith experiment, where a harmless strain became virulent using DNA from killed pathogens).
• Transduction: a bacteriophage mistakenly packages host DNA and delivers it to a new cell — generalized (random genes) or specialized (genes near the integration site).
• Conjugation: a sex pilus draws two cells together and a plasmid copy (classically the F plasmid) crosses by direct contact.

Plasmids as Resistance Vehicles

• Plasmids are self-replicating mobile DNA separate from the chromosome, and they frequently bundle several resistance genes. One conjugation event can hand a recipient simultaneous resistance to unrelated drug classes — for example a beta-lactamase gene alongside a tetracycline efflux pump.
• Identical resistance plasmids appearing in different species (Klebsiella and E. coli on one hospital ward) point to conjugation, the most efficient cross-species route.

Key Takeaways

• Distinguish transformation (naked DNA), transduction (phage), and conjugation (pilus and plasmid).
• Conjugative plasmids spread resistance fastest and furthest.
• Multidrug resistance often rides a single mobile element.
• Horizontal transfer means resistance genes do not respect species boundaries.