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.
Horizontal gene transfer and resistance mechanisms
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