Bacterial genetics: how bacteria swap genes
Bacteria divide by binary fission, yet genes still move sideways between cells. Each route of horizontal gene transfer has one identifying clue.
Distinguish transformation, transduction, conjugation and transposition as routes of bacterial gene transfer and antibiotic resistance spread.
The short version
- Bacteria copy themselves by binary fission, so new genes arrive sideways by horizontal gene transfer.
- Transformation is free DNA taken up from the surroundings, so a DNase stops it.
- Transduction needs a bacteriophage in the middle; a DNase does not stop it, because the DNA is inside the phage coat.
- Conjugation needs direct cell contact through a pilus, so a filter that separates the cells stops it.
- Transposons are jumping genes that move within DNA; they are not a route between cells by themselves.
Bacterial genetics: why genes move sideways
A hospital finds the same antibiotic resistance gene in Klebsiella from one patient and Escherichia coli from another. Bacteria reproduce by splitting in two, so how did one species hand a gene to another?
Bacterial genetics starts from a contrast. Binary fission means offspring copy the parent. New genes arrive sideways, by horizontal gene transfer.[1]
Three classical routes exist: transformation, transduction and conjugation. Mobile genetic elements such as plasmids, bacteriophages and transposons carry the genes. Each route has one question that identifies it.[1]
Transformation: uptake of free DNA
In transformation, the cell takes up naked DNA from its surroundings. Only some bacteria are naturally competent, and the classic exam ones are Streptococcus pneumoniae, Haemophilus influenzae and Neisseria.[2]
The classic proof is Griffith in 1928: harmless unencapsulated pneumococci turned virulent after meeting killed encapsulated ones.[2][6]
Transduction: a bacteriophage carries the DNA
In transduction, a bacteriophage carries bacterial DNA from one cell to the next. There are two kinds.[1][3]
| Type | Phage cycle | What is carried |
|---|---|---|
| Generalised | Lytic: a phage head packages a random piece of bacterial DNA by mistake, then the cell lyses | Any piece of bacterial DNA |
| Specialised | Lysogenic: the phage cuts itself out of the chromosome imprecisely | Neighbouring genes taken along with the phage |
Phage genes matter clinically: the toxins of Corynebacterium diphtheriae and Vibrio cholerae are encoded by prophages.[3][4][6]
Conjugation: cell contact through a pilus
In conjugation, two cells touch through a pilus. The donor carries a plasmid, such as the F plasmid, which encodes the pilus. A single strand of plasmid DNA passes across, and both cells rebuild a double strand.[1][6]
Hfr cells: when the F plasmid joins the chromosome
If the F plasmid integrates into the chromosome, the cell is called Hfr. It still transfers, but now it drags chromosomal genes across.[1][6]
Transfer usually stops before the end, so the recipient stays F⁻.[6]
Back to the hospital: resistance plasmids
Resistance genes on conjugative plasmids are a main way resistance spreads between species. That is the hospital puzzle: a plasmid carrying the gene can pass between Klebsiella and E. coli by conjugation.[1]
Plasmids can carry many resistance genes at once, which is how one conjugation event can make a bacterium resistant to several drugs.[1][5]
Transposition: jumping genes, not a route between cells
A transposon is a jumping gene that moves within DNA, for example from a plasmid into the chromosome.[1][6]
It is not a route between cells by itself, but it can pack resistance genes onto plasmids that then travel.[6]
| Mechanism | Identifying clue | What stops it |
|---|---|---|
| Transformation | Free DNA from the surroundings | A DNase |
| Transduction | A bacteriophage in the middle | Not a DNase (DNA is inside the phage coat) |
| Conjugation | Direct contact through a pilus | A filter separating the cells |
| Transposition | A jumping gene within DNA | Not a route between cells |
Practice question
Original · USMLE styleA researcher grows two bacterial strains in the two arms of a U-shaped tube, separated by a filter. The pores are too small for bacteria but large enough for viruses. Strain A carries a chromosomal gene for resistance to an antibiotic and harbours a temperate bacteriophage. Strain B is susceptible to that antibiotic and never grows on it when cultured alone. DNase is added to both arms. After incubation, bacteria in the strain B arm grow in the presence of that antibiotic.
Common questions
What is horizontal gene transfer in bacteria?
It is the movement of genes sideways between bacterial cells, rather than from parent to offspring by binary fission. The three classical routes are transformation, transduction and conjugation.[1]
How do you tell transformation from transduction?
Transformation uses free DNA, so a DNase destroys it. Transduction uses a bacteriophage, and the DNA inside the phage coat is not stopped by a DNase.[2][3]
What is an Hfr cell?
An Hfr cell has the F plasmid integrated into its chromosome. It transfers chromosomal genes during conjugation, but transfer usually stops before the end, so the recipient stays F⁻.[1][6]
Transcript
Read the full narration, by chapter644 words
0:00The hook: one resistance gene, two species
A hospital finds the same antibiotic resistance gene in two different species. Klebsiella from one patient, Escherichia coli from another, both resistant to the same drug. Bacteria reproduce by splitting in two, so how did one species hand a gene to another? Today: tell apart transformation, transduction, conjugation and transposition.
0:24Binary fission versus horizontal gene transfer, and the colour key
Bacteria divide by binary fission, so offspring copy the parent. New genes arrive sideways, by horizontal gene transfer. Three classical routes exist: transformation, transduction and conjugation. Each has one question that identifies it.
0:42Transformation: naked DNA and competent bacteria
Transformation: the cell takes up naked DNA from its surroundings. Only some bacteria are naturally competent, and the classic exam ones are Streptococcus pneumoniae, Haemophilus influenzae and Neisseria.
0:55Griffith, 1928, and the DNase clue
The classic proof is Griffith in 1928: harmless unencapsulated pneumococci turned virulent after meeting killed encapsulated ones. The clue is free DNA, so a DNase destroys it.
1:10Transduction: generalised and specialised
Transduction: a bacteriophage carries bacterial DNA from one cell to the next. In generalised transduction, during the lytic cycle a phage head packages a random piece of bacterial DNA by mistake, and then the cell lyses. In specialised transduction, a lysogenic phage cuts itself out of the chromosome imprecisely and takes neighbouring genes with it.
1:36Phage-encoded toxins: diphtheria and cholera
Phage genes matter clinically: the toxins of Corynebacterium diphtheriae and Vibrio cholerae are encoded by prophages. The clue is a virus in the middle. A DNase does not stop it, because the DNA is inside the phage coat.
1:53Conjugation: the pilus and the F plasmid
Conjugation: two cells touch, through a pilus. The donor carries a plasmid, such as the F plasmid, which encodes the pilus. A single strand of plasmid DNA passes across, and both cells rebuild a double strand. The clue is direct contact. A filter that separates the cells stops it. If the F plasmid integrates into the chromosome,
2:15Hfr cells
the cell is called Hfr. It still transfers, but now it drags chromosomal genes across. Transfer usually stops before the end, so the recipient stays F minus.
2:30Back to the hospital: resistance plasmids
Resistance genes on conjugative plasmids are a main way resistance spreads between species. That is our hospital puzzle. Plasmids can carry many resistance genes at once, which is how one conjugation event can make a bacterium resistant to several drugs.
2:47Transposition: jumping genes
Last, transposition. A transposon is a jumping gene that moves within DNA, for example from a plasmid into the chromosome. It is not a route between cells by itself, but it can pack resistance genes onto plasmids that then travel.
3:05Practice question: the U-tube experiment
Your turn. A researcher grows two bacterial strains in the two arms of a U-shaped tube, separated by a filter. The pores are too small for bacteria but large enough for viruses. Strain A carries a chromosomal gene for resistance to an antibiotic, and harbours a temperate bacteriophage. Strain B is susceptible, and never grows on that antibiotic alone. DNase is added to both arms. After incubation, bacteria in the strain B arm grow in the presence of that antibiotic. Which process best explains this result? A, conjugation. B, spontaneous mutation. C, transduction. D, transformation. E, transposition. Pause here and decide.
3:59Wrong options first, then the answer
A is out: conjugation needs direct contact, and the filter keeps the cells apart. B is out: strain B never grows on that antibiotic alone, so the resistance depends on strain A. A spontaneous mutation would not need strain A or the filter. D is out: transformation uses free DNA, and the DNase destroys it. E is out: transposition moves DNA within a cell, not between two cells in separate arms. So the answer is C. The phage from strain A can cross the filter and carries the gene inside its head, safe from the DNase: transduction.
4:41Recap and next lesson
Three things to keep. One: transformation is free DNA, transduction is a phage, conjugation is cell contact through a pilus. Two: Hfr transfers chromosomal genes, but the recipient usually stays F minus. Three: transposons are jumping genes; they help resistance genes move onto plasmids. Next, beta-lactamases and how bacteria resist antibiotics.
Sources
Numbers in the text, e.g. [1], point here- Horizontal gene transfer, mobile genetic elements and antibiotic resistance spread (conjugation, transformation, transduction, Hfr, plasmids, transposons). Infect Drug Resist 2026. Europe PMC PMC13053822. europepmc.org/article/PMC/PMC13053822
- Natural transformation, discovery by Griffith in 1928, competence in Streptococcus pneumoniae, Neisseria and Haemophilus influenzae. Front Microbiol. Europe PMC PMC12236230. europepmc.org/article/PMC/PMC12236230
- Prophages, specialised transduction and prophage-encoded toxins. J Bacteriol. Europe PMC PMC13492144. europepmc.org/article/PMC/PMC13492144
- Cholera toxin encoded by the lysogenic phage CTXphi. Europe PMC PMC4672265. europepmc.org/article/PMC/PMC4672265
- Plasmid fusion and multidrug-resistant plasmids. Europe PMC PMC13672794. europepmc.org/article/PMC/PMC13672794
- Bacterial conjugation; Transduction (genetics); Hfr cell; Griffith's experiment; Corynebacterium diphtheriae; Transposable element. Wikipedia (background). en.wikipedia.org/wiki/Bacterial_conjugation · en.wikipedia.org/wiki/Transduction_(genetics) · en.wikipedia.org/wiki/Hfr_cell · en.wikipedia.org/wiki/Griffith%27s_experiment · en.wikipedia.org/wiki/Corynebacterium_diphtheriae · en.wikipedia.org/wiki/Transposable_element
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined): bacterium, plasmid, bacteriophage. NIAID NIH BioArt Source (bioart.niaid.nih.gov), public domain, adapted (recoloured, outlined): bacillus (rod), DNA double helix, resistance plasmid. Original diagrams drawn in code: bacterial cells, the pilus and strand transfer, Hfr integration, phage packaging, prophage, transposon, the U-tube experiment and all handwriting. No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-MICRO-009.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
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- EP 07Encapsulated organisms and the spleen: asplenia and conjugate vaccines5:08
- EP 08Bacterial genetics: how bacteria swap genes5:13
- EP 09Beta-lactamase and antibiotic resistance: three ways bacteria win5:13
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