Plasmids, Mobile Elements and How Resistance Spreads
Resistance genes can occur on chromosomes, plasmids and transposons. Transposons can move genes between DNA locations, while transferable plasmids can carry several resistance genes between cells. Resistance spreads through both bacterial reproduction and horizontal transfer.
What a learner can do afterwards
- Explains why a plasmid can be gained and lost while the chromosome stays the same.
- Uses genetic evidence to explain how one mobile element can deliver several resistance genes together.
- Predicts how plasmid costs, benefits, loss and transfer affect carrier frequency after a selecting drug is withdrawn.
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A plasmid is DNA that can replicate separately from the chromosome, using the cell's machinery. It can be gained by transfer or lost when a daughter fails to inherit it, even if the chromosome is unchanged. Many plasmids carry extra traits, such as drug resistance. Resistance genes can also be chromosomal. Their location must be measured, not inferred from the resistance trait alone.
The previous topic called direct donor-recipient DNA transfer conjugation. Some plasmids can use this route, but not all plasmids can transfer themselves. A transposon is a DNA segment that can change location within DNA or move between DNA molecules. It can move from a chromosome to a plasmid inside one cell. That is transposition. Transfer to another cell is a separate event.
One plasmid can carry several resistance genes. If a transferred copy arrives and those genes function in the recipient, several resistance traits can appear together. For example, an intact plasmid carrying resistance genes A and B can be favored by drug A, retaining the linked B gene too. This is co-selection. Joint resistance alone does not prove shared plasmid cargo; DNA evidence is needed.
A drug can favor cells with a plasmid that protects against it. After withdrawal, frequency may fall if the plasmid slows growth, has no other benefit, is sometimes lost and does not transfer. Withdrawal alone does not guarantee decline. Low cost, other benefits or continued transfer may maintain carriers. Resistant cells also spread by reproduction and transmission, so investigations may need both cell and plasmid histories.
Separate transposition from transfer between cells. Linked genes can travel together; plasmid persistence depends on costs, benefits, loss and transfer.
Lesson explanations, questions and study cards were written for this dataset, with scientific background from Microbiology.
Readings and study cards were written for this dataset, with scientific background from Microbiology. The linked videos are credited to Khan Academy.
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Identify what moved
Several movements can spread resistance. A resistant bacterium can reproduce, passing DNA to its descendants. A resistant bacterium can also reach another host. Separately, resistance DNA can move into a different bacterial lineage. Investigations may therefore need evidence about both bacterial relationships and mobile DNA.
A plasmid can replicate separately from the chromosome while using the host cell's machinery. Some plasmids can transfer by conjugation. Finding a plasmid does not establish that it can transfer itself, and identifying conjugation does not establish a universal speed advantage over cell division.
A transposon can move within a DNA molecule or between DNA molecules. Consider a resistance-carrying transposon moving from a chromosome onto a plasmid in the same bacterium. That changes the gene's location without transferring it to another cell. If the plasmid later transfers, the resistance gene can move between cells in a separate event.
Predict changes in the carrier fraction
Drug withdrawal removes one possible advantage of a resistance plasmid. It does not reveal all the conditions affecting its persistence. Compare carrier and noncarrier growth without the drug. Ask whether daughters sometimes lose the plasmid, whether new transfer creates carriers, and whether another plasmid-carried trait provides a benefit.
Suppose carriers grow more slowly, the plasmid has no other benefit, some daughters lose it and no transfer occurs. These conditions favor a decline in the fraction of carriers. Change the conditions and the prediction may change. Little growth cost, another benefit or continued transfer can help maintain carriers.
Keep population frequency separate from individual loss. A falling fraction of carriers does not mean that every cell loses its plasmid. It also does not establish that all resistance disappears, since resistance genes can have other locations. A prediction should state which population and conditions it concerns.
Use evidence for shared resistance cargo
Several resistance traits appearing together are compatible with one plasmid carrying several genes. Timing alone does not establish that explanation. Check where the genes are located and whether they move on the same element. If one transferred plasmid carries all the genes and they function in the recipient, one event can produce several resistance traits.
Linkage also matters after transfer. An intact plasmid carrying resistance genes for drugs A and B can be favored by exposure to A. The linked B-resistance gene can remain with it even when B is absent. This is co-selection through genetic linkage.
Another explanation for resistance to several drugs is one mechanism that protects against them all. This is cross-resistance. Multiple resistance traits therefore do not, by themselves, prove multiple genes or a shared plasmid. Use genetic and functional evidence to distinguish these possibilities.
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