Intrinsic versus adaptive antibiotic resistance
Intrinsic resistance is natural and shared by every member of a species; adaptive resistance appears only while the antibiotic is present.
Remember
Resistance that is natural and shared by all members of a species is intrinsic, and resistance that is present only during antibiotic exposure and fades when the drug is removed is adaptive.
Memory aid
intrinsic is built in, acquired is picked up for good, and adaptive is borrowed for as long as the drug is present
Use the picture to sort a type of resistance by how long it lasts. 'For good' is a figure of speech, since acquired resistance usually persists.
Bacteria can be resistant in three main ways: intrinsic, acquired and adaptive. Acquired resistance is gained by mutation (a random change in the bacterial DNA) or by horizontal gene transfer (the sharing of genes between bacteria), and it usually persists once it is present.
Intrinsic resistance is a natural property of every member of a bacterial species, one that predates the use of antibiotics as medicines. It does not depend on the drug being present or on new genes being picked up. The drug may have no target in that species (the structure it normally acts on is missing), it may be unable to enter the cell, or it may be pumped out by efflux pumps (membrane proteins that eject drugs). A penicillin has no effect on a microbe without a cell wall, since that wall is its site of action. Because intrinsic resistance is predictable for a given organism and drug, you can avoid a drug that will never work against that organism.
In adaptive resistance the bacteria are resistant only while they are exposed to the antibiotic, because they adapt their physiology (how the cell is working) to the environment. It is non-mutational, meaning the DNA sequence is not changed, and transient, meaning temporary. For example, the bacteria increase their efflux pumps through a change in gene expression (how strongly genes are switched on).
Because this adaptation is unstable, the bacteria become susceptible (sensitive to the drug) again once the antibiotic is removed. Adaptive resistance is one reason why the efficacy of an antibiotic may fall during treatment. The test is whether the resistance reverses: if susceptibility is regained when the drug stops, think adaptive resistance. Intrinsic and acquired resistance are built into the genes, so they do not normally disappear when the drug is withdrawn.
Key facts
- Intrinsic resistance
- a natural property shared by every member of a bacterial species, present whatever the exposure.
- Reasons for intrinsic resistance
- the drug may have no target, may be unable to enter the cell, or may be pumped out by efflux pumps.
- Adaptive resistance
- resistance that exists only under antibiotic exposure and reverses when the drug is removed.
- How adaptive resistance works
- reversible changes in how strongly genes are switched on, such as more efflux pumps, with no change to the DNA sequence.
- Efflux pumps in the two types
- a standing property of the species in intrinsic resistance, but increased temporarily in response to the drug in adaptive resistance.
- Acquired resistance
- gained by mutation or horizontal gene transfer and usually stable once present.
Exam traps
- Intrinsic resistance does not explain resistance that appears during treatment and fades afterwards, because it is a natural property of the species that is present whether or not the drug is around.
- Resistance from a mutation does not make bacteria simply become susceptible again when the drug stops, because the change is in the genes and is passed on to descendants, so it usually persists.
- Cross-resistance (resistance to one antibiotic extending to related drugs) describes the range of drugs affected, so it says nothing about whether susceptibility returns once an antibiotic is withdrawn.
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