Evolution by natural selection is usually something you have to infer from fossils. In a population of bacteria you can watch it happen in a weekend. That is what makes antibiotic resistance both a brilliant teaching example and a genuine medical emergency.
📘 What you need to know
Within any bacterial population there is variation, caused by random mutations.
A chance mutation may make some bacteria resistant to an antibiotic.
When the population is treated, the resistant bacteria do not die.
They then reproduce with less competition, because the non-resistant bacteria are dead.
The resistance genes are passed on at a much greater frequency to the next generation.
Over time the whole population becomes resistant. This is evolution by natural selection.
Bacteria have only one copy of each gene, so a mutation has an immediate effect on that bacterium.
Some bacteria acquire genes coding for beta-lactamase (penicillinase), an enzyme that breaks down penicillin.
Resistance genes on plasmids can be shared between bacteria, even between different species.
Overuse of antibiotics is a selective pressure that speeds the whole process up.
Natural selection, sped up
The single most common mistake here is to say that bacteria “become resistant because of the antibiotic”. They do not. The resistant ones were already there, by chance, before the antibiotic arrived. The antibiotic only decides who survives.
Bacteria have only one copy of each gene, so a mutation takes effect immediately — there is no second copy to mask it. Combined with fast reproduction, that is why resistance appears so quickly.
🧩 How to write this answer every time
There is variation within the bacterial population, caused by random mutation.
By chance, some bacteria are resistant to the antibiotic.
When the antibiotic is applied, the non-resistant bacteria die; the resistant ones survive.
Survivors reproduce with less competition for nutrients and space.
The allele for resistance is passed on with much greater frequency to the next generation.
Over generations, the whole population becomes resistant — evolution by natural selection.
Marks are lost here for language, not knowledge. Never write that bacteria “want” to survive, “learn” to resist or “adapt themselves” to the drug. Write mutation, variation, survive, reproduce, pass on. Those five words are the mark scheme.
Two ways resistance spreads
Route
What happens
Vertical — parent to offspring
A resistant bacterium divides, and every daughter cell inherits the resistance gene
Horizontal — between bacteria
Plasmids carrying resistance genes can be shared between bacteria, including bacteria of a completely different species
That second route is why resistance spreads so much faster than ordinary evolution would suggest. A harmless gut bacterium that picks up a resistance gene can pass it to a pathogen.
A named mechanism: beta-lactamase
Some pathogenic bacteria have become resistant to penicillin by acquiring genes that code for the enzyme beta-lactamase, also called penicillinase. The enzyme breaks penicillin down before it can interfere with cell wall formation.
MRSA — methicillin-resistant Staphylococcus aureus — is resistant to multiple antibiotics at once, which makes the infections it causes extremely difficult to treat.
Why this is getting worse, and what helps
Every time an antibiotic is used, it applies a selective pressure: it kills the non-resistant and leaves the resistant behind. Using antibiotics frequently, or when they were never going to help, simply speeds that selection up.
New resistant strains are emerging constantly. Finding genuinely new antibiotics that bacteria have never been exposed to is expensive and slow, and when antibiotics were first discovered nobody expected to be back here within a century.
Measure
Why it helps
Not prescribing antibiotics for non-serious or non-bacterial infections
Removes the selective pressure where the drug could never have worked anyway
High standards of hygiene in hospitals
Stops resistant strains spreading between vulnerable patients
Minimising routine antibiotic use in agriculture
Reduces selection for resistance in bacteria that can reach humans
Developing new types of antibiotic
Provides drugs that current resistant strains have not been exposed to
Tackling resistance is a top priority for the World Health Organization, because the future treatment of common infections and even minor injuries depends on it. One current approach is screening chemical libraries — large collections of compounds with known antibacterial properties — to find and develop new drugs.
Worked examples
WORKED EXAMPLE
Explain how a population of bacteria becomes resistant to an antibiotic. [4]
1. VariationRandom mutation produces variation in the population, so a few bacteria happen to be resistant.2. SelectionWhen the antibiotic is applied, non-resistant bacteria die but resistant ones survive.3. ReproductionThe survivors reproduce with less competition for nutrients and space.4. InheritanceThe resistance gene is passed on at a much greater frequency, so over generations the whole population is resistant.Variation → selection → reproduction → inheritancesay “this is evolution by natural selection” at the end — often the fourth mark
WORKED EXAMPLE
A patient stops taking their antibiotics as soon as they feel better, three days into a seven-day course. Suggest why this contributes to antibiotic resistance. [3]
Who dies firstThe most susceptible bacteria are killed early, which is why the patient feels better.Who is leftThe bacteria that survive longest are those with the greatest resistance, and stopping early leaves them alive.What happens nextThese reproduce with reduced competition and pass their resistance genes on, so the surviving population is more resistant than the original one.Stopping early selects for the most resistant bacteriafeeling better is not the same as the infection being gone — say so
💡 Exam tip
Always start with variation caused by random mutation. Answers that start with the antibiotic have the story backwards.
Use the phrase selective pressure when explaining why overuse makes things worse.
Name beta-lactamase (penicillinase) if a question asks for a specific mechanism of penicillin resistance.
Mention plasmids when asked how resistance spreads between species.
Finish with the phrase evolution by natural selection — it is often an explicit marking point.
⚠ Common mix-up
Saying the antibiotic causes the mutation. Mutations are random and happen anyway; the antibiotic selects.
Writing that bacteria “become immune”. Immunity is what your body has. Bacteria become resistant.
Saying “the bacteria adapt”. Individual bacteria do not change — the population changes as its composition shifts.
Thinking a resistant person is the problem. It is the bacteria that are resistant, not the patient.
Forgetting the competition point. Resistant bacteria thrive partly because their competitors have just been wiped out.
Up next: Zoonoses — why some diseases stay in one species while others jump the barrier into humans, and what that means for pandemics.
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