IB Biology HLDisease & ImmunityPaper 1 & 2~10 min read
Antibiotic Resistance
Bacteria do not decide to become resistant. The resistant ones were already there by chance, and the antibiotic simply removed everything else. This is natural selection happening fast enough to watch, and the exam wants it described in exactly those terms.
📘 What you need to know
Bacterial populations show variation caused by mutations, as in every species.
A chance mutation may make some bacteria resistant to an antibiotic such as penicillin.
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 genes for resistance are passed on with much greater frequency to the next generation.
Bacteria have only one copy of each gene, so a mutant gene has an immediate effect on the bacterium carrying it.
Over time the whole population becomes resistant. This is evolution by natural selection.
Some bacteria are resistant to penicillin because they have acquired genes coding for the enzyme β–lactamase (penicillinase), which breaks penicillin down.
MRSA is resistant to multiple antibiotics and causes infections that are very difficult to treat.
The mechanism, step by step
Every stage below is worth a mark in a long answer, so learn them as a sequence rather than as one big idea.
This is the same three–step logic as any natural selection question: variation, selection pressure, inheritance. Only the timescale is different.
🧩 The full chain in words
Within a bacterial population there is variation caused by mutations.
A chance mutation makes some bacteria resistant to an antibiotic.
The population is treated with that antibiotic and the resistant bacteria do not die.
They reproduce with less competition from the non–resistant bacteria, which are now dead.
The genes for resistance are passed on at a much greater frequency to the next generation.
Over time the whole population becomes resistant, because resistant bacteria are best suited to that environment.
The single most common lost mark here is writing that bacteria “become resistant because of the antibiotic”. They do not. The mutation is random and comes first. The antibiotic is the selection pressure that decides which bacteria survive to reproduce.
One copy of each gene. Bacteria are haploid, so there is no second allele to mask a mutation. A mutant gene has an immediate effect on the bacterium carrying it, which is one reason resistance appears so quickly compared with evolution in other organisms.
How resistance spreads sideways
Resistance is not only passed from parent to offspring. Bacteria can also share plasmids carrying resistance genes with other bacteria, including bacteria of a different species. That is why a resistance gene can spread far faster than reproduction alone would allow.
Plasmids are small rings of DNA separate from the main bacterial chromosome, and they can be passed between bacteria of both the same and different species.
Resistance to penicillin
Some pathogenic bacteria have become resistant to penicillin because they acquired genes coding for the enzyme β–lactamase, also called penicillinase. This enzyme breaks down penicillin before it can interfere with cell wall formation.
Why this is such a problem
Antibiotic–resistant strains are a major problem in human medicine, and new ones keep emerging.
The main driver is the overuse of antibiotics. Using them frequently applies a selective pressure on bacterial populations, which supports the evolution of resistance.
Finding new antibiotics that bacteria have not met before is expensive and time–consuming.
Some strains, such as methicillin–resistant Staphylococcus aureus (MRSA), are resistant to multiple antibiotics, and the infections they cause are very hard to treat.
When antibiotics were discovered, scientists expected to eradicate bacterial infections. Less than a century later we face a future in which many bacterial infections cannot be treated with current medicines.
What can be done
Measure
Why it helps
Avoid prescribing antibiotics for non–serious or non–bacterial infections
Removes selective pressure where the drug was never going to help anyway
Maintain high standards of hygiene in hospitals
Stops resistant strains spreading between vulnerable patients
Minimise antibiotic use in the routine treatment of animals in agriculture
Reduces the huge reservoir of selection pressure outside medicine
Develop new types of antibiotic
Gives options against bacteria that already resist existing drugs
Where new antibiotics come from now. Tackling resistance is a top priority for the World Health Organization, because resistance makes common infections harder to treat and raises mortality. Researchers currently screen chemical libraries — large collections of compounds with known antibacterial characteristics — to find candidates for new drugs. New techniques opening up new avenues of research is exactly the kind of nature–of–science point examiners like to see mentioned.
Worked examples
WE 1
Explain the development of antibiotic resistance
Explain how a population of bacteria becomes resistant to an antibiotic. (4 marks)
Point 1: variation exists first
Mutations occur randomly in bacterial populations, and by chance some bacteria are already resistant to the antibiotic.
Point 2: selection pressure
When the antibiotic is used, non–resistant bacteria are killed while resistant bacteria survive.
Point 3: reproduction
The survivors reproduce with less competition, so the resistance gene is passed on at a much greater frequency.
Point 4: the outcome
Over generations the proportion of resistant bacteria rises until the whole population is resistant. This is evolution by natural selection.
Random mutation, then selection, then inheritancenever write that the antibiotic causes the mutation — it is the single biggest error on this topic
WE 2
Explain why resistance spreads so fast in bacteria
Suggest two reasons why antibiotic resistance spreads more quickly in bacteria than an adaptation would spread in a mammal population. (2 marks)
Reason 1: short generation time and one gene copy
Bacteria reproduce very rapidly and have only one copy of each gene, so a mutant gene has an immediate effect rather than being masked.
Reason 2: plasmids
Plasmids carrying resistance genes can be shared directly between bacteria, including between different species, so the gene spreads without reproduction.
Fast generations plus direct gene sharingthe plasmid point is the one most students forget, and it is often the harder mark
WE 3
Evaluate a control measure
Explain why doctors are advised not to prescribe antibiotics for a patient with a sore throat caused by a virus. (3 marks)
Point 1: no benefit
Antibiotics have no effect on viruses, so the patient gains nothing from the prescription.
Point 2: the cost
The antibiotic still acts on bacteria in the patient’s body, applying a selective pressure that favours any resistant bacteria present.
Point 3: the consequence
Those resistant bacteria survive and reproduce, so unnecessary prescriptions speed up the evolution of resistance across the population.
No gain for the patient, real cost to everyone elseuse the term selective pressure — it turns a common–sense answer into a biology answer
💡 Exam tips
Always start with variation caused by random mutation, before any antibiotic appears.
Call the antibiotic a selective pressure.
Say resistant bacteria reproduce with less competition — this exact phrase appears in mark schemes.
Finish by naming the process: evolution by natural selection.
Name β–lactamase and MRSA where they fit.
For control measures, give specific ones: prescribing habits, hospital hygiene, agriculture, new antibiotics.
⚠ Common mistakes
Saying bacteria “become resistant to survive”. Mutations are random and have no purpose.
Saying the antibiotic causes the mutation. It selects between bacteria that already differ.
Saying individual bacteria adapt. The population changes; individuals do not become resistant during their lifetime.
Saying the patient becomes resistant. It is the bacteria that are resistant, not the person.
Forgetting plasmids. Resistance genes also move sideways between bacteria.
Saying resistance means the antibiotic is now useless everywhere. Other antibiotics may still work — MRSA is serious precisely because several no longer do.
Up next: Zoonoses. Resistance is one way a bacterial disease becomes harder to control. Another is a pathogen crossing into humans from a completely different species, which is where the next page starts.
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