Resistant bacteria — AQA GCSE Biology
Test yourself on Resistant bacteria with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
Resistant bacteria explained
Evolution by natural selection needs inherited variation and many generations.
Read the full explanation
Bacteria reproduce by simple cell division, so one cell can become a large population in a short time. For example, under favourable conditions some bacteria divide roughly every twenty minutes, so a single cell could in theory produce over a million descendants in a few hours. Each division can copy mutations, and because so many generations occur, useful new alleles spread quickly. If an antibiotic is present, bacteria with a mutation giving resistance survive and reproduce, while non-resistant cells die. The fast reproduction rate therefore speeds up natural selection, allowing resistance to appear and spread rapidly through a population. This explains why resistant strains emerge so quickly in hospitals and farms.
Mutations of bacterial pathogens produce new strains.
A mutation is a random change in the genetic material of a bacterium. When a bacterial pathogen reproduces, most offspring are identical, but occasional mutations alter their characteristics. These changes can produce a new strain, which is a variant of the species with different features, such as the ability to resist an antibiotic or to produce a different toxin. For example, a mutation might change the shape of a protein that an antibiotic normally targets, so the drug no longer binds and the bacterium survives. Mutations occur at random and are not caused by the antibiotic itself; the antibiotic merely selects the strains that already carry a useful mutation. New strains can therefore spread through a population and make an infection harder to treat.
Some strains might be resistant to antibiotics, and so are not killed.
Antibiotics are medicines that kill bacteria or stop their growth. Within a bacterial population, random mutations can give some cells resistance, meaning the antibiotic no longer harms them. When a patient takes an antibiotic, susceptible bacteria die, but resistant cells survive and reproduce. Their offspring inherit the resistance, so the resistant strain becomes more common. For example, a mutation may change the bacterial protein that the antibiotic targets, so the drug cannot bind. This is natural selection in action. Resistance is a serious problem because infections caused by resistant strains are harder to treat, and resistant bacteria can spread between people. Completing prescribed courses and avoiding unnecessary antibiotic use help limit the spread of resistant strains.
They survive and reproduce, so the population of the resistant strain rises.
This statement explains natural selection in bacteria during antibiotic treatment. When an antibiotic is used, most bacteria of a strain are killed, but a few may carry a random mutation that makes them resistant. Those resistant bacteria survive because the antibiotic does not kill them. They then reproduce rapidly by binary fission, passing the resistance allele to their offspring. As a result, the proportion and number of resistant bacteria in the population increase. For example, a patient with a bacterial infection may start with a mostly non-resistant population; after antibiotic treatment, the resistant survivors multiply, so the resistant strain becomes more common. This is evolution by natural selection acting on variation within a bacterial population.
The resistant strain will then spread because people are not immune to it and there is no effective treatment.
This statement explains why a resistant bacterial strain can spread through a population. Humans do not have natural immunity to the resistant strain, so more people can become infected. Because the strain is resistant to antibiotics, there may be no effective treatment to stop the infection or reduce transmission. The bacteria can spread from person to person through contact, droplets, contaminated surfaces or medical equipment. In healthcare settings, this is a serious problem because vulnerable patients may be infected. For example, MRSA can spread in hospitals where patients are already ill. The combination of no immunity and no effective treatment allows the resistant strain to increase in the population.
MRSA is resistant to antibiotics.
MRSA stands for meticillin-resistant Staphylococcus aureus. It is a strain of the bacterium Staphylococcus aureus that is resistant to several antibiotics, including meticillin and often related antibiotics. This means that infections caused by MRSA are difficult to treat because the usual antibiotics do not kill the bacteria. MRSA can cause serious infections, particularly in hospitals or care settings where people may have wounds, catheters or weakened immune systems. The resistance arises from random mutations and is inherited by offspring, so resistant populations increase when antibiotics are used. Understanding MRSA helps explain why inappropriate antibiotic use is dangerous and why infection control is important.
Doctors should not prescribe antibiotics inappropriately, such as treating non-serious or viral infections.
This statement explains why antibiotics should only be prescribed when they are needed. Antibiotics kill bacteria, not viruses, so they are ineffective against viral infections such as colds or flu. Prescribing antibiotics for non-serious or viral infections is inappropriate because it exposes bacteria to the antibiotic unnecessarily. This increases the chance that resistant bacteria survive and reproduce, leading to more resistant strains. Doctors should prescribe antibiotics only for serious bacterial infections where they are likely to be effective. Patients should also complete their prescribed course as directed. Reducing inappropriate use helps slow the development of antibiotic resistance and keeps antibiotics effective for the future.
Patients should complete their course of antibiotics so all bacteria are killed and none survive to mutate and form resistant strains.
Antibiotics kill bacteria, but within a population some bacteria may carry random mutations that make them less affected by the drug. If a patient stops treatment early, when they feel better, the least susceptible bacteria are more likely to survive. These survivors reproduce rapidly by binary fission, passing on the advantageous allele, so the population becomes dominated by resistant bacteria. Completing the prescribed course maintains the antibiotic concentration for long enough to kill even those bacteria with partial resistance, reducing the chance that any survive to mutate further or transfer resistance genes. For example, a patient with a throat infection may feel well after three days, but stopping a seven-day course can leave resistant survivors that later cause an infection that the same antibiotic cannot treat.
The agricultural use of antibiotics should be restricted.
Antibiotics are used in farming to treat and sometimes to prevent disease in livestock, and low doses have historically been added to feed to promote growth. This creates frequent exposure of large numbers of bacteria to antibiotics, increasing the chance that resistant mutants survive and reproduce. Resistance genes can then spread between bacteria, including from farm animals to humans through food, water, direct contact or the environment. Restricting agricultural use means antibiotics are given only when medically necessary, at appropriate doses, and not routinely for growth promotion. This reduces the selection pressure that favours resistant bacteria, helping to keep antibiotics effective for treating human infections. For example, limiting routine preventive dosing in poultry can lower the number of resistant bacteria entering the food chain.
The development of new antibiotics is costly and slow. It is unlikely to keep up with the emergence of new resistant strains.
Discovering and testing a new antibiotic takes many years and large amounts of money. Researchers must find a compound that kills bacteria without harming human cells, then test it in laboratories, animals and clinical trials to check safety, dosage and effectiveness. Many candidate drugs fail, so the cost of each successful antibiotic is very high. Meanwhile, bacteria reproduce rapidly and can acquire resistance through mutation or by transferring resistance genes, so resistant strains can appear quickly. This means the rate at which new resistant strains emerge can outpace the rate at which new antibiotics are developed. For example, a resistant strain may spread through a hospital in months, while a new antibiotic may take more than a decade to reach patients.
Your focus
- Describe how bacteria reproduce and why this produces many generations quickly.
- Explain how fast reproduction speeds up natural selection.
- Apply the idea to explain the rapid spread of antibiotic resistance.
Show all 30 objectives
- Define mutation and strain in the context of bacteria.
- Explain how a mutation can produce a new bacterial strain.
- Distinguish between the random origin of mutations and the selective effect of antibiotics.
- Describe how resistant strains survive antibiotic treatment.
- Explain how natural selection increases the proportion of resistant bacteria.
- Relate antibiotic resistance to difficulties in treating bacterial infections.
- Describe how resistant bacteria survive antibiotic treatment and reproduce.
- Explain why the population of a resistant strain increases over time.
- Apply the concept of natural selection to a named bacterial example such as MRSA.
- Explain why a resistant bacterial strain can spread when people are not immune and treatment is ineffective.
- Describe routes by which resistant bacteria can spread between people.
- Relate the spread of resistant strains to a named example such as MRSA.
- Identify MRSA as a resistant strain of Staphylococcus aureus.
- Explain why MRSA infections are difficult to treat.
- Relate MRSA to the wider issue of antibiotic resistance in healthcare.
- Explain why antibiotics should not be prescribed for viral or non-serious infections.
- Describe how inappropriate antibiotic use contributes to the rise of resistant bacteria.
- Justify the importance of responsible antibiotic prescribing for public health.
- Describe how random mutation and natural selection can produce antibiotic-resistant bacteria.
- Explain why completing a prescribed course of antibiotics reduces the survival of resistant bacteria.
- Apply the idea of selection to a patient scenario and predict the consequence of stopping treatment early.
- Describe how agricultural antibiotic use can increase the number of resistant bacteria.
- Explain how restricting agricultural use can reduce the spread of resistance to humans.
- Evaluate arguments for and against restricting antibiotic use in farming.
- Describe why developing new antibiotics is slow and expensive.
- Explain why new antibiotic development may not keep pace with the emergence of resistant strains.
- Suggest strategies that could help manage the problem of antibiotic resistance.
Resistant bacteria exam tips
Marking Points
- States that bacteria reproduce by simple cell division, producing genetically identical offspring apart from mutations.
- Explains that a fast reproduction rate produces many generations in a short time.
- Links many generations to a greater chance of beneficial mutations arising and being selected.
- Applies the idea to antibiotic resistance, where resistant bacteria survive and reproduce while others die.
- Uses a numerical or everyday example, such as division every twenty minutes, to show rapid population growth.
- Defines a mutation as a random change in the genetic material of a bacterium.
- Explains that mutations can alter bacterial characteristics, such as resistance or toxin production.
- Defines a strain as a variant of a species with different inherited features.
- Links mutation to the formation of new strains that can spread through a population.
- States that mutations arise at random and are not directed by the environment or by antibiotics.
- States that some bacterial strains carry mutations giving resistance to an antibiotic.
- Explains that the antibiotic kills susceptible bacteria but not resistant ones.
- Describes how surviving resistant bacteria reproduce and pass resistance to their offspring.
- Links the process to natural selection, where the antibiotic acts as the selection pressure.
- Explains a consequence, such as infections becoming harder to treat or resistant strains spreading.
- Resistant bacteria are not killed by the antibiotic, so they survive the treatment.
- Surviving resistant bacteria reproduce, often rapidly by binary fission.
- The resistance allele is inherited by offspring, so resistant bacteria become more common.
- The population of the resistant strain rises because selection favours resistant individuals.
- A named example such as MRSA can be used to illustrate the process.
- People are not immune to the resistant bacterial strain, so they can be infected.
- There is no effective antibiotic treatment for the resistant strain.
- Without effective treatment, the infection is harder to control and can spread more easily.
- Spread can occur through contact, droplets, contaminated surfaces or medical equipment.
- This makes resistant strains a serious public health concern, especially in hospitals.
- MRSA is a strain of Staphylococcus aureus.
- MRSA is resistant to antibiotics, including meticillin and often other antibiotics.
- Infections caused by MRSA are difficult to treat because antibiotics may not work.
- MRSA is a particular concern in healthcare settings such as hospitals.
- Resistance is inherited and can spread through bacterial reproduction.
- Antibiotics are effective against bacteria, not viruses.
- Prescribing antibiotics for viral infections such as colds or flu is inappropriate.
- Non-serious infections may not need antibiotics because the immune system can often clear them.
- Inappropriate use increases selection pressure, allowing resistant bacteria to survive and reproduce.
- Doctors should prescribe antibiotics only when there is a clear bacterial infection and a genuine need.
- Antibiotics kill bacteria but do not kill viruses, so they are only prescribed for bacterial infections.
- Random mutations can arise in bacterial populations and some mutations reduce the effect of an antibiotic.
- Stopping a course early allows bacteria that are less susceptible to survive and reproduce by binary fission.
- Surviving resistant bacteria pass on the advantageous allele to their offspring, increasing the proportion of resistant bacteria in the population.
- Completing the full course keeps the antibiotic at an effective concentration for long enough to kill bacteria that are only partially resistant.
- Reducing the number of surviving bacteria lowers the chance of further mutation and of resistance spreading to other bacteria.
- Antibiotics are used in agriculture to treat disease, prevent disease and sometimes to promote growth in livestock.
- Frequent or low-dose use exposes many bacteria to antibiotics and increases selection pressure.
- Resistant bacteria selected in animals can spread to humans through food, water, contact or the environment.
- Restricting use to necessary treatment reduces the number of bacteria exposed and slows the development of resistance.
- Resistance genes can be transferred between bacteria, so reducing agricultural use can limit the spread of resistance.
- Responsible use in agriculture helps preserve the effectiveness of antibiotics for treating human infections.
- Developing a new antibiotic requires many years of research, testing and clinical trials.
- The process is expensive because most candidate compounds fail and because trials must demonstrate safety and effectiveness.
- Bacteria reproduce rapidly and can mutate or acquire resistance genes, so resistant strains can emerge quickly.
- The slow rate of antibiotic development may not match the fast rate at which resistance appears.
- If new antibiotics cannot keep pace, some bacterial infections may become difficult or impossible to treat.
- Reducing unnecessary antibiotic use and developing new drugs are both needed to manage resistance.
Examiner Tips
- 💡Use the phrase reproduce at a fast rate and link it directly to many generations.
- 💡Include the natural selection sequence: variation, selection pressure, survival, reproduction, increased frequency.
- 💡Refer to bacteria as a population, not as one organism evolving.
- 💡Use the term random when describing mutation.
- 💡Distinguish clearly between mutation, which creates variation, and selection, which favours it.
- 💡Give a named example, such as a change in a target protein, to show how a new strain arises.
- 💡Use the sequence: variation, antibiotic selection pressure, survival, reproduction, increased frequency of resistance.
- 💡State clearly that resistant bacteria are not killed by the antibiotic.
- 💡Mention a practical consequence, such as difficulty treating infections, to show wider understanding.
- 💡Use the terms 'survive', 'reproduce' and 'population rises' in a logical sequence.
- 💡Refer to random mutation and natural selection rather than saying bacteria 'adapt' in a vague way.
- 💡Link the process to a real example such as MRSA to show understanding.
- 💡Separate the two reasons clearly: no human immunity and no effective treatment.
- 💡Use a named example such as MRSA to support your answer.
- 💡Mention a transmission route to show how the strain spreads.
- 💡State that MRSA is a bacterium and a strain of Staphylococcus aureus.
- 💡Mention that it is resistant to antibiotics and therefore hard to treat.
- 💡Link MRSA to hospital infections and infection control.
- 💡State clearly that antibiotics do not work on viruses.
- 💡Explain that inappropriate use increases the chance of resistant bacteria surviving.
- 💡Use examples such as treating a cold with antibiotics to show why it is inappropriate.
- 💡Use the terms mutation, natural selection, survival, reproduction and allele when explaining how resistance develops.
- 💡Link each stage of the explanation to the consequence for the patient or population, such as treatment failure or spread of resistance.
- 💡If asked to evaluate a patient's decision, state the decision, give the biological consequence and reach a clear judgement.
- 💡Name specific routes by which resistant bacteria can pass from farm animals to humans.
- 💡Use the phrase selection pressure when explaining why routine agricultural use increases resistance.
- 💡When evaluating a restriction, give one benefit and one cost, then state a supported conclusion.
- 💡Compare the timescale of antibiotic development with the timescale of bacterial reproduction and resistance emergence.
- 💡Use specific stages such as laboratory testing, animal testing and clinical trials when explaining why development is slow.
- 💡When asked to suggest a solution, link it to reducing selection pressure or to funding new drug development.
Common Mistakes
- Saying bacteria mutate because they need to survive; the correction is that mutations arise at random and selection then favours resistant cells.
- Confusing evolution with an individual bacterium changing during its lifetime; the correction is that the population changes over generations.
- Assuming all bacteria are identical; the correction is that mutations produce variation on which selection can act.
- Saying antibiotics cause the mutation; the correction is that mutations occur at random and antibiotics select resistant strains.
- Treating a strain as a different species; the correction is that a strain is a variant within a species.
- Believing all mutations are harmful; the correction is that some are neutral or beneficial to the bacterium.
- Saying the antibiotic makes bacteria resistant; the correction is that resistance arises from random mutation and the antibiotic selects resistant cells.
- Believing resistance develops in an individual bacterium during treatment; the correction is that the population changes over generations.
- Thinking antibiotics work against viruses; the correction is that antibiotics act on bacteria, not viruses.
- Thinking that the antibiotic causes the mutation: the mutation arises randomly before exposure, and the antibiotic selects survivors.
- Saying that bacteria 'learn' to become resistant: resistance is inherited through genes, not learned.
- Confusing the rise of a resistant strain with the rise of all bacteria: the total bacterial population may fall, but the resistant proportion rises.
- Confusing bacterial resistance with human immunity: people are not immune to the bacterium, and the bacterium is resistant to antibiotics.
- Assuming that a resistant strain is always more infectious: resistance to antibiotics does not automatically mean it spreads faster, but lack of treatment allows it to persist and spread.
- Forgetting that spread requires a route of transmission, such as contact or droplets.
- Thinking MRSA is a virus: it is a bacterium.
- Believing MRSA is resistant to all antibiotics: it is resistant to several, but some antibiotics may still be effective.
- Confusing MRSA with a human immune response: resistance is a property of the bacterium.
- Thinking antibiotics can treat viral infections: they cannot kill viruses.
- Believing that stopping a course early is always safe: patients should follow medical advice, and incomplete courses can contribute to resistance.
- Assuming that stronger or more antibiotics are always better: unnecessary antibiotics increase resistance without benefit.
- Thinking that the human body becomes resistant to antibiotics: the error is confusing the patient with the bacteria; the correction is that bacteria become resistant through mutation and natural selection.
- Believing that antibiotics kill viruses: the error is treating a viral infection with antibiotics; the correction is that antibiotics target bacterial structures and processes, so they are ineffective against viruses.
- Assuming that stopping early is safe once symptoms improve: the error is that symptoms can improve while resistant bacteria remain; the correction is that the full prescribed course is needed to reduce surviving bacteria.
- Thinking that agricultural restriction means animals cannot be treated for illness: the error is confusing restriction with a total ban; the correction is that antibiotics should still be used when an animal is genuinely ill, but not routinely or for growth promotion.
- Believing that resistance in farm animals cannot affect humans: the error is ignoring routes of transmission; the correction is that resistant bacteria and resistance genes can reach humans through food, water and contact.
- Assuming that only human medicine causes resistance: the error is overlooking agricultural selection pressure; the correction is that any use of antibiotics, including in farming, can select for resistant bacteria.
- Thinking that a new antibiotic can be created quickly once resistance appears: the error is underestimating development time; the correction is that discovery, testing and trials take many years.
- Believing that resistance only develops after a new drug is widely used: the error is ignoring that bacteria can already carry resistance genes; the correction is that resistance can emerge or spread rapidly once a drug is used.
- Assuming that cost is the only barrier: the error is overlooking scientific and regulatory challenges; the correction is that finding effective, safe compounds and completing trials are also major difficulties.