AQA GCSE Combined Science Foundation (8464), Foundation tier · Biology › Inheritance, variation and evolution › The development of understanding of genetics and evolution
Practise Resistant bacteria. 7 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How bacteria evolve resistance to antibiotics by natural selection, why resistant strains such as MRSA spread, and how to slow down the development of resistance. Very common: 2 to 6 mark 'explain' questions and data on resistant infections.
Key facts
Bacteria evolve rapidly because they reproduce at a fast rate
Mutation → new strain → some resistant → survive the antibiotic → reproduce → resistant population rises
Resistant strains spread: people are not immune and there is no effective treatment
MRSA = a bacterium resistant to antibiotics
Reduce resistance: no antibiotics for viral or non-serious infections; complete the course; restrict agricultural use
New antibiotics are costly and slow to develop, so they are unlikely to keep up
Notes
How resistance develops
diagram
Bacteria can evolve rapidly because they reproduce at a fast rate.
Mutations of bacterial pathogens produce new strains. Some strains might be resistant to antibiotics, and so are not killed.
When an antibiotic is used, the non-resistant bacteria are killed. The resistant bacteria survive and reproduce, so the population of the resistant strain rises. This is natural selection.
The resistant strain then spreads because people are not immune to it and there is no effective treatment.
MRSA is a bacterium that is resistant to antibiotics.
The antibiotic does not cause the mutation. It only kills the bacteria that are not resistant.
Resistance evolves by natural selection. The antibiotic does not cause the mutation.
Slowing the development of resistant strains
Doctors should not prescribe antibiotics inappropriately, such as for non-serious or viral infections. Antibiotics do not kill viruses.
Patients should complete their course of antibiotics, so all the bacteria are killed and none survive to mutate and form resistant strains.
The agricultural use of antibiotics should be restricted.
New antibiotics
The development of new antibiotics is costly and slow.
It is unlikely to keep up with the emergence of new resistant strains.
How to answer each type of question
Explain a way of reducing antibiotic resistance
2 marksGrade 5
Say what should be done (complete the course, don't use antibiotics for viral infections, restrict use in farming).
Explain why: all bacteria killed, or fewer chances for resistant strains to be selected.
Example. A doctor tells a patient to finish their course of antibiotics, even if they feel better. Explain why. [2 marks]
Show the model answerHide the model answer
So that all the bacteria are killed (1), and none survive to mutate and form resistant strains (1).
Explain why antibiotics should not be used for viral infections
2 marksGrade 5
Antibiotics do not kill viruses.
Unnecessary use kills non-resistant bacteria and leaves resistant bacteria to survive and reproduce.
Example. Explain why doctors should not prescribe antibiotics to treat a cold. [2 marks]
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Colds are caused by viruses, and antibiotics do not kill viruses (1). Using antibiotics when they are not needed increases the chance of antibiotic-resistant strains of bacteria developing (1).
Don’t lose marks
Saying bacteria 'become immune'. Bacteria become resistant; immunity is about your white blood cells.
Saying the antibiotic causes the mutation. The mutation happens by chance; the antibiotic kills the non-resistant bacteria.
Saying people become resistant to antibiotics. It is the bacteria that are resistant.
Saying antibiotics can treat viral infections such as colds and flu.
More tips
Memory tricks
Resistance in four steps: mutate, survive, reproduce, spread.
Three ways to slow it: don't use antibiotics for viruses, finish the course, restrict use on farms.
Antibiotics kill bacteria, not viruses.
Exam technique
Use 'resistant', not 'immune', for bacteria.
When explaining how resistance develops, always include both 'survive' and 'reproduce'.
If a graph shows resistant infections rising, link it to the use of antibiotics and natural selection.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name an antibiotic-resistant bacteriumMRSA is resistant to antibiotics.
Grade 4
State ways to reduce antibiotic resistanceDon't prescribe antibiotics for viral or non-serious infections, complete the course, and restrict their use in farming.
Grade 5
Explain why bacteria evolve rapidlyThey reproduce at a fast rate, so there are many generations in a short time.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
The statements describe how a strain of bacteria that is resistant to an antibiotic can develop and spread. They are in the wrong order. A The resistant bacteria survive and reproduce. B A mutation produces a bacterium that is resistant to the antibiotic. C The antibiotic kills the bacteria that are not resistant. D The population of the resistant strain rises. E The resistant strain spreads, because people are not immune to it and there is no effective treatment. Write the letters in the correct order.
B, C, A, D, E
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Some strains of bacteria are resistant to antibiotics.
(a) Which of these is a strain of bacteria that is resistant to many antibiotics? Tick (✓) one box.[1]
HIV
MRSA
Plasmodium
TMV
(b) Which two actions reduce the rate at which antibiotic-resistant strains develop? Tick (✓) two boxes.[2]
Doctors prescribing antibiotics for viral infections
Patients completing their course of antibiotics
Farmers routinely adding antibiotics to animal feed
Restricting the use of antibiotics in agriculture
Patients sharing leftover antibiotics with family members
(c) The development of new antibiotics is unlikely to keep up with the emergence of new resistant strains. Give one reason why.[1]
Show the answer and mark scheme
(a)Answer: MRSA
(b)Answer: Patients completing their course of antibiotics; Restricting the use of antibiotics in agriculture
(c)
developing new antibiotics is costly / expensive
developing new antibiotics is slow / takes many years
bacteria reproduce / evolve quickly
Question 2Medium8 marks
A patient in hospital had an infection caused by a strain of bacteria. The patient was treated with an antibiotic. Most of the bacteria were killed, but a few survived. The bacteria that survived were resistant to the antibiotic.
(a) Explain how a population of bacteria can become resistant to an antibiotic.[4]
(b) The resistant strain spread to other patients in the hospital. Explain why a resistant strain spreads easily.[2]
(c) Explain why patients should complete their course of antibiotics.[2]
Show the answer and mark scheme
(a)
(random) mutation (of bacterial DNA / genes)
produces a new strain / some bacteria that are resistant
when the antibiotic is used, non-resistant bacteria are killed but resistant bacteria survive
resistant bacteria reproduce (rapidly)
(so) the population of the resistant strain rises / the resistance gene is passed on
(b)
people are not immune to the (new) strain
there is no effective treatment (so infected people stay ill / infectious for longer)
(c)
so that all the bacteria are killed
(so) none survive to mutate and form resistant strains / fewer partly resistant bacteria survive to reproduce