Biology · Evolution and Natural Selection
Antibiotic Resistance
Natural selection is happening right now, in the bacteria that cause disease. This lesson explains how antibiotic-resistant bacteria evolve, why this is a major threat to health, and what can be done about it.
Learning Objectives
- 1Explain how the emergence of antibiotic-resistant bacteria supports Darwin's theory of evolution.
- 2Describe the stages by which a population of bacteria becomes resistant.
- 3Explain why antibiotic resistance is a problem, and describe what can be done to slow it down.
- 4Use data and graphs to interpret rates of resistance.
Retrieval practice
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1
What is natural selection?
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The process by which better adapted individuals survive and reproduce, passing on their alleles.
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2
What is a mutation?
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A random change in the DNA base sequence.
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3
What are antibiotics used to treat?
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Bacterial infections.
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4
What is a pathogen?
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A microorganism that causes disease.
Evolution you can watch
Evolution usually takes thousands of years, but bacteria reproduce so fast that it can be seen within a few years. Antibiotics are drugs that kill bacteria or stop them growing, and for decades they have been one of the greatest medical successes. But bacteria evolve resistance to them, and so some antibiotics no longer work. Doctors now describe antibiotic resistance as one of the biggest threats to human health, and it is a clear example of natural selection happening today.
How resistance evolves
The same steps as natural selection, with the antibiotic as the selection pressure.
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1
Variation
A population of bacteria contains variation. By chance, a mutation gives one bacterium resistance to the antibiotic.
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2
Antibiotic used
The antibiotic is taken, and kills most of the non-resistant bacteria.
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3
Survival
The resistant bacterium survives, since the antibiotic cannot harm it.
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4
Reproduction
It reproduces rapidly, dividing about every 20 minutes, and its offspring all carry the allele for resistance.
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5
A resistant population
Soon the population is made mainly of resistant bacteria, and the antibiotic no longer works against them.
How a population becomes resistant
The antibiotic does not cause the mutation. It selects the bacteria that already have it.
How fast can bacteria multiply?
A single resistant bacterium divides every 20 minutes. Calculate how many bacteria there will be after 4 hours, assuming that every cell divides each time and none die.
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- 1 Find the number of divisions 4 hours is 240 minutes, and \(240 \div 20 = 12\) divisions.
- 2 Use the doubling pattern The number doubles each time, so it is \(2^{12}\).
- 3 Calculate \(2^{12} = 4096\) bacteria.
Answer4096 bacteria.
Why resistance matters
Antibiotic resistance threatens many of the treatments that we rely on.
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MRSA
Methicillin-resistant Staphylococcus aureus is resistant to many antibiotics. It causes serious infections in hospitals, especially in surgical wounds.
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Tuberculosis
Some strains of the bacteria that cause TB are resistant to several drugs, and treatment takes many months with a combination of medicines.
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Operations at risk
Surgery, cancer treatment and transplants all rely on antibiotics to prevent infection, so they become more dangerous as resistance spreads.
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New antibiotics are slow to develop
It takes many years and costs a great deal to develop a new antibiotic, and few have been discovered recently.
Slowing down resistance
Resistance cannot be stopped, but it can be slowed by using antibiotics carefully.
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Not for viral infections
Antibiotics do not work against viruses, so they should not be prescribed for colds and flu.
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Finish the course
Patients should complete the course of antibiotics, so that all of the bacteria are killed and not just the weakest.
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Restricting use
Doctors should avoid prescribing antibiotics when they are not needed, and farmers should not use them routinely on healthy animals.
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Hygiene
Handwashing and cleanliness in hospitals reduce the spread of resistant bacteria such as MRSA.
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New antibiotics
Research continues into new antibiotics, and into other ways of fighting bacteria.
Resistant and non-resistant bacteria
Non-resistant bacteria
- Killed or stopped by the antibiotic
- Make up most of the population at first
- Cannot survive when the antibiotic is used
- Do not pass on resistance
Resistant bacteria
- Not harmed by the antibiotic
- Very rare in the population at first
- Survive and reproduce when the antibiotic is used
- Pass on the allele for resistance, which spreads
Case study
MRSA in hospitals
MRSA is a strain of Staphylococcus aureus that is resistant to many antibiotics, including methicillin. It lives on the skin of healthy people, but if it gets into a wound or the blood of a hospital patient it can cause a serious infection that is hard to treat. Hospitals try to control it by screening patients, isolating those with MRSA, and by making sure that staff wash their hands and use alcohol gel. Rates of MRSA in UK hospitals fell sharply after these measures were introduced in the 2000s.
Summary and exam focus
- Antibiotic resistance arises by mutation, which gives variation in a population of bacteria.
- When an antibiotic is used, the resistant bacteria survive and reproduce, so the resistance spreads.
- This is natural selection, and it supports Darwin's theory.
- MRSA and drug-resistant TB are examples that are difficult to treat.
- Resistance is slowed by using antibiotics carefully, finishing courses and good hygiene.
Exam focus
Explain how a population of bacteria can become resistant to an antibiotic, and describe two ways in which the development of resistance can be slowed. (6 marks) (6 marks)
Use the steps of natural selection in order: mutation, antibiotic as the selection pressure, survival and reproduction, and the passing on of the allele. Then give two clear actions, such as finishing a course and not prescribing antibiotics for viral infections, with the reason for each.
Key terms
The words this lesson expects you to use. Each one is linked from the first place it appears above.
- Antibiotic
- A drug that kills bacteria or stops them from growing.
- Antibiotic resistance
- The ability of bacteria to survive an antibiotic that would normally kill them.
- MRSA
- A strain of bacteria that is resistant to many antibiotics, including methicillin.
- Selection pressure
- A factor, such as an antibiotic, that decides which individuals survive.
- Strain
- A genetic variety of a species of microorganism.
- Pathogen
- A microorganism that causes disease.
- Resistant
- Not affected by something, such as an antibiotic.
- Course of treatment
- The full set of doses that a doctor tells a patient to take.
- Mutation
- A random change in the DNA of an organism.
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