15.8 Antibiotic Resistant Bacteria

A free GCSE Biology lesson for AQA (8461) from OBA School: how resistant strains evolve and spread, MRSA, and how we can slow resistance down. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain why bacteria can evolve rapidly
• Describe step by step how an antibiotic-resistant strain develops and spreads
• Describe MRSA and why it is hard to treat
• Describe ways to reduce the rate at which resistant strains develop
• Explain why developing new antibiotics cannot keep up

Key words

• Antibiotic: A medicine, such as penicillin, that kills bacteria inside the body. Antibiotics do not work on viruses.
• Strain: A type of bacterium within a species that differs genetically from others, e.g. a resistant strain.
• Antibiotic resistance: When bacteria are no longer killed by an antibiotic that used to kill them.
• MRSA: Methicillin-resistant Staphylococcus aureus: a strain of bacteria resistant to many antibiotics.
• Binary fission: The way bacteria reproduce, by splitting into two. In good conditions this can happen every 20 minutes or so.

Bacteria evolve rapidly

Bacteria reproduce very quickly, so there are many generations in a short time. Each time DNA is copied there is a small chance of a mutation. With billions of bacteria, some will have new mutations, and a few of these may make a bacterium resistant to an antibiotic.

Worked example: how fast can bacteria multiply?

A bacterium divides every 20 minutes. How many bacteria are there after 4 hours, starting from one?
4 hours = 240 minutes = 240 ÷ 20 = 12 divisions.
Number = 2¹² = 4096.

Answer: 4096 bacteria (12 generations in 4 hours)

How a resistant strain develops and spreads

• 1. Mutations in bacterial DNA produce new strains. Some strains might be resistant to an antibiotic.
• 2. When the antibiotic is used, the non-resistant bacteria are killed. The resistant bacteria survive.
• 3. The survivors reproduce, so the population of the resistant strain rises.
• 4. The resistant strain spreads from person to person, because people are not immune to it and there is no effective treatment.

Key fact: The antibiotic does not cause the mutation. The mutation happens by chance; the antibiotic just kills the competition, so the resistant strain is selected.

The MRSA story

Staphylococcus aureus lives harmlessly on the skin and in the nose of many people. If it gets into a wound it can cause serious infections. Strains resistant to the antibiotic methicillin were first reported in the UK in 1961, soon after the antibiotic came into use. MRSA is now resistant to many antibiotics.

• MRSA spreads most easily in hospitals and care homes, where many people are ill, have wounds or weak immune systems, and antibiotics are used a lot.
• Hospitals reduce spread by hand washing and alcohol gel, cleaning wards, screening patients and isolating infected people.

Slowing down resistance

• Doctors should not prescribe antibiotics for non-serious or viral infections; Why it helps Antibiotics do not kill viruses. Unnecessary use gives resistant bacteria an advantage.
• Patients should complete the full course of antibiotics; Why it helps All the bacteria are killed, so none survive to mutate and form resistant strains.
• Restrict the use of antibiotics in farming; Why it helps Routine use in healthy animals selects for resistant strains, which can pass to humans.

New antibiotics are needed, but developing them is slow and expensive – it takes many years of testing. So new drugs are unlikely to keep up with the appearance of new resistant strains.

Common mistakes

• Saying the bacteria “become immune” or “get used to” the antibiotic. Resistance is caused by a mutation and selected by natural selection.
• Saying people become resistant to antibiotics. It is the bacteria that are resistant.
• Thinking antibiotics treat colds and flu. These are caused by viruses, which antibiotics cannot kill.
• Forgetting the spreading step: resistant strains spread because people have no immunity and there is no effective treatment.

Revision questions

Try these, then check your answers below each one.

Question 1. What does MRSA stand for? (Choose from: Methicillin-resistant Staphylococcus aureus, Multiple-resistant Streptococcus antibiotic, Methicillin-reactive Salmonella, Mutation-resistant Staphylococcus agent.)

Answer: Methicillin-resistant Staphylococcus aureus

Working: MRSA = methicillin-resistant Staphylococcus aureus.

Question 2. True or false? Antibiotics can be used to kill viruses that cause colds and flu.

Answer: False

Working: False. Antibiotics kill bacteria, not viruses.

Question 3. Which order correctly shows how a resistant strain develops? (Choose from: Mutation → antibiotic kills non-resistant bacteria → resistant bacteria reproduce → strain spreads, Antibiotic causes mutation → bacteria become immune → strain spreads → bacteria reproduce, Bacteria get used to the antibiotic → they reproduce → mutation happens → strain spreads, Strain spreads → mutation → antibiotic kills resistant bacteria → reproduction.)

Answer: Mutation → antibiotic kills non-resistant bacteria → resistant bacteria reproduce → strain spreads

Working: A random mutation gives resistance; the antibiotic selects it; survivors reproduce; the strain spreads.

Related lessons

• Previous lesson: 15.7 More About Extinction
• Next lesson: 15.9 Classification
• In this chapter: 15.1 The History of Genetics
• In this chapter: 15.2 Theories of Evolution
• In this chapter: 15.3 Accepting Darwin's Ideas

Continue in the full course

This lesson is section 15.8 (AGCBH1508) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.