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B14.4 Antibiotic Resistant Bacteria

GCSE Combined Science · AQA · Chapter B14: Genetics and Evolution

A free GCSE Combined Science: Trilogy lesson for AQA (8464) 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
  • Explain why antibiotic resistance is evidence for evolution
  • Describe MRSA and how hospitals reduce its spread
  • Describe ways to reduce the development of resistant strains, and why 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.
  • Mutation: A random change in the DNA. Bacteria mutate often because they reproduce so quickly.
  • 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.

Bacteria evolve rapidly

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

Worked example: how fast can bacteria multiply?

A bacterium divides every 25 minutes. How many bacteria are there after 3 hours 20 minutes, starting from one?
3 hours 20 minutes = 200 minutes; 200 ÷ 25 = 8 divisions.
Number = 2⁸ = 256.

Answer: 256 bacteria (8 generations)

Key fact: Because bacteria evolve so fast, we can watch natural selection happening within a few years. This is strong evidence for evolution.

How a resistant strain develops and spreads

  • 1. Mutations in bacterial DNA produce new strains. By chance, some strains are resistant to an antibiotic.
  • 2. When the antibiotic is used, the non-resistant bacteria are killed. The resistant bacteria survive.
  • 3. The survivors reproduce, passing on the resistance gene, so the population of the resistant strain rises.
  • 4. The resistant strain spreads because people are not immune to it and there is no effective treatment.

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

MRSA in hospitals

Staphylococcus aureus lives harmlessly on the skin and in the noses of many people, but it can cause serious infections if it gets into a wound. MRSA is resistant to methicillin and to many other antibiotics, so it is very hard to treat.

MRSA is most dangerous in hospitals, where patients have open wounds and weakened immune systems, and many antibiotics are used. Hospitals have reduced MRSA infections by strict hygiene (hand washing, alcohol gel, cleaning), screening and isolating infected patients, and using antibiotics more carefully.

Slowing down resistance

  • Doctors should not prescribe antibiotics for viral or non-serious 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.
  • The agricultural use of antibiotics should be restricted; Why it helps Giving antibiotics to healthy farm 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. New drugs are unlikely to keep up with the appearance of new resistant strains.

Common mistakes

  • Saying bacteria “become immune” or “get used to” the antibiotic. Resistance comes from a random mutation and is 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.
  • Leaving out 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, Multi-resistant Streptococcus agent, Methicillin-reactive Salmonella, Mutated resistant Staphylococcus antibiotic.)

Answer: Methicillin-resistant Staphylococcus aureus

Working: MRSA = methicillin-resistant Staphylococcus aureus, a strain of bacteria resistant to many antibiotics.

Question 2. The diagram shows a population of bacteria before (stage 1) and after (stages 2 and 3) an antibiotic is used. Which statement explains the change from stage 2 to stage 3? (Choose from: The surviving type P bacteria are resistant; they reproduce and pass on the resistance gene, The antibiotic caused type T bacteria to mutate into type P, Type T bacteria became used to the antibiotic, The patient became immune to the antibiotic.)

Answer: The surviving type P bacteria are resistant; they reproduce and pass on the resistance gene

Working: Type P bacteria already had a mutation giving resistance. The antibiotic killed type T, and the type P survivors reproduced, so the whole population became resistant.

Question 3. True or false? Antibiotics should be given to people with colds and flu to kill the viruses.

Answer: False

Working: False. Antibiotics kill bacteria, not viruses. Using them for viral infections does nothing to help and increases the chance of resistant bacteria.

Related lessons

Continue in the full course

This lesson is section B14.4 (AGCSC1404) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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