B6.2 Antibiotics and Painkillers

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: what drugs are, how antibiotics treat bacterial infections, painkillers, and antibiotic resistance. 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 how antibiotics help to cure bacterial disease, and why they cannot treat viral disease
• Describe what painkillers and other medicines do
• Explain how antibiotic-resistant strains of bacteria arise and spread
• Describe how the spread of resistance can be reduced
• Compare the effectiveness of antibiotics using data

Key words

• Drug: Any substance that has a biological effect on the body. Medicines are drugs used to treat or prevent disease.
• Antibiotic: A medicine that kills bacteria (or stops them growing) inside the body, e.g. penicillin.
• Painkiller: A medicine, e.g. paracetamol or aspirin, that relieves symptoms but does not kill pathogens.
• Resistant: Not affected by an antibiotic; resistant bacteria survive treatment.
• Mutation: A random change in DNA. It can give a bacterium a new feature, such as resistance.
• Strain: A type of bacterium within a species that has particular features, e.g. MRSA.

Drugs and antibiotics

A drug is any substance that changes the way the body works. Some are medicines; others are recreational drugs, which may be legal (e.g. caffeine, nicotine) or illegal.

Antibiotics such as penicillin kill infective bacteria inside the body without harming the body’s own cells. Different antibiotics work in different ways: penicillin weakens the bacterial cell wall so the cell bursts; others block bacterial enzymes or stop bacteria reproducing. Specific bacteria should be treated with a specific antibiotic. Since the 1940s antibiotics have greatly reduced deaths from bacterial disease.

Worked example: comparing clear zones

The clear zone around disc D has a diameter of 18 mm. Area = πr² = π × 9² = 254 mm² (3 s.f.).
The zone around disc B has a diameter of 8 mm. Area = π × 4² = 50.3 mm².
Ratio of areas D : B = 254 ÷ 50.3 = 5.05, so D’s zone is about 5 times larger.

Answer: D is the most effective antibiotic against these bacteria

Why antibiotics do not work on viruses

Viruses reproduce inside the body’s own cells. Antibiotics have no effect on viruses, and it is difficult to develop drugs that kill viruses without also damaging body tissues. So antibiotics are not used for colds and flu, which are caused by viruses.

Key fact: Antibiotics kill bacteria only. They do not kill viruses.

Painkillers and other medicines

Painkillers and other medicines, such as cough mixtures, treat the symptoms of disease. They make the patient feel better but do not kill the pathogen; the immune system still has to destroy it.

• Aspirin; Notes Developed from a chemical in willow bark. Relieves pain, reduces fever and swelling; low doses reduce the risk of heart attacks.
• Paracetamol; Notes Mild painkiller in many cold and flu remedies. Taking more than the stated dose can cause serious liver damage.
• Morphine; Notes A strong painkiller; prescribed by doctors only.

Antibiotic resistance

Bacteria reproduce very quickly. A random mutation can make one bacterium resistant to an antibiotic. When the antibiotic is used, non-resistant bacteria are killed but the resistant one survives and reproduces, so the resistant strain spreads (natural selection). Resistant strains such as MRSA are very hard to treat, and few new antibiotics have been found in recent decades.

• Doctors do not prescribe antibiotics for viral or non-serious infections; Why it helps Fewer chances for resistant bacteria to be selected
• Patients complete the full course; Why it helps All the bacteria are killed, so none survive to form resistant strains
• Restrict antibiotic use in farming; Why it helps Less selection for resistance in farm animals
• Good hygiene in hospitals (hand washing, isolating patients); Why it helps Stops resistant strains such as MRSA spreading

Exam tip: Resistance develops in the bacteria, not in the person. Say “the bacteria become resistant”, never “the patient becomes resistant”.

Common mistakes

• Saying antibiotics can treat flu or colds. These are caused by viruses.
• Saying the antibiotic causes the mutation. Mutations are random; the antibiotic only kills the non-resistant bacteria.
• Saying the patient’s body becomes resistant. It is the bacteria that are resistant.
• Thinking painkillers cure disease. They only relieve symptoms.

Revision questions

Try these, then check your answers below each one.

Question 1. What do antibiotics do? (Choose from: Kill bacteria inside the body, Kill viruses inside body cells, Relieve pain only, Make the body immune.)

Answer: Kill bacteria inside the body

Working: Antibiotics kill infective bacteria (or stop them growing). They have no effect on viruses.

Question 2. True or false? Antibiotics can be used to cure flu.

Answer: False

Working: False. Flu is caused by a virus, and antibiotics do not affect viruses.

Question 3. The diagram shows an agar plate covered in bacteria with four paper discs, each soaked in a different antibiotic. Which antibiotic is most effective against these bacteria? (Choose from: A, B, C, D.)

Answer: B

Working: B has the largest clear zone, so it killed bacteria furthest from the disc. C has no clear zone: the bacteria are resistant to it.

Related lessons

• Previous lesson: B6.1 Vaccination
• Next lesson: B6.3 Discovering Drugs
• In this chapter: B6.4 Developing Drugs

Continue in the full course

This lesson is section B6.2 (AGCSC0602) 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.