OxfordAQA International GCSE Biology: Infection, immunity, vaccination and antibiotics (9201) – Revision Notes

These notes condense section 3.4.7 (Infection and response, parts a to j) of the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams May/June 2018 onwards, including the required practical on disinfectants and antibiotics. The qualification is untiered. The study guide has the full explanations and worked examples. Then try the practice questions. The course hub is OxfordAQA IGCSE Biology and the printable list is the checklist. Not sure where your gaps are? Take the free diagnostics.

Key definitions

Term	Meaning
Pathogen	A microorganism that causes infectious disease (3.4.7 a)
Toxin	A poison made by some bacteria that makes you feel ill
Phagocytosis	A white blood cell ingesting a pathogen
Antibody	A protein made by white blood cells that destroys one particular bacterium or virus
Antitoxin	A substance made by white blood cells that counteracts a toxin
Immunity	Protection from a pathogen because the body can make the correct antibody quickly
Vaccination	Putting small quantities of dead or inactive pathogen into the body to make you immune
Antibiotic	A medicine, such as penicillin, that kills infective bacteria inside the body
Mutation	A change that can make a new strain of a pathogen
MRSA	A bacterial strain resistant to antibiotics

Bacteria compared with viruses (3.4.7 b)

	Bacteria	Viruses
Where	Can reproduce rapidly inside the body	Live and reproduce inside cells
How they harm you	May produce toxins that make you feel ill	Reproducing damages the cell
Treated with antibiotics?	Yes, with a suitable antibiotic	No

Structure of viruses is not required.

Three white blood cell defences (3.4.7 c)

1. Phagocytosis: ingest the pathogen.
2. Antibodies: destroy particular bacteria or viruses.
3. Antitoxins: counteract toxins released by pathogens.

Memory hook: antibodies attack the body invader, antitoxins neutralise its poison.

Immunity and herd immunity (3.4.7 d)

• Antibodies are specific: one pathogen, one antibody.
• Making the right antibody leads to immunity.
• Dead or inactivated pathogens can stimulate antibody production.
• If a large proportion of the population is immune, the spread of the pathogen is very much reduced.

Vaccination (3.4.7 e)

Method in steps:

1. A small quantity of dead or inactive pathogen is injected.
2. The white blood cells are triggered to make antibodies.
3. The person becomes immune without having had the disease.
4. If the real pathogen arrives later, the correct antibody is made rapidly.

MMR protects against measles, mumps and rubella. Schedules and specific side effects are not required.

Evaluating vaccination

For	Against
Protects the individual	May not work for everyone
Builds up the immune proportion, protecting others	Possible reaction, such as soreness or fever
Prevents serious disease	A mutated strain may escape the vaccine

A strong evaluation uses both columns and ends with a judgement.

Antibiotics (3.4.7 f, g)

• Kill infective bacteria inside the body.
• Must be matched to the bacterium: specific antibiotic for specific bacteria.
• Greatly reduced deaths from bacterial disease.
• Cannot kill viruses.
• Difficult to make drugs that kill viruses without damaging body tissue.

Resistance (3.4.7 h, i, j)

Order of events:

1. Mutation produces a new strain with resistance.
2. Antibiotic kills non-resistant individuals.
3. Resistant individuals survive and reproduce.
4. The resistant strain increases in number.
5. Old antibiotics and vaccines may stop working on it.
6. It spreads rapidly because people are not immune and there is no effective treatment.

Overuse and inappropriate use increase the rate at which resistant strains develop. Antibiotics are not currently used for non-serious infections such as mild throat infections. New antibiotics have to be developed because resistant strains keep appearing.

Required practical in brief

Step	Reason
Aseptic technique and sterile equipment	Keeps the culture uncontaminated
Bacterial lawn on agar	Even growth, so clear zones can be seen
Discs with different antibiotics or disinfectants, plus a water control	Compares chemicals fairly
Lid loosely taped, plate upside down	Lets air in; condensation does not drip on the culture
Incubate at about 25 °C in school	Lower risk of growing harmful pathogens
Measure the clear zone	Larger zone means more effective

Area of zone = πr², where r is half the diameter. A control disc with no zone shows the liquid alone had no effect.

Small worked reminders

• Doubling: n divisions give a multiplier of 2ⁿ. A bacterium dividing every 20 minutes has 2⁶ = 64 descendants after 2 hours.
• Percentage fall: (old − new) ÷ old × 100. For 10 000 cases falling to 1500, (10 000 − 1500) ÷ 10 000 × 100 = 85%.
• Not immune: 1200 people with 90% immune leaves 10% of 1200 = 120 not immune.
• Zone area for a diameter of 10 mm: r = 5, so π × 5² = 78.5 mm² (1 d.p.).

Model answer shapes

Explain how a vaccine makes you immune. Dead or inactive pathogen is put into the body. It stimulates white blood cells to produce antibodies. The antibodies are specific to that pathogen. If the real pathogen enters later, the correct antibody is made rapidly, so you do not become ill.

Explain how an antibiotic-resistant strain spreads. Mutation produces a resistant bacterium. Non-resistant bacteria die when the antibiotic is used, while the resistant ones live on. They reproduce, so the resistant strain increases. People are not immune to it and there is no effective treatment, so it spreads rapidly.

Explain why herd immunity helps. A large proportion of the population is immune. An infected person meets few people who can catch the pathogen. The spread is very much reduced, which also protects people who are not immune.

Reading practical results

• No clear zone: the bacteria were not affected by that disc.
• Bigger clear zone: the chemical was more effective against that bacterium.
• Water control with no zone: the liquid on the disc did not kill the bacteria, so any zone elsewhere is due to the chemical.
• One bacterium only tested: do not claim the result applies to all bacteria.
• Repeating and averaging: use the mean zone area from repeat plates, and say which repeat plates were used.
• A disinfectant that makes a large zone is not a medicine: it is used on surfaces and equipment, not inside the body.

Command words

• State: give the fact with no explanation.
• Describe: set out what happens, step by step.
• Explain: give the reason, using because or so.
• Calculate: show the working and include units.
• Evaluate: weigh advantages and disadvantages, then judge.
• Suggest: use what you know in an unfamiliar context.

Must-know distinctions

• Antibody or antitoxin: antibody acts on the pathogen, antitoxin on its toxin.
• Vaccine or antibiotic: a vaccine prevents disease by building immunity, an antibiotic treats bacterial disease once you have it.
• Bacteria or viruses: antibiotics work on bacteria only.
• Resistant bacteria or resistant person: bacteria are resistant, people are immune.
• Disinfectant or antibiotic: a disinfectant is used on surfaces and equipment, an antibiotic is a medicine used inside the body.

Quick self-test

1. What word describes a microorganism that causes infectious disease?
2. Give one way bacteria harm you and one way viruses harm you.
3. Name the process in which a white blood cell takes in a pathogen.
4. What do antitoxins do?
5. Why does a dead pathogen in a vaccine still lead to immunity?
6. Name the three diseases the MMR vaccine protects against.
7. Why are antibiotics no use for a viral infection?
8. A bacterium divides every 20 minutes. How many bacteria come from one after 2 hours?
9. Why are antibiotics not used for a mild throat infection?
10. A clear zone has a diameter of 10 mm. What is its area to 1 decimal place?

Answers

1. Pathogen.
2. Bacteria may produce toxins. Viruses live and reproduce inside cells and damage them.
3. Phagocytosis.
4. They counteract the toxins released by pathogens.
5. It stimulates white blood cells to make antibodies, so the correct antibody can be made rapidly if the real pathogen arrives.
6. Measles, mumps and rubella.
7. Antibiotics cannot kill viral pathogens, which live inside body cells.
8. 6 divisions give 2⁶ = 64.
9. To slow the rate at which resistant strains develop.
10. π × 5² = 78.5 mm².

Last-day checks

• Can you say in one sentence what a pathogen is?
• Can you give the three white blood cell defences and what each acts on?
• Can you explain, step by step, how a vaccine gives immunity?
• Can you write the five-step story of how a resistant strain takes over?
• Can you name one reason antibiotics are not used for mild throat infections?
• Can you calculate a percentage change and a zone area without hesitating?

Where marks are usually lost

• Saying antibiotics kill viruses or work on every infection.
• Writing “the bacteria become immune” instead of “resistant”.
• Saying a vaccine contains the living disease rather than dead or inactive forms.
• Describing resistance without the sequence: mutation, survival, reproduction, rise of the resistant strain.
• Writing “antibodies kill toxins” or “antitoxins kill bacteria”.
• Evaluating vaccination with one side only.
• In the practical, forgetting a control disc or giving diameters when area is asked for.
• Leaving out units, for example mm² for area.

Official syllabus

OxfordAQA International GCSE Biology (9201), section 3.4.7 Infection and response and the required practical on disinfectants and antibiotics. Specification Version 5.2, for exams May/June 2018 onwards, published by OxfordAQA.