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

This guide covers 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. The qualification is untiered, so every candidate meets all of it. It includes the required practical on disinfectants and antibiotics. Use it with the revision notes and the practice questions. The course hub is OxfordAQA IGCSE Biology and the printable list is the checklist. To find weak spots first, try the free diagnostics.

What you need to be able to do

Section	You must be able to
3.4.7 a	Define pathogens as microorganisms that cause infectious disease
3.4.7 b	Explain how bacteria and viruses make you ill (structure of viruses is not required)
3.4.7 c	Describe phagocytosis, antibodies and antitoxins
3.4.7 d	Explain specific antibodies, immunity and why a high proportion of immune people slows spread
3.4.7 e	Explain vaccination, name the MMR diseases, evaluate being vaccinated
3.4.7 f, g	Explain what antibiotics treat, why they must match the bacterium, and why they do not work on viruses
3.4.7 h, i, j	Explain mutation, resistant strains, MRSA, overuse, and why new antibiotics are needed
Required practical	Investigate disinfectants and antibiotics on uncontaminated cultures of microorganisms

Vaccination schedules and the side effects of specific vaccines are not required.

3.4.7 a and b: pathogens and how they harm you

A pathogen is a microorganism that causes infectious disease. Two kinds matter here: bacteria and viruses.

Bacteria and viruses can both reproduce rapidly inside the body. They harm you in different ways.

• Bacteria can produce poisons called toxins. The toxins are what make you feel ill.
• Viruses live and reproduce inside your cells. Making new viruses damages the cell.

You do not need to know the structure of a virus. You do need the contrast: bacteria make toxins, viruses damage the cells they live in.

Worked example 1: how fast can bacteria multiply? Bacteria reproduce by dividing in two. Suppose a wound contains 5 bacteria and each one divides every 30 minutes. How many are there after 6 hours?

1. Number of divisions = 6 hours ÷ 0.5 hours = 12.
2. Each division doubles the number, so the multiplier is 2¹² = 4096.
3. Number of bacteria = 5 × 4096 = 20 480.

The lesson is speed. A small number of bacteria becomes a large number within hours, which is why the body needs a fast defence and why early treatment matters.

3.4.7 c: how white blood cells defend you

White blood cells help to defend against pathogens in three ways.

Defence	What happens	Works against
Phagocytosis	The white blood cell ingests (takes in) the pathogen	Pathogens in general
Antibodies	Proteins that destroy particular bacteria or viruses	One specific pathogen each
Antitoxins	Counteract the toxins released by pathogens	Toxins, mostly from bacteria

Do not mix up the last two. Antibodies act on the pathogen. Antitoxins act on the poison the pathogen releases. A good answer names the defence and says what it acts on.

3.4.7 d: specific antibodies, immunity and herd immunity

Your immune system makes specific antibodies: an antibody that destroys one pathogen will not destroy a different one. Making the right antibody leads to immunity from that pathogen. In some cases, dead or inactivated pathogens are enough to stimulate antibody production, which is the idea behind vaccines.

If a large proportion of the population is immune to a pathogen, its spread is very much reduced. This is often called herd immunity. Each infected person meets mostly immune people, so the pathogen finds it hard to move on. People who are not immune, for example because they cannot be vaccinated, gain some protection.

Worked example 2: herd immunity. A town of 8000 people has a vaccination programme and 95% are immune to a disease.

1. Immune people = 0.95 × 8000 = 7600.
2. Not immune = 8000 − 7600 = 400.
3. Only 5% of the people an infected person meets can catch the disease, so the spread is very much reduced.

3.4.7 e: vaccination

People can be immunised by putting small quantities of dead or inactive forms of the pathogen into the body. This is vaccination. The vaccine stimulates white blood cells to produce antibodies that destroy the pathogen.

Later, if the real pathogen enters, the body can rapidly make the correct antibody, in the same way as if you had already had the disease. You become immune without having been ill.

The MMR vaccine protects children against measles, mumps and rubella.

Evaluating vaccination

You may be asked to evaluate the advantages and disadvantages of being vaccinated against a particular disease. Give both sides and then reach a judgement.

Advantages	Disadvantages
Protects you from the disease	A vaccine does not always work, so some people stay unprotected
Raises the proportion of immune people, which slows spread and protects those who cannot be vaccinated	A person may have a reaction to a vaccine, such as soreness or a fever
Prevents serious illness and the effects of the toxins or cell damage	The pathogen may mutate, so the vaccine may no longer work on a new strain

Keep side-effect points general. The specification does not require the side effects of particular vaccines.

Worked example 3: judging a programme from data. In a region, cases of a disease were 2400 per 100 000 people in the year before a vaccination programme and 150 per 100 000 in the year after.

1. Fall in cases = 2400 − 150 = 2250 per 100 000.
2. Percentage fall = 2250 ÷ 2400 × 100 = 93.75%, so about 94%.
3. Judgement: the data support the programme. A single year does not prove the vaccine alone caused the fall, because other changes might have happened, so you would want several years of data.

3.4.7 f and g: antibiotics

Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body. Their use has greatly reduced deaths from infectious bacterial diseases.

Two rules follow.

• Specific bacteria should be treated with specific antibiotics. An antibiotic that works on one type of bacterium may not work on another.
• Antibiotics cannot kill viral pathogens. So an antibiotic is no use for a cold or flu. It is difficult to develop drugs that kill viruses without also damaging the body’s tissues, because viruses live inside your own cells.

3.4.7 h: mutation and new strains

Pathogens mutate, and mutation produces new strains. Think about what happens when an antibiotic is used.

1. A population of bacteria contains a few individuals that are resistant to the antibiotic because of a mutation.
2. The antibiotic kills the individuals of the non-resistant strain.
3. The individual resistant bacteria survive and reproduce.
4. The population of the resistant strain rises.

Antibiotics and vaccinations may no longer be effective against a new resistant strain. The new strain spreads rapidly because people are not immune to it and there is no effective treatment. For this specification, knowledge of how resistance develops is limited to the fact that pathogens mutate, producing resistant strains.

Worked example 4: how a rare resistant strain takes over. A patient has 2 million bacteria and 2 of them are resistant. An antibiotic kills all of the non-resistant bacteria. The 2 survivors divide every 20 minutes for 5 hours.

1. Divisions = 5 × 60 ÷ 20 = 15.
2. Multiplier = 2¹⁵ = 32 768.
3. Resistant bacteria = 2 × 32 768 = 65 536.

A strain that began as 1 in a million is now the whole infection, and the same antibiotic will not remove it.

3.4.7 i and j: MRSA, overuse and new antibiotics

Many strains of bacteria, including MRSA, have developed resistance to antibiotics. Overuse and inappropriate use of antibiotics has increased the rate at which resistant strains develop. Each time an antibiotic is used, any resistant bacteria are left with less competition and can reproduce.

For this reason antibiotics are not currently used to treat non-serious infections such as mild throat infections. This slows the rate at which resistant strains develop.

Because resistant strains keep appearing, new antibiotics have to be developed. A good answer to “why do we need new antibiotics?” links the steps: overuse, resistant strain, old antibiotic fails, new antibiotic needed.

Required practical: disinfectants and antibiotics on cultures

You investigate the effect of disinfectants and antibiotics on uncontaminated cultures of microorganisms. A typical method is below. Follow your own teacher’s safety instructions.

1. Use sterile equipment and aseptic technique so that no other microorganisms get in. A flame-sterilised loop, a sterile spreader and sterile agar are common.
2. Spread the bacterium evenly over the agar to make a lawn.
3. Place sterile paper discs soaked in different antibiotics or disinfectants on the lawn. Include a control disc soaked in sterile water.
4. Fit the lid loosely, for example with a couple of small pieces of tape, so that air can enter. Do not seal it fully.
5. Incubate the plate upside down. School work is normally done at a temperature of about 25 °C, which is below human body temperature, so that dangerous pathogens are less likely to grow.
6. After the incubation time, measure the diameter of each clear zone around a disc.

A clear zone means the bacteria were killed or could not grow near that disc. A larger zone means the chemical was more effective against that bacterium.

Worked example 5: comparing zones. Antibiotic discs P and Q gave clear zones of diameter 16 mm and 9 mm. Disc R gave no zone. Compare the antibiotics using area.

1. Area = πr², and r is half the diameter.
2. P: r = 8 mm, area = π × 8² = 201.1 mm² (1 d.p.).
3. Q: r = 4.5 mm, area = π × 4.5² = 63.6 mm² (1 d.p.).
4. P’s zone has about 3.2 times the area of Q’s.
5. R shows no zone, so the bacteria were not affected by it.

Conclusion: P was the most effective of the three against this bacterium. You cannot say P is best against all bacteria, because only one was tested.

Reasons for variation to control: the same bacterium, the same agar depth, the same disc size and volume of liquid, the same incubation time and temperature. Risk assessment matters because cultures could contain harmful microorganisms: wash hands, disinfect the bench, and do not open sealed plates.

Common errors

• Writing that antibiotics kill viruses, or that they “kill the infection” for any disease.
• Saying bacteria “become resistant” as if each one chooses to. A mutation produces resistant individuals, and these survive and reproduce.
• Saying people “become resistant”. It is the bacteria that are resistant.
• Mixing up antibodies and antitoxins.
• Writing that vaccines contain the live disease. They contain dead or inactive forms.
• Giving only advantages when asked to evaluate.
• Comparing zones by diameter when the question asks for area.

Next steps

Test yourself with the practice questions, then use the revision notes close to the exam.

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.