Bacterial diseases — AQA GCSE Biology
Test yourself on Bacterial diseases with AQA GCSE practice questions.
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Bacterial diseases explained
Salmonella is a bacterium that causes food poisoning when it is swallowed in contaminated food or transferred onto food during unhygienic preparation.
Read the full explanation
The bacteria multiply in the gut and release toxins that irritate the digestive system, causing symptoms such as cramps, diarrhoea, vomiting and fever. Common sources include undercooked poultry, eggs and meat, while cross-contamination occurs when raw food touches cooked or ready-to-eat food, or when hands, boards and knives are not washed. Prevention therefore focuses on cooking food thoroughly, storing raw and cooked foods separately, washing hands and surfaces, and keeping food at safe temperatures. Most people recover, but severe cases can cause dehydration, so rehydration is important.
In the UK, poultry are vaccinated against Salmonella to control the spread.
Salmonella is a bacterium that can cause food poisoning in humans, typically after eating contaminated food such as undercooked chicken or eggs. In the UK, poultry such as chickens and turkeys are routinely vaccinated against Salmonella. Vaccination introduces a harmless form of the pathogen, so the bird's immune system produces antibodies and memory cells without becoming ill. If the bird later meets Salmonella, its immune response is rapid and the bacterium is destroyed, so fewer birds carry the pathogen. This reduces the chance that contaminated meat or eggs reach consumers, breaking the chain of transmission and controlling the spread of the disease. The vaccination programme is therefore a preventive control measure rather than a treatment for infected birds.
Fever, abdominal cramps, vomiting and diarrhoea are caused by the bacteria and the toxins they secrete.
When bacteria such as Salmonella enter the body, they multiply in the gut and release toxins. The toxins irritate the lining of the digestive system and affect the body's normal processes. This produces the symptoms of food poisoning: fever, abdominal cramps, vomiting and diarrhoea. Fever is a general response to infection as the body raises its temperature; cramps result from irritation and altered gut movement; vomiting and diarrhoea help expel the bacteria and toxins but also cause loss of water and ions. Understanding that both the bacteria and their toxins cause symptoms explains why food poisoning can continue even after the bacteria are killed, and why rehydration is an important part of care.
Gonorrhoea is a sexually transmitted disease (STD) with symptoms of a thick yellow or green discharge from the vagina or penis and pain on urinating.
Gonorrhoea is a bacterial sexually transmitted disease spread through unprotected sexual contact, including vaginal, oral and anal sex. The bacterium infects the moist linings of the reproductive tract. In many people it causes a thick yellow or green discharge from the vagina or penis, and pain when urinating. Some people, especially women, may have mild or no symptoms, so they can pass the infection on without knowing. Diagnosis involves testing a sample of the discharge or a swab, and treatment usually involves antibiotics. Because it is an STD, control depends on prompt treatment, avoiding unprotected sex while infected, and informing partners so they can also be tested and treated.
It is caused by a bacterium and was easily treated with the antibiotic penicillin until many resistant strains appeared.
Gonorrhoea is caused by a bacterium, so it can be treated with antibiotics. Penicillin was once a very effective treatment, and most infections were cleared easily. However, bacteria reproduce rapidly and mutations can arise that make some individuals resistant to penicillin. When antibiotics are used, non-resistant bacteria are killed while resistant bacteria survive and reproduce, passing on the resistance. Over time, resistant strains become more common, so penicillin is no longer a reliable treatment for gonorrhoea. This is an example of natural selection in bacteria and shows why antibiotics should be used carefully and why new antibiotics or different treatment combinations are needed.
Gonorrhoea is spread by sexual contact.
Gonorrhoea is a communicable disease caused by the bacterium Neisseria gonorrhoeae. It is transmitted when mucous membranes or body fluids from an infected person contact those of an uninfected person during sexual activity, including vaginal, anal and oral sex. The bacterium cannot survive long outside the body, so direct sexual contact is the main route. It does not spread by casual contact such as sharing cups or touching surfaces. Many infected people, especially women, show no symptoms, so they can pass it on unknowingly. Symptoms, when present, include a thick yellow or green discharge from the penis or vagina and a burning sensation when urinating. A concrete example is an uninfected person having unprotected sex with an infected partner; the bacteria transfer to the uninfected person's urethra or cervix and multiply there.
The spread can be controlled by treatment with antibiotics or the use of a barrier method of contraception such as a condom.
Because gonorrhoea is caused by a bacterium, it can be treated with antibiotics that kill the pathogen or stop its growth. Treating an infected person reduces the chance of passing the bacterium to others. Barrier methods of contraception, such as condoms, physically prevent the exchange of body fluids during sexual contact, so the bacterium cannot move from an infected person to an uninfected person. Using a condom correctly every time reduces transmission. A concrete example is a person diagnosed with gonorrhoea who completes a prescribed course of antibiotics and avoids unprotected sex until cleared; their partner uses condoms to avoid reinfection. Both treatment and barrier methods together give stronger control than either alone.
Your focus
- State that Salmonella is a bacterium spread by contaminated food.
- Describe how unhygienic preparation causes cross-contamination.
- Explain how cooking and hygiene measures reduce the risk of food poisoning.
Show all 21 objectives
- State that Salmonella is a bacterium that causes food poisoning and that UK poultry are vaccinated against it.
- Explain how vaccination makes poultry immune and reduces the number of birds carrying Salmonella.
- Relate reduced carriage in poultry to lower risk of human infection and control of spread.
- List the symptoms of food poisoning caused by bacteria such as Salmonella.
- Explain how bacteria and the toxins they secrete produce fever, cramps, vomiting and diarrhoea.
- Recognise that vomiting and diarrhoea can cause dehydration and that rehydration is important.
- Define gonorrhoea as a bacterial STD and describe its main symptoms.
- Explain how gonorrhoea is transmitted and why asymptomatic carriers matter for control.
- Outline measures that reduce the spread of gonorrhoea, including treatment and partner notification.
- State that gonorrhoea is caused by a bacterium and was treated with penicillin.
- Explain how mutation and natural selection lead to penicillin-resistant strains of gonorrhoea bacteria.
- Discuss the consequences of antibiotic resistance for treating gonorrhoea and the need for alternative treatments.
- Identify gonorrhoea as a bacterial disease spread by sexual contact.
- Describe how the bacterium is transmitted from one person to another.
- Distinguish sexual transmission from casual contact routes.
- Describe how antibiotics treat gonorrhoea.
- Explain how barrier methods such as condoms reduce transmission.
- Compare the roles of treatment and prevention in controlling the spread.
Bacterial diseases exam tips
Marking Points
- Salmonella is a bacterium that causes food poisoning.
- Infection occurs when contaminated food containing the bacteria is eaten.
- Unhygienic preparation can transfer bacteria onto food, for example via hands, utensils or surfaces.
- Symptoms include diarrhoea, vomiting, cramps and fever caused by toxins.
- Prevention includes thorough cooking, hand washing, separating raw and cooked food and safe storage.
- Salmonella is a bacterium that causes food poisoning in humans, often linked to eating contaminated poultry or eggs.
- Vaccination gives poultry immunity without causing the disease, because the vaccine contains a harmless form of the pathogen.
- Immune memory means vaccinated birds destroy Salmonella quickly on later exposure, so they are less likely to carry and shed the bacterium.
- Fewer infected birds means less contaminated meat and eggs reach consumers, reducing human cases and controlling spread.
- Vaccination is a preventive control measure applied to the poultry population, not a cure for birds already infected.
- Bacteria such as Salmonella multiply in the digestive system and secrete toxins.
- Toxins irritate the gut lining and disrupt normal digestion, contributing to cramps, vomiting and diarrhoea.
- Fever is a response to infection and the presence of toxins in the body.
- Vomiting and diarrhoea are symptoms caused by the infection and toxin action, and they can lead to dehydration.
- Both the bacteria themselves and the toxins they secrete are responsible for the symptoms, not one alone.
- Gonorrhoea is caused by a bacterium and is transmitted sexually, so it is classified as a sexually transmitted disease (STD).
- Typical symptoms include a thick yellow or green discharge from the vagina or penis.
- Another typical symptom is pain on urinating.
- Transmission occurs through unprotected sexual contact, and an infected person may be asymptomatic and still pass on the bacterium.
- Control involves prompt antibiotic treatment, partner notification and avoiding unprotected sexual contact during infection.
- Gonorrhoea is caused by a bacterium, so antibiotics such as penicillin can kill the pathogen.
- Penicillin was originally an effective and easily used treatment for gonorrhoea.
- Mutations can produce bacteria that are resistant to penicillin.
- Antibiotic use selects for resistant bacteria, which survive and reproduce, so resistant strains become more common.
- Resistance means penicillin is no longer reliably effective, so alternative antibiotics or treatment combinations are needed.
- Gonorrhoea is caused by a bacterium, not a virus or fungus.
- Transmission occurs through sexual contact, including vaginal, anal and oral sex.
- The pathogen is passed from an infected person to an uninfected person during sexual activity.
- It is not spread by casual contact such as sharing food, touching surfaces or airborne droplets.
- Asymptomatic carriers can transmit the infection without knowing they are infected.
- Antibiotics are effective because gonorrhoea is caused by a bacterium.
- Antibiotics kill the bacterium or inhibit its growth, reducing the source of infection.
- Barrier methods such as condoms physically prevent sexual transmission.
- Correct and consistent use of a condom is needed for effective control.
- Combining treatment with barrier contraception reduces the spread more than either measure alone.
Examiner Tips
- 💡Name a specific food source such as undercooked chicken or eggs to make the answer concrete.
- 💡Use the term cross-contamination when explaining how bacteria reach ready-to-eat food.
- 💡Link each symptom to the effect of bacterial toxins on the digestive system.
- 💡Link the vaccination of poultry directly to reduced human food poisoning, not just to healthier birds.
- 💡Use the terms immune system, antibodies and memory cells to show the biological mechanism.
- 💡If asked to evaluate control measures, compare vaccination with hygiene measures such as cooking food thoroughly and refrigerating it correctly.
- 💡Name the four symptoms accurately and link each to bacterial infection or toxin action.
- 💡Use the word toxins when explaining how bacteria cause harm, as this is a key idea in this section.
- 💡When describing treatment, mention rehydration because vomiting and diarrhoea cause loss of water and ions.
- 💡State the two named symptoms precisely: discharge from the vagina or penis, and pain on urinating.
- 💡Link the sexual transmission route to control measures such as using protection and treating partners.
- 💡Use the abbreviation STD correctly and be ready to explain what it means.
- 💡Explain resistance using the sequence: mutation, antibiotic selection, survival and reproduction of resistant bacteria.
- 💡Avoid saying that antibiotics create mutations; mutations occur naturally and the antibiotic selects the resistant bacteria.
- 💡Link resistance to the need for careful antibiotic use and the development of new treatments.
- 💡State the pathogen type clearly: gonorrhoea is bacterial, so antibiotics can treat it.
- 💡Use the phrase 'sexual contact' rather than vague terms like 'contact' when describing transmission.
- 💡Link the mode of transmission to control measures: because it spreads sexually, barrier contraception reduces risk.
- 💡Name the specific barrier method, for example a condom, rather than saying 'contraception' alone.
- 💡Explain the mechanism: antibiotics kill bacteria; condoms stop body fluid contact.
- 💡Use the word 'control' rather than 'cure' when describing measures that reduce spread.
Common Mistakes
- Calling Salmonella a virus; correction: Salmonella is a bacterium, so it can be treated with antibiotics in serious cases and is controlled by hygiene and cooking.
- Thinking food poisoning is caused by food going off only; correction: Salmonella can be present in food that looks and smells normal, so thorough cooking is essential.
- Ignoring cross-contamination; correction: bacteria can transfer from raw meat to salad via a chopping board, so separate boards and washing are needed.
- Thinking the vaccine treats a bird that already has Salmonella; correction: vaccination prevents disease by preparing the immune system before exposure.
- Confusing vaccination with antibiotic treatment; correction: vaccines stimulate immunity, whereas antibiotics kill bacteria in an infected individual.
- Assuming vaccination guarantees zero Salmonella in all poultry products; correction: it greatly reduces the risk but hygiene and cooking remain important.
- Saying only the bacteria cause symptoms and ignoring toxins; correction: both the bacteria and the toxins they secrete cause the symptoms.
- Confusing a symptom with a cause, for example calling diarrhoea the cause of food poisoning; correction: diarrhoea is a symptom caused by the infection and toxins.
- Believing fever is caused directly by the food itself; correction: fever is a response to the bacterial infection and its toxins.
- Thinking gonorrhoea is caused by a virus; correction: it is caused by a bacterium, which is why antibiotics can be used.
- Assuming everyone with gonorrhoea has obvious symptoms; correction: some people have mild or no symptoms but can still transmit the infection.
- Believing gonorrhoea can be spread by casual contact such as sharing food; correction: it is spread sexually, through unprotected sexual contact.
- Saying the person becomes resistant to the antibiotic; correction: it is the bacteria that become resistant, not the human.
- Thinking resistance develops because the body gets used to the antibiotic; correction: resistance arises from random mutation followed by natural selection when the antibiotic is used.
- Believing antibiotics work against all infections; correction: antibiotics kill bacteria, not viruses, and resistance can reduce their effectiveness against some bacteria.
- Thinking gonorrhoea is caused by a virus; the correction is that it is caused by the bacterium Neisseria gonorrhoeae.
- Believing it can be caught from toilet seats or shared towels; the correction is that it is spread by sexual contact, not by casual contact.
- Assuming a person must have symptoms to pass it on; the correction is that asymptomatic people can still transmit the infection.
- Suggesting antibiotics work against viruses; the correction is that antibiotics target bacteria such as Neisseria gonorrhoeae.
- Thinking any contraception prevents gonorrhoea; the correction is that only barrier methods such as condoms block transmission, while hormonal methods do not.
- Believing one treatment is enough without preventing reinfection; the correction is that barrier methods are also needed to stop reinfection and spread.