5.7 Bacterial Diseases

A free GCSE Biology lesson for AQA (8461) from OBA School: salmonella food poisoning and gonorrhoea: symptoms, spread, treatment and control. 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

• Describe the symptoms of Salmonella food poisoning and explain what causes them
• Explain how the spread of Salmonella is controlled, from farm to plate
• Describe the symptoms, spread, treatment and control of gonorrhoea
• Explain why gonorrhoea is now harder to treat with antibiotics

Key words

• Salmonella: A type of bacterium that causes food poisoning.
• Toxin: A poison released by bacteria that damages tissues and causes symptoms.
• Gonorrhoea: A sexually transmitted disease (STD) caused by a bacterium.
• STD: Sexually transmitted disease: a communicable disease spread by sexual contact.
• Resistant: Describes bacteria that are no longer killed by a particular antibiotic.
• Barrier method: A form of contraception, such as a condom, that stops body fluids passing between partners.

Salmonella food poisoning

Salmonella bacteria live in the guts of many animals, including chickens. They are spread when people eat food containing the bacteria, or food prepared in unhygienic conditions. Undercooked chicken and eggs are common sources.

Symptoms usually start between 12 hours and 3 days after eating the food:

• fever
• abdominal (stomach) cramps
• vomiting
• diarrhoea

Key fact: These symptoms are caused by the bacteria and the toxins they secrete. Most people recover in about a week. It can be serious in babies, older people and people with weak immune systems.

Controlling the spread of Salmonella

In the UK, poultry are vaccinated against Salmonella, so fewer chickens carry it and fewer eggs and chicken products contain it. Good food hygiene breaks the chain at every step.

Cases are usually higher in warm months. Bacteria grow faster in warm food, and barbecues and picnics make it harder to keep raw and cooked food apart and cold.

Worked example: how fast bacteria grow at room temperature

A chicken salad contains 25 Salmonella bacteria. On a warm day they divide every 30 minutes. How many are there after 2 hours 30 minutes?
150 ÷ 30 = 5 divisions. 25 × 2⁵ = 25 × 32 = 800.

Answer: 800 bacteria

Gonorrhoea

• Symptoms; Gonorrhoea A thick yellow or green discharge from the vagina or penis, and pain on urinating. Some people have no symptoms, so they may pass it on without knowing. If untreated it can cause infertility.
• How it spreads; Gonorrhoea By sexual contact.
• Treatment; Gonorrhoea Antibiotics. It was originally treated with penicillin, until many resistant strains appeared. Doctors now use other antibiotics.
• Control; Gonorrhoea Treating infected people (and their partners) with antibiotics, and using a barrier method of contraception such as a condom. Testing helps find people without symptoms.

Key fact: When an antibiotic is used a lot, any bacteria that happen to be resistant survive and reproduce. Over time most of the population becomes resistant. This is why penicillin no longer works for most gonorrhoea.

Some bacteria also infect plants. For example, crown gall disease causes large growths (galls) on plant stems and roots.

Common mistakes

• Saying the symptoms of Salmonella are caused only by the bacteria. The toxins they secrete also cause symptoms.
• Saying "the Salmonella vaccine is given to people". In the UK it is given to poultry.
• Saying gonorrhoea is caused by a virus. It is bacterial, so antibiotics can treat it.
• Saying the bacteria "get used to" an antibiotic. Resistant bacteria survive and reproduce; the others are killed.

Revision questions

Try these, then check your answers below each one.

Question 1. What causes the symptoms of Salmonella food poisoning? (Choose from: The bacteria only, The bacteria and the toxins they secrete, A virus in the food, Stomach acid.)

Answer: The bacteria and the toxins they secrete

Working: Fever, abdominal cramps, vomiting and diarrhoea are caused by the bacteria and the toxins they secrete.

Question 2. True or false? Gonorrhoea is easily treated with penicillin today.

Answer: False

Working: False. Gonorrhoea was first treated with penicillin, but many resistant strains appeared. Other antibiotics are now used.

Question 3. Why are chickens in the UK vaccinated against Salmonella? (Choose from: To make the chickens grow faster, So fewer chickens carry the bacteria, and fewer people get food poisoning, So the meat can be eaten raw, To kill viruses in eggs.)

Answer: So fewer chickens carry the bacteria, and fewer people get food poisoning

Working: Vaccinated chickens are less likely to carry Salmonella, so fewer eggs and chicken products contain it.

Related lessons

• Previous lesson: 5.6 Viral Diseases
• Next lesson: 5.8 Diseases Caused by Fungi and Protists
• In this chapter: 5.1 Health and Disease
• In this chapter: 5.2 Pathogens and Disease
• In this chapter: 5.3 Growing Bacteria in the Lab

Continue in the full course

This lesson is section 5.7 (AGCBH0507) of our free GCSE Biology 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 Biology course.