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B5.5 Bacterial Diseases

GCSE Combined Science · AQA · Chapter B5: Communicable Diseases

A free GCSE Combined Science: Trilogy lesson for AQA (8464) 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
  • Calculate numbers of bacteria after a number of divisions

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

Bacteria are prokaryotic cells. They live inside their hosts but not inside cells. Many release toxins as they grow. Salmonella bacteria live in the guts of many animals, including chickens. People are infected when they eat food that contains the bacteria: food that is undercooked, kept too long or prepared in unhygienic conditions.

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, but it can be serious in babies and older people.

Controlling the spread of Salmonella

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

Cases are higher in warm months: bacteria divide faster in warm food, and food is often left out at barbecues and picnics.

Worked example: how fast bacteria multiply

A rice salad contains 30 Salmonella bacteria. At 30 °C they divide every 30 minutes. How many are there after 3 hours?
180 ÷ 30 = 6 divisions. 30 × 2⁶ = 30 × 64 = 1920.

Answer: 1920 bacteria

Gonorrhoea

  • Symptoms; Gonorrhoea A thick yellow or green discharge from the vagina or penis, and pain (a burning feeling) on urinating.
  • How it spreads; Gonorrhoea By sexual contact.
  • Treatment; Gonorrhoea Antibiotics. It was originally treated with penicillin, until many resistant strains appeared. Other antibiotics are now used.
  • Control; Gonorrhoea Treating infected people with antibiotics, and using a barrier method of contraception such as a condom.

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.

Not all bacteria are harmful. Many bacteria in the gut help with digestion and compete with pathogens, so removing all the bacteria from the body would not be healthy.

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, Too much stomach acid.)

Answer: The bacteria and the toxins they secrete

Working: The symptoms (fever, abdominal cramps, vomiting and diarrhoea) are caused by the bacteria and the toxins they secrete.

Question 2. The bar chart shows cases of Salmonella food poisoning in one region over a year. In which month were there most cases? (Choose from: June, July, August, September.)

Answer: July

Working: July has the tallest bar (133 cases). Cases are higher in summer because bacteria divide faster in warm food, and food is often left out at barbecues and picnics.

Question 3. True or false? Penicillin is still the main antibiotic used to treat gonorrhoea today.

Answer: False

Working: False. Gonorrhoea was once treated with penicillin, but many resistant strains have appeared, so other antibiotics are now used.

Related lessons

Continue in the full course

This lesson is section B5.5 (AGCSC0505) 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.

 

 

 

 

 

 

 

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