Lesson T1.3.5

T1.3.5 Bacteria in the digestive system Quiz: KS3 Biology, Unit 1

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson T1.3.5, Bacteria in the digestive system: 20 multiple choice questions for the KS3 Biology (National Curriculum), Unit 1: Structure and function of living organisms, written with Revision Ninja.

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The 20 questions

  1. Which group of microorganisms lives in the human large intestine and helps digestion?

    • Gut bacteria
    • Moulds that grow on bread
    • Parasitic worms
    • Viruses that cause colds

  2. What do beneficial gut bacteria help the body to do?

    • Produce red blood cells in the large intestine
    • Replace the acid that the stomach normally makes
    • Break down some food that the body cannot digest alone
    • Digest all protein without needing any enzymes at all

  3. Which of these is made partly by bacteria in the large intestine?

    • Bile, which is made in the gall bladder
    • Stomach acid, made by the small gut
    • Some vitamins, such as vitamin K
    • Red blood cells made in the gut

  4. Why can antibiotics sometimes upset the digestive system?

    • They can kill useful gut bacteria as well as harmful ones
    • They increase the number of gut bacteria too much
    • They stop the stomach producing acid
    • They block the small intestine

  5. Which of these describes a harmful bacterium in the gut?

    • One that digests fibre
    • One that causes food poisoning
    • One that helps absorb water
    • One that makes vitamin K

  6. What is a good way to keep beneficial gut bacteria healthy?

    • Eating fibre-rich foods
    • Avoiding all vegetables
    • Taking antibiotics every day
    • Eating only refined sugar

  7. Why is the large intestine a suitable place for bacteria to live?

    • It has no water and is kept sterile by bile
    • It contains no food, so bacteria cannot grow
    • It is very acidic and dry, with little oxygen
    • It is warm and has food that has not been digested

  8. Which organ is the main home for bacteria in the human gut?

    • The oesophagus
    • The large intestine
    • The stomach
    • The mouth

  9. Why does stomach acid kill most bacteria in food?

    • Acid makes bacteria reproduce faster in the gut
    • Acid makes bacteria stronger and more resistant
    • Acid is too strong for most bacteria to survive
    • Acid gives bacteria the energy they need to grow

  10. Which statement about gut bacteria is correct?

    • Gut bacteria only live in the mouth and throat
    • All gut bacteria cause disease and must be removed
    • Bacteria in the gut can be useful or harmful
    • Gut bacteria never help with digestion of food

  11. Yoghurt contains live bacteria. What might eating it do?

    • Stop the digestive system from working properly
    • Replace the need for digestive enzymes in the gut
    • Kill all bacteria in the stomach before they reach the gut
    • Support the balance of helpful bacteria in the gut

  12. Why do bacteria in the gut need to be kept in balance?

    • Balanced bacteria stop digestion
    • Balanced bacteria cause no effect at all
    • Balanced bacteria remove the need for food
    • Too many harmful bacteria can cause illness

  13. What type of organism is a bacterium?

    • A multicellular plant cell
    • A group of tissues in an organ
    • A single-celled microorganism
    • A type of organ system

  14. A patient takes antibiotics for an infection and then has diarrhoea. Which explanation is most likely?

    • The antibiotics blocked the mouth so food could not pass
    • The antibiotics increased the amount of bile in the gut
    • The antibiotics stopped the stomach from producing any acid
    • The antibiotics have reduced the number of helpful gut bacteria

  15. Why are gut bacteria described as useful rather than simply present?

    • They are useful only in the mouth, where digestion starts
    • Some help digestion and vitamin production, so the body benefits
    • They only help when a person is unwell or has an infection
    • They replace all of the digestive enzymes the body makes

  16. Why do gut bacteria need a supply of food from the diet?

    • They get their food from the stomach acid around them
    • They use nutrients from undigested food to grow and reproduce
    • They make their own food using sunlight in the gut
    • They need only water and oxygen to survive in the gut

  17. Which two things can gut bacteria help the body with?

    • Movement and vision
    • Growth and sweating
    • Breathing and blood clotting
    • Digestion and vitamin production

  18. A pupil eats yoghurt with live cultures. Which organ is most directly affected by these bacteria?

    • The heart
    • The brain
    • The large intestine
    • The lungs

  19. What would most likely happen if the bacteria in the large intestine were completely killed?

    • Red blood cells would stop forming
    • Digestion of some fibre would be reduced
    • Stomach acid would stop being made
    • Bile production would stop completely

  20. Which description best fits gut bacteria?

    • Enzymes that digest proteins in the stomach and gut
    • Cells that make bile and store it in the liver
    • Microorganisms that live in the digestive system
    • Organs that absorb nutrients from the digestive tract

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