Microorganisms — Bacteria, Viruses, Fungi, Protists
In 2017, CSIRO researchers confirmed that the human gut contains up to 1.5 kg of bacteria — more than 500 different species all living and feeding inside one person.
Printable Worksheets
Print or save as PDF — or build a custom worksheet from any module's questions.
Q1 · List three things you think bacteria do that are useful, and three things they do that are harmful.
Q2 · A dead gum tree leaf falls in a forest. Six months later it has rotted away to soil. What organisms did the rotting? Why is that important?
● Know
- The four big microorganism groups: bacteria, viruses, fungi, protists
- That bacteria are single-celled and have no nucleus
- That viruses are not made of cells and most biologists say they are not alive
● Understand
- Why microbes are mostly useful (decomposers, gut bacteria, yoghurt)
- How disease-causing microbes (pathogens) affect humans
- The role of decomposers in recycling nutrients in ecosystems
● Can do
- Match a named microbe to its correct group
- Give a useful and a harmful example for each group
- Explain why decomposers are essential to an ecosystem
- Microorganism
- Bacterium
- Pathogen
- Decomposer
- Protist
- A microbe that causes disease
- A single-celled organism WITH a nucleus
- A living thing too small to see without a microscope
- A single-celled organism with NO nucleus
- An organism that breaks down dead plants and animals
Scrape the inside of your cheek with a cotton bud, smear it on a slide, and stain it — under a microscope you will see thousands of tiny oval shapes scattered across the view, each one a bacterium living on your body right now. They are everywhere — in the soil, in the air, in your gut, on your skin and inside almost every other living thing.
Bacteria do a lot of useful jobs:
- Gut bacteria in your intestines help you digest food (you have about 1.5 kg of them inside you right now).
- Soil bacteria break down dead matter and fix nitrogen from the air so plants can use it.
- Yoghurt bacteria (Lactobacillus) turn milk into yoghurt.
- Cheese bacteria help give cheese its flavour.
Some bacteria are pathogens — they cause disease:
- Salmonella — food poisoning.
- Streptococcus — strep throat.
- Mycobacterium tuberculosis — tuberculosis.
Bacterial diseases can often be treated with antibiotics (medicines that kill bacteria but not human cells).
A virus is a tiny package of genetic material wrapped in a protein coat. It is not made of cells. On its own it can't move, grow, eat or respire — so most biologists say it is not alive. (You met this idea in Lesson 1.)
Viruses only do one thing: when they get inside a living cell, they hijack the cell's machinery to make copies of themselves. That's how they spread and cause disease.
Examples:
- Influenza virus — the flu.
- SARS-CoV-2 — COVID-19.
- Varicella zoster — chickenpox.
- HIV — causes AIDS.
Antibiotics do not work on viruses (because viruses are not bacteria, and antibiotics target bacterial cells). Vaccines are the best defence — they teach your immune system to recognise the virus before you get sick.
A virus is just material in a coat. It is not made of . To make copies, it must a living cell. are our best defence.
Fungi range from tiny single-celled yeasts to huge networks underground. They get their food by absorbing nutrients — they cannot photosynthesise (which is why they are NOT plants).
The three kinds you should know:
- Yeasts — single-celled fungi. The yeast in bread dough releases CO₂ that makes the dough rise. The yeast in beer turns sugar into alcohol.
- Moulds — the fuzzy growth you see on old bread or fruit. Penicillium mould was used by Alexander Fleming to discover the antibiotic penicillin.
- Multi-celled fungi — mushrooms, toadstools and the huge underground fungal networks that connect tree roots in a forest.
Along with bacteria, fungi are the planet's main decomposers. Without them, dead leaves and dead animals would never rot. Forests would be buried in dead matter and the nutrients locked inside would never return to the soil.
Some fungi are harmful — ringworm and tinea (athlete's foot) are caused by fungi infecting human skin.
Protists are single-celled organisms with a nucleus. The nucleus is what makes them different from bacteria. Most live in water (a drop of pond water has dozens of them swimming around).
| Protist | What it does |
|---|---|
| Amoeba | A blob-shaped protist that moves by stretching out "feet" (pseudopods). Common in freshwater. |
| Paramecium | Slipper-shaped, covered in tiny hairs (cilia) that it uses to swim. Eats bacteria. |
| Euglena | Single-celled with chloroplasts — can photosynthesise like a plant and eat like an animal. |
| Plasmodium | The protist that causes malaria — spread by mosquitoes. |
So protists range from harmless pond-water swimmers to one of the deadliest pathogens in human history (Plasmodium has killed billions over the centuries).
Wrong: "All microbes are dangerous and you should kill them all." Most microbes are harmless or useful. Your gut bacteria help you digest food and your skin bacteria help keep harmful microbes out.
Right: Most microbes are harmless or useful. Only a small number are pathogens.
Wrong: "I have the flu, so my doctor will give me antibiotics." Flu is caused by a virus. Antibiotics only work on bacterial infections. Taking antibiotics for a virus does nothing and helps create antibiotic-resistant bacteria.
Right: Antibiotics treat bacterial infections — not viral ones.
Wrong: "A mushroom is a plant." Fungi cannot photosynthesise — they absorb nutrients. Their cells walls are made of chitin (the same stuff as insect exoskeletons), not cellulose. They are their own kingdom.
Right: Fungi are NOT plants. They are their own kingdom.
Imagine all the decomposing fungi and bacteria in a forest suddenly disappeared. Predict: what would happen to the forest over the next 10 years? Give two specific things you'd expect to see.
How close was your prediction?
Nice — you saw that decomposers are essential to the whole forest, not just dead stuff.
Good — most people underestimate just how essential decomposers are. They keep every ecosystem running.
At the start of the lesson you were asked why a spoonful of yoghurt containing billions of bacteria isn't scary.
Now you know about the four microbe groups, write your full answer. What kind of bacteria are in yoghurt, and why are they harmless? How is that different from a pathogen?
Q1. Name the four big microbe groups and give one example of each. (3 marks)
Q2. Give two ways bacteria are useful and two ways bacteria can be harmful. (4 marks)
Q3. Explain why decomposers are essential to every ecosystem. Use the example of a forest with no fungi or bacteria, and use at least three lesson terms. (4 marks)
Answers
▾MCQ 1
C — Viruses are not made of cells (just genetic material in a protein coat). Most biologists say they are not alive. Bacteria, fungi and protists are all made of cells and are alive.
MCQ 2
A — The flu is caused by a virus. Antibiotics work by attacking parts of a bacterial cell, so they have no effect on viruses. Vaccines and the body's own immune system are how we beat the flu.
MCQ 3
D — Fungi and bacteria in a forest are the main decomposers. They break down dead leaves, branches and animals so the nutrients return to the soil for new plants to use.
MCQ 4
B — Plasmodium is a protist (single-celled, has a nucleus). It causes malaria and is spread by mosquito bites — one of the most deadly diseases in human history.
MCQ 5
C — The defining difference is the nucleus. Bacteria have no nucleus (their DNA floats free); protists have a nucleus.
Short Answer 1
Model answer: Bacteria (e.g. Lactobacillus in yoghurt, Salmonella, gut bacteria). Viruses (e.g. influenza, SARS-CoV-2, chickenpox virus). Fungi (e.g. mushroom, bread yeast, Penicillium mould). Protists (e.g. Amoeba, Paramecium, Plasmodium). Any sensible example per group is fine.
Short Answer 2
Model answer: Useful: (1) Gut bacteria help us digest food and make some vitamins. (2) Lactobacillus turns milk into yoghurt and cheese; soil bacteria break down dead matter and fix nitrogen for plants. Harmful: (1) Salmonella causes food poisoning. (2) Streptococcus causes strep throat; Mycobacterium tuberculosis causes TB. Any two valid examples on each side are fine.
Short Answer 3
Model answer: Decomposers — mostly fungi and bacteria — break down dead plants and animals and release the nutrients (like nitrogen, phosphorus, carbon) back into the soil. Living plants need these nutrients to keep growing, so without decomposers the soil would run out. In a forest with no fungi or bacteria, fallen gum leaves and dead branches would just pile up forever — within years the forest floor would be metres deep in dead matter. Meanwhile, new trees couldn't grow because no nutrients would be coming back into the soil, and the entire ecosystem would collapse from the bottom up. So even though microbes can sometimes be pathogens, decomposers are essential to every ecosystem on Earth.