Microorganisms: Bacteria, Fungi, Viruses, and Protozoa

A Today you'll learn

Classify microorganisms into bacteria, fungi, protozoa, and viruses.
Highlight beneficial roles of microbes in curd making, baking, and medicine.
Understand how pathogenic microbes spread disease and learn preventive hygiene methods.

B Let's understand

Microorganisms (microbes) are microscopic single-celled or simple multicellular organisms visible only through a microscope. They include Bacteria, Fungi, Protozoa, and Viruses. Many microbes are extremely beneficial to humans and ecosystems. For example, Lactobacillus bacteria turn milk into curd; yeast (a fungus) ferments sugars in bread and cake making; and Penicillium fungus produces life-saving penicillin antibiotics. Other microbes act as decomposers, recycling dead organic matter into fertile soil nutrients. However, harmful microbes (pathogens) cause diseases such as cholera, malaria (caused by the Plasmodium protozoan transmitted by female Anopheles mosquitoes), ringworm, and viral flu.

C See it step by step

Example 1: How does yeast help dough rise when baking bread?

Step 1: Yeast is mixed with warm water, flour, and sugar in dough.
Step 2: Yeast cells feed on sugars and respire anaerobically (fermentation).
Step 3: This reaction produces bubbles of carbon dioxide gas.
Step 4: Trapped carbon dioxide expands, causing the dough to rise and become spongy and soft.

Example 2: Why is pasteurisation used for milk before distribution?

Step 1: Raw milk can contain disease-causing bacteria.
Step 2: Milk is heated to approximately 72°C for 15–30 seconds and rapidly chilled.
Step 3: This sudden temperature shock kills harmful bacteria without altering milk flavour or nutrition.

D Your turn

Which microorganism transforms milk into curd?

Look back at "Let's understand" above for a clue.

E Ready to practise?

Microorganisms include bacteria

Microorganisms include bacteria, fungi, protozoa, algae, and viruses.

Key idea 2

Lactobacillus bacteria are used in curd making; yeast is used in baking and brewing.

Key idea 3

Microscopic fungi like Penicillium produce antibiotics that kill harmful bacteria.

Key idea 4

Malaria is caused by a protozoan transmitted by the female Anopheles mosquito.

Boiling

Boiling, pasteurisation, and refrigeration preserve foods by killing or suppressing microbes.

Warm up

1. Name the bacterium responsible for turning milk into curd.

2. Which micro-organism is used in baking bread to make it rise?

3. Name an antibiotic made from a fungus.

Practise

4. Which protozoan causes malaria, and which insect carries it?

5. Why are viruses unique compared to other microorganisms?

6. How do decomposer microbes keep our environment clean?

Challenge

7. State two effective ways to prevent food spoilage by microorganisms at home.

8. Why must hands be washed thoroughly with soap before eating?

Download worksheet (PDF)

Question 1: Name the bacterium responsible for turning milk into curd.

Answer: Lactobacillus

Question 2: Which micro-organism is used in baking bread to make it rise?

Answer: Yeast (a microscopic fungus)

Question 3: Name an antibiotic made from a fungus.

Answer: Penicillin (from Penicillium fungus)

Question 4: Which protozoan causes malaria, and which insect carries it?

Answer: Plasmodium protozoan, carried by the female Anopheles mosquito

Question 5: Why are viruses unique compared to other microorganisms?

Answer: Viruses reproduce only inside the living cells of a host organism

Question 6: How do decomposer microbes keep our environment clean?

Answer: They decompose dead plants and animal waste into simple soil nutrients

Question 7: State two effective ways to prevent food spoilage by microorganisms at home.

Answer: Refrigeration/freezing and boiling (or airtight canning/drying/salting)

Question 8: Why must hands be washed thoroughly with soap before eating?

Answer: To wash away pathogenic bacteria and viruses that could cause digestive infections