BiologyICSE Class 9 · Economic Importance of Bacteria and Fungi

Uses and Harms of Bacteria and Fungi (ICSE Class 9 Biology)

Updated 17 October 2026 · Fact-checked

Bacteria help in medicine (antibiotics, vaccines), in agriculture through nitrogen fixation and in industries such as tea curing and leather tanning, but also spoil food and cause diseases. Fungi such as yeast and Penicillium are useful in baking, brewing and making antibiotics, though some spoil food or cause disease.

Notice that the same group of living things helps us in medicine, farming, industry and baking, yet also spoils food and causes disease.

Illustration drawn for QuestionBuddy.

Understand Uses and Harms of Bacteria and Fungi

1. Bacteria in medicine. *Antibiotics* are chemicals made by micro-organisms that kill or stop the growth of bacteria; streptomycin comes from a bacterium (*Streptomyces*) and penicillin from the fungus *Penicillium*. *Vaccines* contain weakened or killed germs that make the body produce antibodies. *Serums* contain ready-made antibodies from an animal or a recovered person and give quick short protection.

2. Bacteria in agriculture. Nitrogen makes up most of the air but plants cannot use it directly. Bacteria complete the *nitrogen cycle*:

*Nitrogen fixation*: *Rhizobium* in root nodules of pulse crops (legumes) and free-living *Azotobacter* convert nitrogen into compounds.
*Nitrification*: nitrifying bacteria convert ammonia to nitrites and nitrates, which plants absorb.
*Denitrification*: some bacteria convert nitrates back to nitrogen gas.
Decomposer bacteria break down dead matter and return nutrients to the soil.

3. Bacteria in industry. Bacteria help in *curing of tea* (fermentation of tea leaves) and in *tanning of leather* (removal of fat and hair from hides). Bacteria also turn milk into curd (*Lactobacillus*) and help to make vinegar.

4. Harmful bacteria. They spoil food (milk turns sour, meat putrefies), cause diseases such as cholera, typhoid and tuberculosis, and some can be misused as biological weapons (for example anthrax). Food is preserved by refrigeration, salting, drying and heating.

5. Useful fungi. *Yeast* ferments sugar to give alcohol and carbon dioxide, used in bread making (dough rises), brewing and wine making. *Penicillium* gives the antibiotic penicillin. Mushrooms are edible fungi. Fungi also decompose dead matter.

6. Harmful fungi. They spoil bread and fruit, cause skin diseases such as ringworm and athlete's foot, and some fungi cause plant diseases.

Uses and Harms of Bacteria and Fungi: key facts

Antibiotics: Penicillin from Penicillium | Streptomycin from Streptomyces
Nitrogen cycle: $F i x a t i o n : R hi z o bi u m, A z o t o ba c t er ∣ N i t r i f i c a t i o n ∣ D e ni t r i f i c a t i o n ∣ D ec a y$
Bacteria in industry: Curing of tea
Tanning of leather
Curd by Lactobacillus
Fungi: $Y e a s t : b r e a d, a l co h o l ∣ P e ni c i l l i u m : p e ni c i l l in ∣ M u s h r oo m s : f oo d$

Uses and Harms of Bacteria and Fungi in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Rising dough

Suppose dough kneaded with a pinch of yeast and sugar doubles in size after a few hours in a warm place.

Explain the rise.

Try it first, then show the solutionSolution

Yeast ferments the sugar.
This releases carbon dioxide, which gets trapped as bubbles in the dough.
Heat in baking makes the gas expand and the bread soft.

Answer: Yeast respiration releases carbon dioxide that raises the dough.

Memory hook: Yeast breathes bubbles.

Tricks to remember Uses and Harms of Bacteria and Fungi

Short memory aids. Say each one aloud once, then use it on a question the same day.

R for Root
Rhizobium lives in Root nodules.
Use it when: Nitrogen fixation.
Y-B-B
Yeast: Bread and Brewing.
Use it when: Yeast uses.

Uses and Harms of Bacteria and Fungi: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Antibiotic sources · 3 marks · Short answerBoard level

Name the micro-organism that gives (a) penicillin, (b) curd, (c) root nodules in pulses. [3]

Try it first, then show the solutionSolution

(a) The fungus Penicillium.
(b) Lactobacillus.
(c) Rhizobium.

Answer: (a) Penicillium (b) Lactobacillus (c) Rhizobium.

Example 2: Vaccine versus serum · 2 marks · Short answerHarder board level

Distinguish between a vaccine and a serum on two points. [2]

Try it first, then show the solutionSolution

Vaccine contains weakened or killed germs; serum contains ready-made antibodies.
Vaccine gives long-lasting protection after some days; serum gives immediate but short protection.

Answer: Vaccine: weakened germs, long protection; serum: antibodies, quick short protection.

Example 3: Nitrogen cycle · 4 marks · Long answerHarder board level

Explain how nitrogen in air reaches a plant as nitrate and then returns to the air. [4]

Try it first, then show the solutionSolution

Rhizobium and Azotobacter fix nitrogen into compounds.
Nitrifying bacteria convert ammonia to nitrites then nitrates.
Plants absorb nitrates; animals eat plants.
Denitrifying bacteria convert nitrates to nitrogen gas, returned to the air.

Answer: Fixation, nitrification, absorption by plants, then denitrification back to nitrogen gas.

Example 4: Preserving food · 3 marks · Short answerHarder board level

A family stores milk in a refrigerator rather than a cupboard. Explain using the action of bacteria. [3]

Try it first, then show the solutionSolution

Bacteria in milk multiply quickly in warm conditions.
Cold slows their growth.
So milk stays fresh for longer.

Answer: Low temperature slows bacterial growth and delays spoilage.

Example 5: Case: legume field · 6 marks · Case-basedHOTS / competency

Case. A farmer grows pulses after a field was used for rice and finds he needs less nitrogen fertiliser. (i) Name the bacteria involved. [1] (ii) Where do they live? [1] (iii) What do they do? [2] (iv) Why is this called crop rotation benefit? [2]

Try it first, then show the solutionSolution

(i) Rhizobium.
(ii) Root nodules of pulses.
(iii) Fix atmospheric nitrogen into nitrogen compounds (ammonia), which nitrifying bacteria later convert to nitrates and which plants can use.
(iv) The soil is enriched in nitrogen for the next crop.

Answer: (i) Rhizobium (ii) Root nodules (iii) Fix nitrogen (iv) Soil gains nitrogen for later crops.

Where students lose marks in Uses and Harms of Bacteria and Fungi

The slips examiners see most often, and what to write instead.

Mistake: Writing that Rhizobium is a fungus
Names are mixed.
Do this: Rhizobium is a bacterium; Penicillium is a fungus.
Usually costs: 1 mark.
Mistake: Stating that vaccines contain antibodies
Vaccines and serums are confused.
Do this: Vaccines contain weakened germs; serums contain antibodies.
Usually costs: 1 mark.
Mistake: Leaving out the cycle steps
Only fixation is written.
Do this: Add nitrification and denitrification.
Usually costs: 2 marks.

Exam tips

Always name the organism with the product.
Draw the nitrogen cycle as a flowchart.
Separate useful from harmful in two lists.
Use scientific names in italics.

Practice questions on Uses and Harms of Bacteria and Fungi

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of these is a fungus?
(a) Rhizobium
(b) Penicillium
(c) Lactobacillus
(d) Azotobacter
Hint
Antibiotic.
Answer
(b) Penicillium.
Q2 · 2 marks · Very short answerBoard level
Name two harmful effects of bacteria. [2]
Hint
Food and disease.
Answer
Food spoilage and diseases such as cholera and typhoid.
Q3 · 2 marks · Very short answerBoard level
Give one use each of yeast and mushrooms. [2]
Hint
Brewing/food.
Answer
Yeast: baking and brewing; mushrooms: food.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Pulse crops enrich the soil with nitrogen.
Reason (R): Rhizobium in their root nodules fixes atmospheric nitrogen.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Nodule.
Answer
(a).
Q5 · 2 marks · Very short answerHarder board level
Explain why antibiotics do not work against viral infections such as cold. [2]
Hint
Targets.
Answer
Antibiotics act against bacteria, not viruses, so they do not cure a cold.
Q6 · 2 marks · Very short answerHarder board level
What does curing of tea mean? State the role of bacteria. [2]
Hint
Fermentation.
Answer
It is the fermentation of tea leaves; bacteria improve flavour.
Q7 · 3 marks · Short answerHarder board level
Why are bacteria called decomposers and why is this role important? [3]
Hint
Recycling.
Answer
They break down dead plants and animals into simple substances that return to the soil, so nutrients are recycled.
Q8 · 3 marks · Long answerHOTS / competency
Suppose a bakery finds that the bread has failed to rise. List two possible reasons related to yeast and explain. [3]
Hint
Dead yeast.
Answer
The yeast may be dead from overheating, or there may be too little sugar for fermentation; in both cases little carbon dioxide forms.
Q9 · 4 marks · Case-basedHOTS / competency
Case. A hospital keeps a bottle labelled penicillin. (i) Give the source organism. [1] (ii) Is it an antibiotic or a vaccine? [1] (iii) Why should it not be taken for a viral fever? [2]
Hint
Chemical.
Answer
(i) Penicillium. (ii) An antibiotic. (iii) It acts against bacteria, not viruses.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Source of penicillin?Show answer
Penicillium.
2.Nitrogen fixing bacterium in pulses?Show answer
Rhizobium.
3.Gas that raises dough?Show answer
Carbon dioxide.
4.Bacteria that make curd?Show answer
Lactobacillus.
5.One bacterial disease?Show answer
Cholera.

Uses and Harms of Bacteria and Fungi: frequently asked questions

Are all bacteria harmful?

No, many are useful.

What is fermentation?

Breakdown of sugar into alcohol or acid without oxygen.

Why are vaccines given in advance?

They train the body to make antibodies.