Economic Importance of Bacteria and Fungi ICSE Class 9: Useful and Harmful Microbes

What Economic Importance of Bacteria and Fungi is about

Weightage: CISCE publishes no chapter weightage for ICSE Biology. Economic Importance of Bacteria and Fungi is topic 5, Economic importance of Bacteria and Fungi, of the Class IX part of the 2028 syllabus; the Class IX paper (80 marks, 2 hours) is set and marked by the school.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2028 (Class IX)

Key concepts

Bacteria in medicine: antibiotics, vaccines and serums

Antibiotics: chemicals made by some microbes that kill or stop the growth of other microbes, especially bacteria. Several are obtained from soil bacteria called Streptomyces, e.g. streptomycin (used against tuberculosis), tetracycline and chloramphenicol. (Penicillin, the first antibiotic, comes from a fungus.)

Vaccines: preparations of killed or weakened germs (or their products) that, when given, make the body produce antibodies, so the person becomes immune without suffering the disease. Bacteria are grown to make vaccines such as BCG (tuberculosis), and vaccines against typhoid, cholera, whooping cough and tetanus (from its toxin).

Serums (antiserums): a liquid part of blood containing ready-made antibodies (antitoxins). Bacterial toxins (e.g. of tetanus or diphtheria) are injected in small doses into an animal such as a horse; it makes antibodies, and the serum separated from its blood is given to patients for immediate protection (for instance, anti-tetanus serum after a deep wound).

Worked example

A child with a deep, dirty wound is given an anti-tetanus serum injection, not the vaccine. Why?

Try it first, then show the solutionSolution

1. A vaccine makes the body produce its own antibodies, which takes days or weeks.
2. Serum gives ready-made antibodies (antitoxin) at once.

Answer: Immediate protection is needed.

Bacteria in agriculture: the nitrogen cycle

Air is about 78% nitrogen, but plants cannot use free nitrogen (N₂); they absorb nitrates (and ammonium) from the soil. Bacteria change nitrogen from one form to another.

1. Nitrogen-fixing bacteria convert atmospheric nitrogen into nitrogen compounds (ammonia, then other compounds) that plants can use.

• Symbiotic: Rhizobium in the root nodules of leguminous plants (pea, beans, gram, groundnut).
• Free-living in soil: Azotobacter; cyanobacteria such as Nostoc and Anabaena (in rice fields).

2. Ammonifying (decay) bacteria break down proteins in dead plants, animals and wastes into ammonia.
3. Nitrifying bacteria convert ammonia into nitrates:

• Nitrosomonas: ammonia → nitrites
• Nitrobacter: nitrites → nitrates

4. Plants absorb nitrates to make proteins; animals get nitrogen by eating plants.
5. Denitrifying bacteria (e.g. Pseudomonas) convert nitrates back into free nitrogen, which returns to the air, especially in waterlogged, poorly aerated soils.

(Lightning also fixes some nitrogen.)

Worked example

Why do farmers often grow a crop of gram or beans before a crop of wheat?

Try it first, then show the solutionSolution

1. Legumes have Rhizobium in their root nodules that fix atmospheric nitrogen.
2. The soil becomes richer in nitrogen compounds, especially when the roots are left to decay.
3. The next crop gets more nitrogen and needs less fertiliser.

Answer: Legumes enrich the soil with nitrogen.

Bacteria in industry: curing of tea and tanning of leather

Curing of tea: freshly plucked tea leaves are moistened and heaped; bacteria (and natural enzymes) act on them during fermentation, developing the characteristic flavour, aroma and colour of tea before the leaves are dried.

Tanning of leather: raw animal hides are soaked; bacterial action helps loosen hair and remove fat and flesh, and softens the hide, preparing it to be tanned into leather.

Other familiar uses of bacteria include curd formation from milk (lactic acid bacteria) and vinegar making (acetic acid bacteria), and retting of jute fibres, where bacteria rot away the soft stem tissue to free the fibres.

Worked example

Name the role of bacteria in (a) curing of tea, (b) tanning of leather.

Try it first, then show the solutionSolution

1. (a) Fermentation of leaves to develop flavour and aroma.
2. (b) Loosening hair and removing fat from hides; softening them.

Answer: As above.

Harmful bacteria: food spoilage, diseases and bio-weapons

Spoilage of food: bacteria growing on food decompose it, giving bad smells, sour tastes and slime, and some produce toxins that cause food poisoning (e.g. botulism from badly canned food; stomach upsets from contaminated food). Milk sours, meat and fish rot.

Diseases:

• In plants: citrus canker (lemon, orange), bacterial blight of rice, wilt and soft rot of potato.
• In animals: anthrax of cattle and sheep, tuberculosis, mastitis; in humans cholera, typhoid, tuberculosis, tetanus, diphtheria and pneumonia.

Bio-weapons (biological weapons): the deliberate use of disease-causing microbes or their toxins to kill or harm people, animals or crops, e.g. spores of the anthrax bacterium or the bacterium of plague. They can spread silently and widely; their development and use are banned by an international treaty (the Biological Weapons Convention).

To prevent spoilage: refrigeration, boiling or pasteurisation, canning, drying, salting, sugaring and adding preservatives, which kill bacteria or slow their growth.

Worked example

Why is milk boiled or pasteurised before it is stored?

Try it first, then show the solutionSolution

1. Heating kills most bacteria in milk.
2. With fewer bacteria, souring and spoilage are delayed, and disease germs are destroyed.

Answer: To kill bacteria and delay spoilage.

Useful role of fungi

Breweries: yeast, a single-celled fungus, ferments sugars anaerobically into alcohol (ethanol) and carbon dioxide, used to make beer and wine and industrial alcohol.

Bakeries: yeast mixed with flour dough ferments the sugars and gives off carbon dioxide, whose bubbles make the dough rise, giving soft, spongy bread; the alcohol evaporates during baking.

Cheese processing: certain moulds (species of Penicillium) are added to some cheeses to ripen them and give them special flavour, texture and blue or white veins (e.g. Roquefort and Camembert).

Mushroom cultivation: edible mushrooms (e.g. the button mushroom, oyster mushroom) are grown on composted straw or other farm waste in dark, moist, warm sheds. They are a good food, rich in protein, vitamins and minerals, low in fat; growing them turns farm waste into food and income.

Fungi also give antibiotics (penicillin from Penicillium notatum, discovered by Alexander Fleming) and act as decomposers, recycling nutrients.

(The manufacturing processes are not required.)

Worked example

Which product of yeast fermentation is important in bakeries and which in breweries?

Try it first, then show the solutionSolution

1. Bakeries: carbon dioxide (makes dough rise).
2. Breweries: alcohol (ethanol).

Answer: CO₂ for bread; alcohol for beer and wine.

Economic Importance of Bacteria and Fungi in real life

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

Topics in this chapter

• Uses and Harms of Bacteria and Fungi

Economic Importance of Bacteria and Fungi: key facts

Nitrogen fixation

$N_2(\text{air})\stackrel{\text{Rhizobium, Azotobacter}}{\to }\text{nitrogen compounds}$

Usable by plants.

Nitrification

$\text{ammonia}\stackrel{\text{Nitrosomonas}}{\to }\text{nitrites}\stackrel{\text{Nitrobacter}}{\to }\text{nitrates}$

Nitrifying bacteria.

Denitrification

$\text{nitrates}\stackrel{\text{Pseudomonas}}{\to }{N}_2$

Returns nitrogen to air.

Yeast fermentation

$C_6{H}_{12}{O}_6\to 2C_2{H}_{5}OH+2C{O}_2$

Brewing (alcohol), baking (CO₂).

Economic Importance of Bacteria and Fungi: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Economic Importance of Bacteria and Fungi

The question patterns to expect, and the method that works for each.

Tricks to remember Economic Importance of Bacteria and Fungi

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

• Nitro-SOME then Nitro-BACK-ter

  Nitrosomonas makes Some nitrite first; Nitrobacter Brings nitrate after.

  Use it when: Order of nitrifying bacteria.

• De-nitrify = De-lete nitrate

  Denitrifying bacteria delete nitrates back to N₂ gas.

  Use it when: Denitrification.

• Vaccine Very slow but Very long; Serum Sudden but Short

  Vaccine: active, slow, lasting. Serum: passive, quick, short-lived.

  Use it when: Vaccine versus serum.

• Yeast: Bread gets the Bubbles, Brewery gets the Booze

  CO₂ for bakers, alcohol for brewers.

  Use it when: Fungi in industry.

Economic Importance of Bacteria and Fungi: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 2 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Economic Importance of Bacteria and Fungi

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

• ✗Mistake: Nitrifying and denitrifying mixed

  The names look alike.

  Wrong working

  Denitrifying bacteria convert ammonia into nitrates.

  Right working

  Nitrifying: ammonia → nitrites → nitrates. Denitrifying: nitrates → N₂.

  ✓Do this: 'De' means removing nitrogen from the soil.

  Usually costs: 1 mark.

• ✗Mistake: Penicillin from bacteria

  All antibiotics are assumed to come from bacteria.

  Wrong working

  Penicillin is obtained from Streptomyces.

  Right working

  Penicillin from the fungus Penicillium; streptomycin from the bacterium Streptomyces.

  ✓Do this: Pen-icillin from Pen-icillium.

  Usually costs: 1 mark.

• ✗Mistake: Vaccine and serum confused

  Both are injections against disease.

  Wrong working

  Serum contains weakened germs.

  Right working

  Serum has ready-made antibodies; vaccine has killed or weakened germs.

  ✓Do this: Vaccine teaches; serum supplies.

  Usually costs: 1 mark.

• ✗Mistake: Writing manufacturing steps for bread or beer

  The syllabus says processes of manufacture are not required.

  Wrong working

  A page on the brewing process.

  Right working

  One line: yeast ferments sugar into alcohol and CO₂.

  ✓Do this: Give the role of the fungus only.

  Usually costs: Time.

Practice questions on Economic Importance 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.

1. Q1 · 1 mark · MCQExam level

   Denitrifying bacteria convert:
   (a) N₂ into ammonia
   (b) ammonia into nitrites
   (c) nitrates into N₂
   (d) nitrites into nitrates

   Hint

   Back to the air.

   Answer

   (c)

2. Q2 · 1 mark · MCQExam level

   The fungus used in baking bread is:
   (a) Penicillium
   (b) yeast
   (c) mushroom
   (d) Rhizobium

   Hint

   Bubbles.

   Answer

   (b)

3. Q3 · 3 marks · Very short answerExam level

   Name: (a) a free-living nitrogen-fixing bacterium, (b) the antibiotic from Penicillium, (c) a bacterial disease of cattle.

   Hint

   Azoto-...

   Answer

   (a) Azotobacter (b) Penicillin (c) Anthrax

4. Q4 · 2 marks · Short answerExam level

   Give two ways in which bacteria are harmful to humans.

   Hint

   Food, disease.

   Answer

   They spoil food and cause food poisoning; they cause diseases such as cholera, typhoid and tuberculosis (also crop diseases, bio-weapons).

5. Q5 · 3 marks · Short answerHarder exam level

   Explain why a deep wound contaminated with soil is treated with anti-tetanus serum, while children are given the tetanus vaccine as a routine.

   Hint

   Immediate versus long-term.

   Answer

   After the wound, ready-made antitoxin in the serum neutralises any tetanus toxin at once. The routine vaccine makes the child's body produce its own antibodies, giving long-lasting protection before any injury.

6. Q6 · 2 marks · Short answerHarder exam level

   Give two methods of preventing food spoilage by bacteria and explain how each works.

   Hint

   Cold, heat, dry, salt.

   Answer

   Refrigeration: low temperature slows bacterial growth. Boiling or pasteurisation: heat kills bacteria (also salting/sugaring draw water out of microbes; drying removes water they need).

7. Q7 · 2 marks · Short answerHarder exam level

   How are mushrooms useful as food, and why is mushroom farming considered environment-friendly?

   Hint

   Protein; farm waste.

   Answer

   Mushrooms are rich in protein, vitamins and minerals and low in fat. They are grown on farm waste like straw, which would otherwise be burnt, and the spent compost can be used as manure.

8. Q8 · 3 marks · Short answerHarder exam level

   Arrange in the correct order of the nitrogen cycle and name the bacteria at each arrow: nitrates, atmospheric nitrogen, nitrites, ammonia (from dead organisms).

   Hint

   Ammonia first in the soil.

   Answer

   Atmospheric N₂ → (nitrogen-fixing bacteria) → nitrogen compounds; dead organisms → (ammonifying bacteria) → ammonia → (Nitrosomonas) → nitrites → (Nitrobacter) → nitrates → (denitrifying bacteria, e.g. Pseudomonas) → atmospheric N₂.

9. Q9 · 3 marks · Short answerHarder exam level

   What are bio-weapons? Name one bacterium that has been feared as a bio-weapon and state why international law bans such weapons.

   Hint

   Anthrax.

   Answer

   Bio-weapons are disease-causing microbes or their toxins used deliberately to harm people, animals or crops. Example: the anthrax bacterium (its spores). They are banned because they can cause mass disease, spread uncontrollably and harm innocent people and the environment.

10. Q10 · 3 marks · Short answerHOTS / competency

    A milk sample stored at 30 °C contains 1000 bacteria per mL, and the bacteria double every 30 minutes. How many per mL will there be after 2 hours? Why would the same milk stored at 4 °C have far fewer?

    Hint

    4 doublings.

    Answer

    2 hours = 4 doublings: 1000 × 2⁴ = 16 000 per mL. At 4 °C bacterial growth and division are greatly slowed, so numbers rise very little.

11. Q11 · 3 marks · Short answerHOTS / competency

    A farmer stops all fertiliser use and grows only wheat year after year on the same field; yields fall steadily. A neighbour alternates wheat with beans and his yields stay steady. Explain, using the nitrogen cycle.

    Hint

    Removal versus fixation.

    Answer

    Each wheat harvest removes nitrogen from the soil in the grain, and nothing replaces it, so nitrates fall. Beans are legumes whose root-nodule Rhizobium fixes atmospheric nitrogen, restoring soil nitrogen for the next wheat crop.

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. 1.Bacterium in root nodules of legumes?Show answer

   Rhizobium.

2. 2.Free-living nitrogen-fixing soil bacterium?Show answer

   Azotobacter.

3. 3.Bacterium that converts ammonia to nitrites?Show answer

   Nitrosomonas.

4. 4.Antibiotic from Streptomyces used against TB?Show answer

   Streptomycin.

5. 5.Who discovered penicillin?Show answer

   Alexander Fleming.

6. 6.Bacterial disease of citrus plants?Show answer

   Citrus canker.

7. 7.Fungus used in baking?Show answer

   Yeast.

8. 8.What are bio-weapons?Show answer

   Disease-causing microbes or their toxins deliberately used to harm people, animals or crops.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you name the bacteria at each step of the nitrogen cycle?
• ☐Can you give three differences between a vaccine and a serum?
• ☐Can you state the role of bacteria in tea curing and leather tanning?
• ☐Can you name two bacterial diseases each of plants and animals?
• ☐Can you explain how yeast helps bakers and brewers?

Economic Importance of Bacteria and Fungi: frequently asked questions