Microorganisms — REET Level 2 Science Notes
This page covers microorganisms for REET Level 2 (Maths-Science group) — their types, useful and harmful examples, diseases, and preservation methods — in exam-focused form. Based on NCERT Class 8, this topic matters for REET's direct fact-matching questions.
- 1 question in REET 2024
- Class 6-8 level content
- 6 min read
How REET actually asks this topic
REET 2024 carried a direct, fact-based question from this topic — which fungus is used to make an antibiotic. In other words, REET focuses mainly here on matching the name of a microorganism precisely to its use or the disease it causes. Since only one official question is available from this topic, this pattern has been generalised based on the content of NCERT Class 8's chapter 'Microorganisms: Friend and Foe'. So this page doesn't stop at Penicillium alone — it covers all five groups of microorganisms, their uses, the diseases they cause, and preservation methods in equal detail, so you're ready for any name-matching question.
What are microorganisms?
Microorganisms are extremely small living things that cannot be seen with the naked eye; you need a microscope to see them. They are found everywhere — in air, water, soil, food, and even inside our own bodies — and they can survive in extremely harsh conditions, from very hot springs to icy regions.
Microorganisms are mainly divided into five groups — bacteria, fungi (moulds and yeasts), protozoa, algae, and viruses. Among these, viruses occupy the most unusual position: a virus stays inactive outside a living (host) cell and can only reproduce once it's inside one, which is why it's considered to sit on the borderline between the living and the non-living.
Credit for first observing microorganisms goes to Antonie van Leeuwenhoek, who used a simple microscope he built himself to see moving organisms in a drop of water in the 17th century. Bacteria can be spherical, rod-shaped, or spiral in shape; they are counted among the simplest single-celled microorganisms, though in size, viruses are even smaller — they are considered the smallest of all microorganisms.
Useful microorganisms
| Microorganism | Use / product |
|---|---|
| Lactobacillus (bacterium) | Curdling milk into curd/yoghurt |
| Yeast — Saccharomyces cerevisiae (fungus) | Raising bread dough and alcoholic fermentation |
| Rhizobium (bacterium) | Fixing atmospheric nitrogen in the root nodules of leguminous plants |
| Penicillium (fungus) | Producing the antibiotic penicillin |
REET 2024 asked exactly this match (Penicillium → antibiotic).
Common diseases caused by microorganisms
| Disease | Causative microorganism | Group |
|---|---|---|
| Common cold | Rhinovirus | Virus |
| Tuberculosis (TB) | Mycobacterium tuberculosis | Bacterium |
| Cholera | Vibrio cholerae | Bacterium |
| Malaria | Plasmodium (spread by the female Anopheles mosquito) | Protozoan |
The main routes by which disease spreads — air, water, contaminated food, direct contact, or a carrier (such as a mosquito).
Methods of food preservation
- Adding salt or sugar
- Draws out the moisture microorganisms need to grow, stopping them from multiplying — used in pickles and jams
- Pasteurisation
- Heating milk to about 70°C for 15-30 seconds and then cooling it immediately, which kills harmful microorganisms while keeping milk's taste and nutrition intact.
- Refrigeration (keeping cold)
- Low temperature slows the metabolism and growth rate of microorganisms, so food stays safe for longer
- Boiling / drying
- Destroys microorganisms by removing heat or moisture, such as sun-drying grains and spices
Industrial and agricultural uses of microorganisms
- Vinegar production
- Certain bacteria oxidise alcohol into acetic acid (vinegar)
- Biogas production
- Anaerobic bacteria break down cow dung and organic waste to produce combustible biogas, which is useful as a cooking fuel
- Biofertilizers
- Microorganisms like Rhizobium and blue-green algae boost the soil's nitrogen fertility, reducing dependence on chemical fertilisers
- Decomposition of waste
- Microorganisms break down the remains of dead plants and animals into simpler substances, enriching the soil with nutrients
Role in the nitrogen cycle and in vaccines
Bacteria like Rhizobium live in nodules formed in the roots of leguminous plants and convert atmospheric nitrogen into a form plants can use, boosting soil fertility and reducing the farmer's need for chemical fertiliser.
A vaccine made from weakened or dead microorganisms helps the body develop immunity (antibodies) against that disease, so it can fight off a real infection faster in the future. Historically, Edward Jenner developed the first-ever vaccine, against smallpox, laying the foundation for modern vaccination.
How to teach this to children
Classroom activity
Run a simple demonstration in class of curd setting or bread dough rising, so children can see useful microorganisms at work. For curdling, take slightly warm milk, add a spoonful of curd, and show the result a few hours later.
Teaching aids
Show pictures of bacteria, fungi, protozoa, algae and viruses on a chart to explain differences in size and structure. If a microscope is available at school, showing a drop of pond water or curd gives students a real experience of seeing microorganisms directly.
Common misconception
Many students assume all microorganisms are harmful 'germs'; make it clear that most microorganisms are harmless or beneficial, and only some are disease-causing (pathogenic). Everyday food examples like curd, bread and cheese are an effective way to correct this misconception.
Assessment idea
Have students make lists of 'useful' and 'harmful' microorganisms and explain their reasoning — this checks both classification and application. You can also ask them which preservation method they use at home to keep food from spoiling.
Memory trick
Remember the 'P' link — Penicillium gives Penicillin (antibiotic), both starting with 'P'. Similarly, link Rhizobium to 'Root + Nitrogen' — it fixes nitrogen in the roots.
REET 2024 question (with solution)
Question 142
An example of fungus used in making antibiotics
- Aम्यूकर
- Bराइजोपस
- Cपेनिसिलियम
- Dपक्सिनिया
Explanation
पेनिसिलियम कवक से पेनिसिलिन नामक प्रतिजैविक (antibiotic) बनाया जाता है, जिसकी खोज अलेक्जेंडर फ्लेमिंग ने की थी। म्यूकर और राइजोपस ब्रेड मोल्ड हैं तथा पक्सिनिया गेहूँ में रतुआ रोग उत्पन्न करने वाला कवक है।
Practice questions
Question 1
Which of the following is an example of a fungus?
- Aयीस्ट
- Bअमीबा
- Cविब्रियो कॉलेरी
- Dक्लैमाइडोमोनास
Explanation
Yeast (Saccharomyces cerevisiae) is a fungus. Amoeba is a protozoan, Vibrio cholerae is a bacterium, and Chlamydomonas is an alga.
Question 2
Which bacterium helps in setting milk into curd?
- Aराइजोबियम
- Bलैक्टोबैसिलस
- Cपेनिसिलियम
- Dराइजोपस
Explanation
Lactobacillus is the bacterium that multiplies in milk and turns it into curd. Rhizobium fixes nitrogen, while Penicillium and Rhizopus are fungi.
Question 3
Where is Rhizobium bacterium mainly found?
- Aफलीदार पौधों की जड़ ग्रंथिकाओं में
- Bपत्तियों की सतह पर
- Cमानव आंत में
- Dतालाब के पानी में
Explanation
Rhizobium lives in the root nodules of leguminous plants, where it fixes atmospheric nitrogen, enriching the soil.
Question 4
Which of the following diseases is caused by a virus?
- Aक्षय रोग
- Bहैजा
- Cचिकन पॉक्स
- Dमलेरिया
Explanation
Chickenpox is caused by the varicella-zoster virus. Tuberculosis and cholera are caused by bacteria, and malaria by a protozoan (Plasmodium).
Question 5
Which activity would be most suitable for teaching Class 8 students about "beneficial and harmful microorganisms"?
- Aकेवल पाठ्यपुस्तक पढ़वाना
- Bदही जमने व ब्रेड में खमीर उठने का सरल प्रदर्शन कराना
- Cपरीक्षा में सीधे प्रश्न पूछना
- Dकेवल चित्र दिखाना
Explanation
A direct demonstration (curd setting, dough rising) helps children experience the role of useful microorganisms for themselves, which is more effective than just reading or looking at pictures.
Question 6
In pasteurization, milk is heated to approximately what temperature for a few seconds and then cooled immediately?
- Aलगभग 30°C
- Bलगभग 70°C
- Cलगभग 100°C
- Dलगभग 150°C
Explanation
In pasteurisation, milk is heated to about 70°C for 15-30 seconds and then cooled immediately, which destroys harmful microorganisms.
Question 7
Which scientist first discovered antibiotics?
- Aलुई पाश्चर
- Bएडवर्ड जेनर
- Cअलेक्जेंडर फ्लेमिंग
- Dरॉबर्ट कोच
Explanation
Alexander Fleming discovered penicillin, the first antibiotic, from the fungus Penicillium, in 1928.
Question 8
Which of the following pairs is NOT correctly matched?
- Aयीस्ट - एल्कोहल किण्वन
- Bराइजोबियम - नाइट्रोजन स्थिरीकरण
- Cपेनिसिलियम - प्रतिजैविक
- Dप्लाज़्मोडियम - जीवाणु
Explanation
Plasmodium is a protozoan, not a bacterium; it causes malaria. The other three pairs are correctly matched.
Question 9
If a student thinks that "all microorganisms are harmful," which example would a teacher give to best correct this misconception?
- Aमलेरिया परजीवी
- Bदही बनाने वाला लैक्टोबैसिलस
- Cचेचक का विषाणु
- Dक्षय रोग जीवाणु
Explanation
Giving the example of a beneficial bacterium like the Lactobacillus that makes curd makes it clear that not all microorganisms are harmful — many are useful too.
Question 10
Against which disease did Edward Jenner develop the first-ever vaccine?
- Aपोलियो
- Bचेचक
- Cहैजा
- Dक्षय रोग
Explanation
Edward Jenner developed history's first vaccine, against smallpox.
Frequently asked questions
How many types of microorganisms are there?+
Microorganisms are divided into five main groups — bacteria, fungi, protozoa, algae, and viruses. Each group differs in structure, mode of reproduction, and dependence on a host.
What is penicillin made from?+
Penicillin is made from the fungus Penicillium; this antibiotic was discovered in 1928 by Alexander Fleming, and it is considered one of the greatest discoveries in medical science.
What is pasteurisation?+
The process of heating milk to about 70°C for 15-30 seconds and then cooling it immediately, which destroys harmful microorganisms while keeping milk's nutritional value nearly intact.
Should a virus be considered living or non-living?+
A virus stays inactive outside a host cell and only reproduces once inside one, which is why it's considered to sit on the borderline between living and non-living. On its own, it behaves almost like an inert crystal.
How many questions come from microorganisms in REET?+
There's no fixed number; REET 2024 carried 1 question from this topic. In the science section, this topic usually accounts for around 1-2 questions, so the lists of useful and harmful microorganisms are worth learning well.
Are all bacteria harmful?+
No, most bacteria are harmless or beneficial — such as Lactobacillus (curd) and Rhizobium (nitrogen fixation); only some bacteria are disease-causing.
Read next
Change Log
- 17 September 2026 — English page published — mirrors the fact-checked Hindi page, independently bilingual-accuracy checked