Microorganisms are living things so small that we need a microscope to see them. Exams ask a fixed set of questions about them: which microbe makes curd, which one makes bread rise, who found penicillin, which microbe causes malaria. The answers do not change. This post explains microbes in simple words, with tables of useful and harmful microbes, followed by 15 previous year questions.
Key Points (Quick Revision)
- Five main groups: bacteria, fungi, protozoa, algae and viruses.
- Lactobacillus turns milk into curd. Yeast makes bread rise.
- Alexander Fleming found penicillin in 1928, from the fungus Penicillium.
- Rhizobium in the roots of pulses fixes nitrogen in the soil.
- Malaria is caused by a protozoan, Plasmodium, spread by the female Anopheles mosquito.
1. What microbes are
Microbes, or microorganisms, are tiny living things. Most of them have just one cell. Some, like many fungi, have many cells. They live everywhere - in soil, water, air, and inside our own body.
The first person to see them was Antonie van Leeuwenhoek, using his own simple microscope in the 1670s. He is called the Father of Microbiology. The study of microbes is called microbiology.
Most microbes are harmless. Many are useful. Only some cause disease. Those that cause disease are called pathogens.
2. The five groups
| Group | Simple description | Example |
|---|---|---|
| Bacteria | Single cells with no proper nucleus | Lactobacillus, Rhizobium |
| Fungi | Can be single-celled or many-celled; no chlorophyll | Yeast, bread mould, mushroom, Penicillium |
| Protozoa | Single cells with a nucleus, often move around | Amoeba, Paramecium, Plasmodium |
| Algae | Have chlorophyll and make their own food | Chlamydomonas, Spirogyra |
| Viruses | Not cells at all; can grow only inside a living host | Flu virus, polio virus, HIV |
Viruses are asked about often, because they are different from the rest. Outside a living cell, a virus does nothing - it is like a non-living particle. Inside a host cell, it makes copies of itself. That is why viruses are said to lie on the border between living and non-living.
3. Useful microbes
| Use | Microbe | How it helps |
|---|---|---|
| Curd | Lactobacillus (bacteria) | Turns the sugar in milk into lactic acid, and the milk sets |
| Bread, idli, dosa | Yeast (fungus) | Gives out carbon dioxide gas, which makes the dough rise |
| Alcohol and wine | Yeast | Turns sugar into alcohol - this is called fermentation |
| Vinegar | Acetobacter (bacteria) | Turns alcohol into acetic acid |
| Antibiotics | Penicillium (fungus) and some bacteria | Make medicines that kill harmful bacteria |
| Nitrogen in soil | Rhizobium (bacteria) | Lives in the root nodules of pulses and fixes nitrogen from the air |
| Biogas | Methanogens (bacteria) | Break down cow dung and waste to make methane gas |
| Cleaning sewage | Many bacteria | Break down waste in sewage treatment plants |
Three of these come most often. Lactobacillus and curd. Yeast and bread. And Rhizobium and nitrogen - this is why farmers grow pulses like gram and moong to make the soil richer, without adding much fertiliser.
The story of penicillin is asked directly. In 1928, Alexander Fleming noticed that a mould had grown on one of his bacteria plates, and the bacteria around it had died. The mould was Penicillium. The medicine made from it, penicillin, was the first antibiotic. Remember that antibiotics work against bacteria, not viruses - they do not cure a cold or flu.
4. Harmful microbes and the diseases they cause
| Group | Disease in humans |
|---|---|
| Bacteria | Tuberculosis, cholera, typhoid, tetanus, diphtheria |
| Viruses | Common cold, flu, polio, measles, chickenpox, COVID-19, AIDS, rabies |
| Protozoa | Malaria (Plasmodium), amoebic dysentery (Entamoeba), kala-azar (Leishmania) |
| Fungi | Ringworm, athlete's foot |
Some diseases spread through a carrier, called a vector. The female Anopheles mosquito carries malaria. The Aedes mosquito carries dengue and chikungunya. Houseflies carry the germs of cholera and typhoid on their legs to food.
Microbes also harm plants and food. Rust of wheat is a fungal disease. Citrus canker is bacterial. Microbes spoil food by growing on it and producing harmful substances - which is why food goes bad.
5. How we keep food safe from microbes
- Salt and sugar - pull water out of microbes, so they cannot grow. Used in pickles and jams.
- Oil and vinegar - stop microbes from growing in pickles.
- Heating and cooling - cooking kills most microbes; the fridge slows their growth.
- Pasteurisation - milk is heated to about 70°C for 15 to 30 seconds and then cooled quickly. This kills harmful bacteria. It is named after Louis Pasteur.
- Chemical preservatives - sodium benzoate and sodium metabisulphite are used in jams, squashes and sauces.
- Airtight packing - keeps microbes out.
6. Previous year questions
- Which bacteria turns milk into curd? — Lactobacillus
- Which microbe is used to make bread rise? — Yeast
- Which gas makes bread dough rise? — Carbon dioxide
- Who discovered penicillin? — Alexander Fleming, in 1928
- Penicillin is obtained from which microbe? — Penicillium (a fungus)
- Which bacteria fix nitrogen in the root nodules of pulses? — Rhizobium
- Who is called the Father of Microbiology? — Antonie van Leeuwenhoek
- Malaria is caused by which microbe? — Plasmodium (a protozoan)
- Which mosquito spreads malaria? — Female Anopheles
- Which mosquito spreads dengue? — Aedes
- Vinegar is made using which bacteria? — Acetobacter
- The process of turning sugar into alcohol by yeast is called what? — Fermentation
- Ringworm is caused by which type of microbe? — Fungus
- Pasteurisation is named after whom? — Louis Pasteur
- Do antibiotics work against viruses? — No, only against bacteria
7. How to revise this topic
Learn the five groups first - bacteria, fungi, protozoa, algae, viruses - with one example each. Then learn why viruses are different: they grow only inside a living cell.
Next, learn the useful microbes as pairs: Lactobacillus-curd, yeast-bread, Acetobacter-vinegar, Rhizobium-nitrogen, Penicillium-penicillin. Say each pair out loud. Most questions come from these five.
For diseases, remember the group of the germ. Malaria is protozoan, not viral. TB is bacterial. Flu and polio are viral. Ringworm is fungal. Options often mix these up.
Finally, keep the three names apart: Leeuwenhoek saw microbes first, Pasteur gave pasteurisation, Fleming found penicillin.
8. Frequently Asked Questions
Which microbe turns milk into curd?
Lactobacillus, a type of bacteria. It turns the sugar in milk into lactic acid, which makes the milk set into curd. A spoonful of old curd is added to warm milk because it already has Lactobacillus in it.
Why are viruses said to be on the border of living and non-living?
Because outside a living cell, a virus shows no sign of life - it does not grow, eat or move. But inside a host cell, it makes many copies of itself, which is a feature of living things.
Who discovered penicillin and how?
Alexander Fleming in 1928. He saw that a mould called Penicillium had grown on a plate of bacteria and killed the bacteria around it. The medicine made from it, penicillin, was the first antibiotic.
How does Rhizobium help farmers?
Rhizobium bacteria live in small swellings on the roots of pulse plants like gram and moong. They take nitrogen from the air and fix it in the soil. This makes the soil richer, so farmers need less fertiliser.
Do antibiotics cure viral diseases?
No. Antibiotics work only against bacteria. They do not cure viral diseases like the common cold, flu or COVID-19. Taking them without need can make bacteria resistant to them.
For more General Science topics in simple language, see our Immunity and Vaccines and Human Diseases posts, or browse the General Science section. Preparing for a specific exam? Start with the SSC CGL guide or join the 100 Hour GS Course.