Microorganisms

Overview

Microorganisms are living things too small to see with the naked eye, typically requiring a microscope for observation. This topic forms a core part of the Science section in WB TET Paper II, connecting biology concepts with everyday life applications like food preservation, disease prevention and agriculture. Questions often test your ability to classify microorganisms, distinguish beneficial from harmful types and recall specific examples with their uses or diseases.

For the exam, you must know the four main groups (bacteria, viruses, fungi and protozoa), understand how microorganisms help in food production and nitrogen fixation, and identify common diseases caused by each type. Expect 2–4 questions that may include matching diseases to pathogens, identifying useful microorganisms or explaining preservation methods.

Key Concepts

• **Classification by size and structure**: Microorganisms include bacteria (single-celled, no true nucleus), viruses (non-cellular, need a host to reproduce), fungi (may be single-celled like yeast or multicellular like mushrooms) and protozoa (single-celled with a nucleus).

• **Viruses are not truly "living"**: They cannot reproduce on their own and need to invade a living cell — this makes them unique among microorganisms.

• **Bacteria can be useful or harmful**: The same group that causes diseases like cholera also includes species that make curd, fix nitrogen and decompose waste.

• **Fungi as decomposers and food sources**: Fungi break down dead organic matter, returning nutrients to soil; some like mushrooms are edible while others cause diseases in plants and humans.

• **Nitrogen-fixing bacteria**: Rhizobium lives in root nodules of leguminous plants and converts atmospheric nitrogen into usable forms for plants — a key concept linking microorganisms to agriculture.

• **Antibiotics from microorganisms**: Penicillin, the first antibiotic, comes from the fungus Penicillium — microorganisms fighting microorganisms.

• **Disease transmission routes**: Microorganisms spread through air (droplet infection), water (contaminated drinking water), food (spoiled or uncooked food), direct contact and vectors like mosquitoes and houseflies.

• **Preservation methods work by stopping microbial growth**: Techniques like refrigeration, boiling, pasteurisation, adding salt/sugar and using chemical preservatives all aim to kill microorganisms or prevent their multiplication.

Formulas / Key Facts

| Fact | Detail | |------|--------| | Discovery of bacteria | Antonie van Leeuwenhoek (1670s) using a simple microscope | | Discovery of penicillin | Alexander Fleming (1928) from Penicillium notatum | | Pasteurisation temperature | Milk heated to about 70°C for 15–30 seconds | | Common bacterial diseases | Tuberculosis, cholera, typhoid, tetanus | | Common viral diseases | Common cold, influenza, dengue, polio, COVID-19, chickenpox | | Common fungal diseases | Ringworm, athlete's foot (humans); rust, smut (plants) | | Common protozoan diseases | Malaria (Plasmodium), dysentery (Entamoeba) | | Nitrogen-fixing bacteria | Rhizobium (in legume roots), Azotobacter (free-living in soil) | | Fermentation agents | Yeast (alcohol, bread); Lactobacillus (curd, pickles) | | Vaccine principle | Weakened or killed pathogens introduced to build immunity |

Worked Examples

**Example 1: Classifying by type**

*Question*: Identify whether each is caused by bacteria, virus or fungus — (a) Malaria (b) Ringworm (c) Typhoid (d) Polio

*Solution*:

• Malaria — caused by Plasmodium, a protozoan (not bacteria/virus/fungus)
• Ringworm — caused by fungi (dermatophytes)
• Typhoid — caused by bacteria (Salmonella typhi)
• Polio — caused by a virus (poliovirus)

*Tip*: If the question gives only three options, malaria won't appear or will be marked "none of these."

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**Example 2: Useful microorganisms**

*Question*: How is curd formed from milk?

*Solution*: 1. A small amount of curd (containing Lactobacillus bacteria) is added to warm milk. 2. Lactobacillus multiplies and converts lactose (milk sugar) into lactic acid. 3. Lactic acid causes milk proteins to coagulate, turning liquid milk into thick curd. 4. The process works best at warm temperature (around 30–40°C).

*Key point*: This is fermentation — a process where microorganisms break down sugars in the absence of oxygen.

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**Example 3: Food preservation**

*Question*: Why does adding salt or sugar preserve food?

*Solution*:

• Salt and sugar are hygroscopic — they draw water out of food and out of any microorganisms present.
• Without adequate water, bacteria and fungi cannot grow or reproduce.
• This is why pickles (high salt), jams and murabbas (high sugar) last for months without refrigeration.

Common Mistakes

• **Confusing bacteria and viruses**: Students often list viral diseases as bacterial or vice versa. *Fix*: Memorise a short list — TB/cholera/typhoid = bacteria; cold/flu/dengue/polio = virus.

• **Thinking all microorganisms are harmful**: Many questions test beneficial uses. *Fix*: Remember the "3 Fs" of useful microbes — Fermentation (yeast, Lactobacillus), Fertiliser (nitrogen-fixers) and pharmaceuticals (Penicillium).

• **Forgetting protozoa**: Students focus on bacteria/virus/fungi and miss protozoa entirely. *Fix*: Always recall malaria (Plasmodium) and amoebic dysentery (Entamoeba) as protozoan examples.

• **Mixing up pasteurisation and sterilisation**: Pasteurisation kills most pathogens but not all; sterilisation kills everything. *Fix*: Pasteurised milk still needs refrigeration; sterilised items (like surgical tools) do not.

• **Assuming antibiotics work on viruses**: Antibiotics kill bacteria only; viruses require antivirals or vaccines. *Fix*: This is a frequently asked concept — antibiotics are useless against common cold or dengue.

Quick Reference

• **Four groups**: Bacteria, Viruses, Fungi, Protozoa
• **Lactobacillus** → curd; **Yeast** → bread, alcohol
• **Rhizobium** → nitrogen fixation in legume roots
• **Penicillin** → antibiotic from Penicillium fungus
• **Preservation methods**: refrigeration, boiling, pasteurisation, salt/sugar, chemical preservatives
• **Antibiotics do NOT work on viruses**