Story of Microorganisms — AP Class 8 Biological Science
'Microorganisms are everywhere — in the air, water, soil, and even inside your body. Most are harmless, many are helpful, and some cause disease.'
1. What are Microorganisms?
Microorganisms (or microbes) are tiny living organisms that can only be seen through a microscope. They are found everywhere — soil, water, air, food, and inside our bodies.
| Type | Size Range | Can we see it? |
|---|---|---|
| Bacteria | 0.5–5 micrometres | Only under microscope |
| Fungi (yeast) | 5–10 micrometres | Only under microscope |
| Protozoa | 10–100 micrometres | Only under microscope |
| Algae | Varies (some visible) | Some visible as green scum |
| Viruses | 0.02–0.3 micrometres | Only under electron microscope |
'Remember: Viruses are considered 'borderline' — they are NOT truly living because they cannot reproduce outside a host cell.'
2. The Five Major Groups of Microorganisms
Group 1: Bacteria
- Structure: Single-celled, prokaryotic (no nucleus), cell wall present
- Shapes: Cocci (round), Bacilli (rod), Spirilla (spiral)
- Example: Lactobacillus — converts milk into curd
- Other Examples: Rhizobium (nitrogen fixation), E. coli (in intestine)
Group 2: Fungi
- Structure: Eukaryotic, can be single-celled (yeast) or multi-celled (mould, mushroom)
- Example: Yeast (Saccharomyces) — used in bread-making, fermentation
- Other Examples: Penicillium (antibiotic), Aspergillus
- 'Important: Mushrooms and moulds are also fungi — they grow on dead organic matter.'
Group 3: Protozoa
- Structure: Single-celled, eukaryotic, no cell wall
- Movement: Using pseudopodia (Amoeba), cilia (Paramecium), or flagella (Euglena)
- Example: Plasmodium — causes malaria
- Habitat: Mostly aquatic (ponds, lakes, damp soil)
Group 4: Algae
- Structure: Eukaryotic, contain chlorophyll (can photosynthesise)
- Types: Chlamydomonas (single-celled), Spirogyra (filamentous), seaweed (multi-cellular)
- Importance: Produce 50–80% of Earth's oxygen
- AP Context: Algal blooms common in Andhra Pradesh water bodies due to fertiliser runoff
Group 5: Viruses
- Structure: Protein coat (capsid) + genetic material (DNA or RNA)
- NOT cellular — no cytoplasm, no organelles, no respiration
- Cannot reproduce outside a living host cell
- Examples: Common cold virus, HIV, COVID-19 (SARS-CoV-2), Polio virus
- 'Viruses are obligate parasites — they need a host cell to multiply.'
3. Friendly Microorganisms — How They Help Us
In Food Production
| Microorganism | Product | Process |
|---|---|---|
| Lactobacillus bacteria | Curd / Yogurt | Converts lactose to lactic acid → milk thickens |
| Yeast (Saccharomyces) | Bread, Dosa, Idli | Fermentation produces CO₂ → dough rises |
| Yeast | Alcoholic drinks (beer, wine) | Fermentation of sugars → ethanol + CO₂ |
| Acetobacter | Vinegar | Converts ethanol to acetic acid |
'AP Context: Idli and dosa fermentation is a traditional process in every Andhra kitchen — microorganisms are essential in our daily food.'
In Medicine
- Antibiotics: Substances produced by microorganisms that kill or stop the growth of other microorganisms
- Penicillin: Discovered by Alexander Fleming (1928) from the fungus Penicillium notatum
- Streptomycin: From Streptomyces bacteria — used for tuberculosis
- Vaccines: Weakened or dead pathogens injected to stimulate antibody production
- Examples: Polio vaccine (oral drops), BCG (tuberculosis), COVID-19 vaccines
In Agriculture
| Microorganism | Function |
|---|---|
| Rhizobium (bacteria) | Nitrogen fixation — lives in root nodules of legumes, converts N₂ into usable form |
| Azotobacter, Azospirillum | Free-living nitrogen-fixing bacteria in soil |
| Mycorrhiza (fungi) | Help plants absorb water and minerals from soil |
| Cyanobacteria (Blue-green algae) | Also fix atmospheric nitrogen |
'AP Board Focus: Rhizobium and nitrogen fixation is a commonly asked topic (2 marks). Remember — legumes like groundnut, pulses, and beans are widely grown in Andhra Pradesh.'
In the Environment
- Decomposers (bacteria and fungi): Break down dead plants and animals → nutrients return to soil
- Sewage treatment: Bacteria break down organic waste in water treatment plants
4. Harmful Microorganisms — Diseases
Diseases in Humans
| Pathogen Type | Disease | Causing Agent | Mode of Transmission |
|---|---|---|---|
| Virus | Common cold | Rhinovirus | Airborne (sneezing, coughing) |
| Virus | COVID-19 | SARS-CoV-2 | Airborne droplets |
| Virus | Chickenpox | Varicella zoster | Airborne / Contact |
| Virus | Polio | Poliovirus | Contaminated food/water |
| Virus | AIDS | HIV | Blood, sexual contact |
| Bacteria | Typhoid | Salmonella typhi | Contaminated food/water |
| Bacteria | Cholera | Vibrio cholerae | Contaminated water |
| Bacteria | Tuberculosis (TB) | Mycobacterium tuberculosis | Airborne |
| Bacteria | Tetanus | Clostridium tetani | Wound infection |
| Protozoa | Malaria | Plasmodium | Female Anopheles mosquito bite |
| Protozoa | Amoebic dysentery | Entamoeba histolytica | Contaminated food/water |
| Fungi | Ringworm | Trichophyton | Skin contact |
| Fungi | Athlete's foot | Trichophyton | Contaminated surfaces |
'AP Exam Tip: Know at least ONE disease from each pathogen type — virus (cold/COVID), bacteria (cholera/typhoid), protozoa (malaria), fungi (ringworm).'
Diseases in Plants
| Disease | Caused by | Affected Crop |
|---|---|---|
| Citrus canker | Bacteria | Lemon, Orange |
| Rust of wheat | Fungus | Wheat |
| Yellow vein mosaic | Virus | Bhindi (Okra) |
| Late blight of potato | Fungus | Potato |
Disease in Animals
- Anthrax: Bacterial disease in cattle (also transmissible to humans — zoonotic)
- Foot and Mouth Disease: Viral disease in cattle, buffalo, sheep, goats
5. Food Preservation Methods
'Microorganisms spoil food by breaking it down. Preservation methods either KILL microbes or STOP their growth.'
| Method | Principle | Examples |
|---|---|---|
| Heating | High temperature kills microbes | Pasteurisation (milk), boiling, canning |
| Cooling / Freezing | Low temperature slows microbial growth | Refrigerator, freezer |
| Adding Salt (Salting) | Salt draws out water by osmosis — dehydrates microbes | Pickles, fish, meat |
| Adding Sugar | High sugar concentration draws water out | Jams, jellies, squashes |
| Drying / Dehydration | Removes water — microbes need water to grow | Cereals, pulses, dried fish, raisins |
| Chemical Preservatives | Chemicals inhibit microbial growth | Sodium benzoate (sauces), Sodium metabisulphite (pickles) |
| Vacuum Packing | Removes oxygen — many microbes need oxygen | Packed nuts, chips |
| Pasteurisation | Heated to 70°C for 15 seconds, then rapidly cooled | Milk (kills pathogens without spoiling taste) |
'Pasteurisation was invented by Louis Pasteur — the father of microbiology.'
6. Louis Pasteur — The Father of Microbiology
- Nationality: French (1822–1895)
- Contributions:
- Pasteurisation — heating liquids to kill harmful microbes while preserving flavour
- Germ Theory of Disease — proved that microorganisms cause disease
- Rabies vaccine — developed the first vaccine for rabies
- Disproved spontaneous generation — showed that life comes from pre-existing life (with Swan-neck flask experiment)
- Quote: 'In the fields of observation, chance favours only the prepared mind.'
7. Worked Examples
Example 1: Identify the Microorganism
Scenario: A microbe is used to ferment idli batter. It is single-celled, eukaryotic, and produces CO₂.
Answer: Yeast (Saccharomyces cerevisiae).
Example 2: Disease and Pathogen
Question: Which microorganism causes malaria, and how does it spread?
Answer: Malaria is caused by the protozoan Plasmodium. It is transmitted through the bite of a female Anopheles mosquito (the vector).
Example 3: Preservation Method
Question: Why does adding salt to fish preserve it?
Answer: Salt draws out water from the fish and from microbial cells through osmosis. Microorganisms need water to grow — without water, they cannot multiply and the fish stays preserved.
Example 4: Pathogen Type
Question: Is the common cold caused by bacteria or a virus?
Answer: The common cold is caused by a virus (Rhinovirus). Antibiotics will NOT work against it — antibiotics target bacteria, not viruses.
8. Common Mistakes
| Mistake | Fix |
|---|---|
| Thinking all bacteria are harmful | Most bacteria are harmless or beneficial (Lactobacillus, Rhizobium) |
| Calling viruses 'living' | Viruses are non-living outside a host — they do not respire, grow, or metabolise |
| Confusing pasteurisation with sterilisation | Pasteurisation kills pathogens but not all microbes; sterilisation kills ALL microbes |
| Saying antibiotics cure viral diseases | Antibiotics work ONLY against bacteria — not viruses (common cold, flu) |
| Forgetting the vector for malaria | The Anopheles mosquito (female) is the vector — not the cause |
| Confusing food poisoning with infection | Food poisoning = toxins released by microbes in food; Infection = microbes growing in body |
9. Exam Focus (AP Board BSEAP Class 8)
| Topic | Marks | Question Type |
|---|---|---|
| Five groups of microorganisms | 2 | Very short / Short answer |
| Friendly microbes — Rhizobium, Lactobacillus, Yeast | 2–4 | Short answer / Table |
| Diseases table (pathogen + disease) | 4 | Table completion |
| Food preservation methods | 4 | Long answer / List |
| Louis Pasteur — contributions | 2 | Short answer |
| Penicillin discovery (Fleming) | 1–2 | Very short answer |
| Nitrogen fixation and Rhizobium | 2 | Short answer |
| Viruses — why are they unique? | 2 | Short answer |
10. Quick Self-Test
Q1. Name the five groups of microorganisms. A1. Bacteria, Fungi, Protozoa, Algae, Viruses.
Q2. Which microorganism is used to make curd from milk? A2. Lactobacillus (bacterium).
Q3. Who discovered penicillin? A3. Alexander Fleming (1928) from Penicillium notatum fungus.
Q4. What is pasteurisation? A4. A process of heating liquids to 70°C for 15 seconds then rapidly cooling — kills pathogens without spoiling taste.
Q5. Which microorganism fixes nitrogen in legume roots? A5. Rhizobium (bacterium).
Q6. Give two food preservation methods. A6. (1) Adding salt (pickles). (2) Refrigeration/Cooling.
Q7. Why are viruses not considered living organisms? A7. They cannot reproduce, respire, or metabolise outside a living host cell.
Q8. Name the vector that transmits malaria. A8. Female Anopheles mosquito.
Q9. What does an antibiotic do? A9. It kills or stops the growth of bacteria (does NOT work on viruses).
Q10. Who is known as the 'father of microbiology'? A10. Louis Pasteur.
11. Summary
- Microorganisms include bacteria, fungi, protozoa, algae, and viruses
- Friendly uses: Food (curd, bread), Medicine (antibiotics, vaccines), Agriculture (nitrogen fixation), Environment (decomposition)
- Harmful effects: Cause diseases in humans, plants, and animals
- Food preservation: Heat, cold, salt, sugar, drying, chemicals, vacuum packing
- Louis Pasteur invented pasteurisation and disproved spontaneous generation
- 'Key difference: Viruses are non-living — they need a host. Bacteria are living cells that can grow on their own.'
'Not all microbes make you sick — many make your life possible. From the curd you eat to the oxygen you breathe, microorganisms are your invisible partners in life.'
