Chapter 2: Microorganisms Friend and Foe

 

Introduction to Microorganisms Friend and Foe:

These notes of Microorganisms Friend and Foe, cover the entire chapter comprehensively for CBSE Class 8 exams. Revise diagrams and examples thoroughly!

1.Microorganisms:

• Microorganisms (microbes) are very tiny living organisms that cannot be seen with the naked eye.
• They can only be seen under a microscope.
• They are present everywhere – air, water, soil, inside our body, on our skin, and even inside other animals and plants.
• Size: Generally 1 micron (1/1000 mm) to a few microns.
• Discovered by Antonie van Leeuwenhoek (called them “animalcules”).

2. Major Groups of Microorganisms

There are five major groups:Microorganisms: Friend and Foe

1. Bacteria
   • Single-celled organisms.
   • Shapes: Spherical (cocci), rod-shaped (bacilli), spiral (spirilla), comma-shaped (vibrio).
   • Can survive in extreme conditions (hot springs, ice, deep inside earth).
   • Examples: Lactobacillus (curd formation), Rhizobium (nitrogen fixation), E. coli (in intestine).
2. Viruses
   • Non-living outside a host, living inside a host.
   • They reproduce only inside living cells.
   • Cause diseases like common cold, influenza, dengue, AIDS, polio, chickenpox.
   • Very small (20–400 nm); visible only under electron microscope.
3. Fungi
   • Mostly multicellular (except yeast which is unicellular).
   • Do not contain chlorophyll → cannot make their own food → saprophytic or parasitic.
   • Examples: Yeast, Moulds (Penicillium), Mushrooms, Bread mould (Rhizopus).
4. Protozoa
   • Single-celled eukaryotic organisms.
   • Mostly found in water bodies.
   • Move with the help of pseudopodia (Amoeba), cilia (Paramecium), or flagella.
   • Examples: Amoeba, Paramecium, Plasmodium (causes malaria).
5. Algae
   • Mostly aquatic, contain chlorophyll → can photosynthesise.
   • Unicellular (Chlamydomonas) or multicellular (Spirogyra, Seaweeds).
   • Examples: Spirogyra, Chlorella, Anabaena.

3. Where do Microorganisms Live?

• Microbes are found in all kinds of habitats: → Extremely hot (thermophiles) → Extremely cold (psychrophiles) → High salinity (halophiles) → Acidic or alkaline environments
• Soil: Bacteria, fungi, protozoa.
• Water: Algae, protozoa, some bacteria.
• Air: Spores of fungi and bacteria, viruses.
• Inside human body: Around 10¹³ human cells vs 10¹⁴ bacterial cells (mostly useful).

4. Microorganisms as Friends (Useful Microorganisms)

A. Commercial Use

1. Making curd: Lactobacillus (LAB – Lactic Acid Bacteria) converts milk sugar (lactose) into lactic acid → milk proteins coagulate → curd.
2. Making alcohol and wine: Yeast (Saccharomyces cerevisiae) ferments sugar → alcohol + CO₂ (anaerobic respiration).
3. Bread making: Yeast ferments dough → releases CO₂ → dough rises → soft and spongy bread.
4. Production of antibiotics: Penicillium notatum → Penicillin (discovered by Alexander Fleming).

B. In Agriculture

1. Nitrogen fixation:
   • Free-living: Azotobacter, Anabaena.
   • Symbiotic: Rhizobium in root nodules of leguminous plants (pea, gram, beans).
   • Converts atmospheric N₂ into usable compounds.
2. Blue-green algae (cyanobacteria) increase soil fertility.

C. In Environment

1. Decomposers: Bacteria and fungi decompose dead plants and animals → convert complex organic matter into simple substances → maintain nutrient cycle (carbon, nitrogen cycle).
2. Sewage treatment: Aerobic and anaerobic bacteria break down organic waste.

5. Microorganisms as Foe (Harmful Microorganisms)

A. Human Diseases

• Bacterial: Tuberculosis, Typhoid, Cholera, Tetanus, Anthrax.
• Viral: Polio, Chickenpox, Measles, Hepatitis, AIDS, Dengue.
• Protozoan: Malaria (Plasmodium), Amoebic dysentery, Sleeping sickness.
• Fungal: Ringworm, Athlete’s foot.

B. Animal Diseases

• Anthrax (cattle), Foot and mouth disease.

C. Plant Diseases

• Citrus canker (bacteria), Rust of wheat (fungi), Yellow vein mosaic of bhindi (virus).

D. Food Spoilage

• Bacteria and fungi grow on food (bread, pickles, fruits) in moist and warm conditions → produce bad smell and taste → food becomes unfit.

6. Disease Transmission (Carriers)

• Housefly: Carries germs from garbage and animal excreta to food.
• Female Anopheles mosquito: Carries Plasmodium (malaria parasite).
• Female Aedes mosquito: Dengue and chikungunya virus.

7. Methods of Food Preservation

To stop growth of harmful microbes:

1. Pasteurisation: Milk is heated to 70°C for 15–30 seconds and then suddenly chilled → kills harmful bacteria.
2. Drying: Removes moisture (sun-drying of grains, fish, spices).
3. Salting: High salt concentration (pickles, fish).
4. Adding sugar: High sugar concentration (jams, jellies).
5. Adding oil and vinegar: Prevents air contact (pickles).
6. Refrigeration/Freezing: Low temperature slows microbial growth.
7. Canning and vacuum packing.

8. Nitrogen Cycle (Role of Microorganisms)

• Atmospheric nitrogen (78%) cannot be used directly by plants.
• Steps:
  1. Nitrogen fixation: Rhizobium, Azotobacter, lightning → convert N₂ into nitrates.
  2. Plants absorb nitrates → make proteins.
  3. Animals eat plants → get nitrogen.
  4. Death and decay → decomposers (bacteria, fungi) convert proteins → ammonia.
  5. Nitrifying bacteria (Nitrosomonas, Nitrobacter) convert ammonia → nitrites → nitrates.
  6. Denitrifying bacteria return nitrogen back to atmosphere.

Key Diagrams to Draw (Important for Exams)

1. Shapes of bacteria (cocci, bacilli, spirilla, vibrio).
2. Amoeba, Paramecium.
3. Bread mould (Rhizopus) showing sporangia.
4. Nitrogen cycle (flow chart).
5. Fermentation by yeast.

Important Examples Table

Quick Revision Points

• Microbes are both friends and foes depending on where and how they act.
• Antibiotics work only against bacteria, not viruses.
• Vaccines give immunity against many viral and bacterial diseases.
• Louis Pasteur discovered pasteurisation and rabies vaccine.
• Edward Jenner developed the first vaccine (smallpox).

 

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