Class 8 Science: Microorganisms – Friend and Foe

Because they are so tiny, we cannot see them with our naked eyes. We need a microscope to observe them. That is why they are called microorganisms or microbes.

In this tutorial, we will explore who these tiny organisms are, why some of them are our best friends, why others can make us sick, and how we protect our food from them.

1. The Invisible World: Major Groups of Microorganisms

Imagine microorganisms as an invisible city with five distinct neighborhoods. Each neighborhood has its own unique type of citizens:

                  ┌─────────────────────────────────┐
                  │      MICROORGANISMS (MICROBES)  │
                  └────────────────┬────────────────┘
                                   │
     ┌───────────────┬─────────────┼───────────────┬───────────────┐
     ▼               ▼             ▼               ▼               ▼
 Bacteria          Fungi        Protozoa         Algae          Viruses
 (Single-celled) (Plant-like) (Animal-like) (Photosynthetic) (Host-dependent)

1. Bacteria (Singular: Bacterium)

• What are they? Single-celled, simple organisms found almost everywhere—in soil, water, air, and inside our body!
• Shapes: They come in different shapes, such as rod-shaped (Bacilli) or spiral-shaped (Spirilla).
• Examples: Lactobacillus, Rhizobium.

2. Fungi (Singular: Fungus)

• What are they? Non-green, plant-like organisms. Since they lack chlorophyll, they cannot make their own food; instead, they absorb nutrients from decaying organic matter.
• Examples: Bread mould (Rhizopus), Yeast, Penicillium, Mushrooms.

3. Protozoa (Singular: Protozoan)

• What are they? Single-celled organisms that exhibit animal-like behaviors, such as moving around and capturing food. They mostly live in water or moist environments.
• Examples: Amoeba, Paramecium, Plasmodium (causes malaria).

4. Algae (Singular: Alga)

• What are they? Simple, plant-like organisms containing chlorophyll. They perform photosynthesis and produce a large portion of the Earth's oxygen.
• Examples: Chlamydomonas, Spirogyra.

5. Viruses: The Special Exception

Viruses are microscopic, but they are different from other microorganisms.

• The Living vs. Non-Living Dilemma: A virus behaves like a non-living object outside a host. However, once it enters a host organism (human, animal, plant, or bacterium), it uses the host cell's machinery to reproduce!
• Examples: Influenza virus, HIV, Poliovirus, Coronavirus.

2. Microorganisms as Friends (Friendly Microbes)

Despite their bad reputation for causing diseases, the vast majority of microbes are beneficial to humans and the environment.

A. Making Food Items

• Curd and Cheese: Curd contains a bacterium called ** Lactobacillus **. When added to warm milk, it multiplies and converts the sugar in milk (lactose) into lactic acid, turning milk into curd.
• Baking Bread and Cakes: Yeast is a fungus used in baking. When added to dough, yeast reproduces rapidly and releases Carbon Dioxide ($CO_2$) gas during respiration. The gas bubbles fill the dough, making it expand and become light and fluffy.

B. Commercial Uses (Fermentation)

Microorganisms are used on a large scale to produce alcohol, wine, and acetic acid (vinegar).

• What is Fermentation? The process of converting sugar into alcohol by yeast is called fermentation.
• Historical Note: Louis Pasteur discovered fermentation in 1857.

C. Medicinal Uses

1. Antibiotics

When you fall sick, a doctor might prescribe antibiotic tablets or capsules.

• Definition: Antibiotics are medicines prepared from microorganisms that kill or stop the growth of disease-causing microbes.
• Examples: Streptomycin, Tetracycline, Erythromycin.
• Historical Note: In 1929, Alexander Fleming discovered the first antibiotic, Penicillin, from a green mould named Penicillium notatum.

Important Caution: Antibiotics must be taken only on the advice of a qualified doctor. Unnecessary use of antibiotics can make them less effective in the future and can kill beneficial bacteria inside your body.

2. Vaccines

Vaccines protect us from deadly infectious diseases like Polio, Measles, Smallpox, and Hepatitis.

• How do vaccines work?
  1. A dead or weakened form of a pathogen (disease-causing microbe) is introduced into the body.
  2. The body's immune system responds by producing antibodies to fight off the invader.
  3. The body remembers how to fight that specific microbe. If the real pathogen attacks later, the immune system destroys it immediately.
• Historical Note: Edward Jenner discovered the vaccine for Smallpox in 1798.

D. Soil Fertility and Environmental Cleanup

• Nitrogen Fixation: Plants cannot take in nitrogen gas directly from the air. Bacteria like Rhizobium (living in the root nodules of leguminous plants like peas and beans) and certain blue-green algae convert atmospheric nitrogen into usable nitrogen compounds in the soil.
• Cleaning the Environment: Decomposers (bacteria and fungi) break down dead plants, animals, and organic waste into simple substances. This recycles nutrients back into the soil and cleans up the environment.

3. Microorganisms as Foes (Harmful Microbes)

Microbes that cause diseases in humans, animals, and plants are called pathogens.

A. Communicable Diseases in Humans

Microbial diseases that spread from an infected person to a healthy person through air, water, food, or physical contact are called communicable diseases.

Disease Carriers (Vectors)

Some insects act as carriers of disease-causing microbes:

1. Housefly: Sits on garbage and excreta, picking up pathogens. When it sits on uncovered food, it transfers these pathogens.
2. Female Anopheles Mosquito: Carries the parasite of Malaria (Plasmodium).
3. Female Aedes Mosquito: Carries the Dengue virus.

Garbage/Waste ──(Fly picks up pathogen)──> Uncovered Food ──(Ingestion)──> Illness

B. Disease-Causing Microbes in Animals and Plants

• Animal Diseases:
  • Anthrax: A dangerous bacterial disease affecting humans and cattle (discovered by Robert Koch in 1876).
  • Foot and Mouth Disease: A viral disease common in cattle.
• Plant Diseases:
  • Citrus Canker: Caused by Bacteria (spread through Air).
  • Rust of Wheat: Caused by Fungi (spread through Air/Seeds).
  • Yellow Vein Mosaic of Bhindi (Okra): Caused by Virus (spread by Insects).

4. Food Preservation: Keeping Microbes at Bay

When cooked food is left uncovered in warm, humid conditions, bacteria and fungi grow on it, causing food spoilage. Spoiled food smells bad, tastes terrible, and can lead to food poisoning.

To keep food safe, we must prevent microbial growth using these common preservation methods:

1. Chemical Method

Preservatives like Sodium benzoate and Sodium metabisulphite are added to jams, squashes, and ketchups to inhibit microbial growth.

2. Preservation by Common Salt

Salt draws out moisture from cells via osmosis, creating a dry environment where bacteria cannot survive.

• Used for: Meat, fish, amla, raw mangoes, and tamarind.

3. Preservation by Sugar

Sugar reduces the moisture content of food, inhibiting the growth of bacteria that spoil food.

• Used for: Jams, jellies, and squashes.

4. Preservation by Oil and Vinegar

Bacteria cannot live in an acidic or oily environment.

• Used for: Vegetables, fruits, meat, and pickles.

5. Heat and Cold Treatments

• Boiling: Kills most microorganisms present in water or milk.
• Refrigeration: Low temperatures stop the growth and activity of microbes (though it does not kill them).

6. Pasteurization

A special heat treatment used primarily for milk:

• The Process: Milk is heated to about 70°C for 15 to 30 seconds and then suddenly chilled and stored.
• Why? Sudden chilling prevents the growth of any surviving microbes.
• Discovered by: Louis Pasteur.

7. Storage and Packing

Dry fruits, snacks, and chips are sealed in airtight, nitrogen-filled packets to prevent contact with air and moisture.

Summary Sheet

• Microorganisms are grouped into Bacteria, Fungi, Protozoa, Algae, and Viruses.
• Useful Roles: Making curd/bread, producing antibiotics/vaccines, fixing nitrogen, decomposing waste.
• Harmful Roles: Pathogens cause diseases in humans, animals, and plants.
• Vectors: Insects like mosquitoes (Anopheles, Aedes) and houseflies spread diseases.
• Preservation Techniques: Chemical preservatives, salting, sugaring, oil/vinegar, pasteurization, and airtight sealing.

Common Student Mistakes to Avoid

1. Confusing Key Terminology: Interchanging closely related scientific terms (e.g. mass vs. weight, reflection vs. refraction, or oxidation vs. reduction).
2. Incomplete Chemical Equations or Formulas: Forgetting to balance chemical equations or omitting physical states (s, l, g, aq) in reaction steps.
3. Diagram Labeling Errors: Drawing scientific diagrams without proper arrows showing light rays, electric current flow, or organ functions.
4. Neglecting SI Units in Physics Problems: Calculating work, force, or energy without converting values into standard SI units first.

Practice Questions with Solutions

Question 1: Conceptual Understanding

Why does dough rise when yeast is added to a mixture of flour, sugar, and warm water? Name the process used in making alcohol using yeast.

Solution:

1. Dough Rising: Yeast is a fungus that reproduces rapidly in the presence of sugar and warmth. As yeast respires, it produces Carbon Dioxide ($CO_2$) gas. The bubbles of $CO_2$ gas get trapped in the dough, causing it to swell up, expand, and rise. This makes baked products like bread and cakes light and fluffy.
2. Process Name: The process of converting sugar into alcohol by yeast is called Fermentation.

Question 2: Application-Based Problem

Rohan’s mother boils milk before storing it in the refrigerator. However, for packaged milk bags marked "Pasteurized," she opens and uses them directly without boiling. Explain the scientific reason behind both actions.

Solution:

1. Boiling Raw Milk: Raw milk contains bacteria from the environment. Boiling heats the milk to high temperatures, killing these heat-sensitive microbes so the milk does not spoil quickly. Storing it in the refrigerator afterwards slows down the growth of any remaining microbes.
2. Using Pasteurized Packaged Milk Directly: Pasteurized milk has already undergone a controlled heat treatment: it was heated to around 70°C for 15–30 seconds and then suddenly chilled. This process kills harmful microbes without altering the quality of milk. Since it is free from harmful microorganisms, it is safe to consume directly without boiling.

Question 3: Matching & Prevention

Identify the causative organism (microbe type) and primary transmission route for the following diseases, and provide one prevention step for each:

1. Malaria
2. Tuberculosis
3. Cholera

Solution:

• 1. Malaria:

  • Causative Microbe: Protozoan (Plasmodium)
  • Transmission Route: Vector (Female Anopheles Mosquito)
  • Prevention: Use mosquito nets/repellents and clear standing water to prevent mosquito breeding.

• 2. Tuberculosis (TB):

  • Causative Microbe: Bacteria (Mycobacterium tuberculosis)
  • Transmission Route: Air (droplets from coughing/sneezing)
  • Prevention: Vaccination (BCG vaccine) and keeping the infected patient in isolation.

• 3. Cholera:

  • Causative Microbe: Bacteria (Vibrio cholerae)
  • Transmission Route: Contaminated water or food
  • Prevention: Maintain proper personal hygiene, consume properly cooked food, and drink clean, boiled water.

Exam Preparation & Frequently Asked Questions (FAQ)

Q1. How should I revise Microorganisms: Friend and Foe for the Class 8 Science examination?

Focus on mastering core textbook definitions, practicing 3-4 numerical problems daily with pen and paper, and reviewing previous year CBSE/NCERT board exam questions.

Q2. What are the key concepts that carry maximum marks in this chapter?

Pay special attention to core definitions, step-by-step derivations, solved textbook examples, and practical real-world applications outlined in your NCERT curriculum.

Q3. How can I avoid losing marks in long answer questions?

Always structure your answers with clear subheadings, write step-by-step working for numerical problems, state given values clearly, and highlight your final answers with correct SI units.