Understanding Micro Organisms: Friends and Foes

This study material is being generated based on the NCERT curriculum for grade 8 science students.

Microorganisms, or microbes, are ubiquitous, microscopic living organisms that play a dual role as both essential allies and formidable adversaries to humankind. This briefing synthesizes their fundamental nature, classification, and profound impact on various aspects of life. Beneficially, they are indispensable in food production (e.g., curd, bread, alcohol), medicine (e.g., antibiotics, vaccines), environmental cleanup through decomposition, and agriculture via nitrogen fixation to enhance soil fertility. Conversely, harmful microorganisms, known as pathogens, are responsible for a wide range of diseases in humans, animals, and plants, including tuberculosis, malaria, and citrus canker. They also cause significant economic and health problems through food spoilage and poisoning. Understanding the mechanisms of both their beneficial actions and harmful effects has led to critical innovations in food preservation, disease prevention, and industrial processes, underscoring their integral role in the planet’s ecosystem and human society.
Review for NCERT Curriculum

World of microorganisms
Beneficial role of microorganisms
The harmful impact of microorganisms
Control and prevention strategies
The nitrogen cycle
Knowledge Review quiz
Suggested answers

1. The World of Micro Organisms

1.1. Definition and Classification

Microorganisms are living organisms of such small size that they cannot be seen with the unaided eye. While some, like bread mold, are visible with a magnifying glass, most require a microscope for observation. They are classified into four primary groups:

Bacteria: Single-celled organisms. Examples include Lactobacillus and the bacteria responsible for diseases like tuberculosis and typhoid.
Fungi: Can be single-celled or multicellular. Examples include yeast, bread mould, Penicillium, and Aspergillus.
Protozoa: Single-celled organisms. Examples include Amoeba, Paramecium, and the Plasmodium parasite that causes malaria.
Algae: Can be single-celled or multicellular. Examples include Chlamydomonas and Spirogyra.

Viruses are also microscopic but are distinct from other microbes. They are considered non-living outside a host and can only reproduce inside the cells of a host organism, which can be a bacterium, plant, or animal. Viruses are responsible for common ailments like the cold and flu, as well as serious diseases such as polio and chicken pox.

1.2. Habitat and Characteristics

Microorganisms are exceptionally hardy and can be found in every conceivable environment. Their habitats range from ice-cold climates to hot springs, and from deserts to marshy lands. They are present in air, water, soil, and are also found living inside the bodies of other animals, including humans. Some microbes grow on other organisms, while others exist freely in the environment. They may be unicellular (single-celled), like bacteria and protozoa, or multicellular, like many types of algae and fungi.

The Beneficial Role of Microorganisms (“Friends”)

Microorganisms are utilized for numerous beneficial purposes, playing a critical role in industry, medicine, agriculture, and the environment.

2.1. Food and Beverage Production

Dairy and Baking: The bacterium Lactobacillus is essential for converting milk into curd. Bacteria are also used in making cheese and pickles. Yeast is fundamental to the baking industry; its rapid reproduction and production of carbon dioxide during respiration cause dough for bread, pastries, and cakes to rise, increasing its volume.
Fermentation: The process of converting sugar into alcohol, known as fermentation, was discovered by Louis Pasteur in 1857. Yeast is used for the large-scale commercial production of alcohol, wine, and acetic acid (vinegar) by growing it on natural sugars found in grains (barley, wheat, rice) and fruit juices.

2.2. Medicinal Applications

Antibiotics: These medicines, sourced from microorganisms like bacteria and fungi, kill or halt the growth of disease-causing microbes. Alexander Fleming’s 1929 discovery of penicillin from a green mould was a landmark achievement. Other common antibiotics include streptomycin, tetracycline, and erythromycin. It is crucial that antibiotics are taken only on a doctor’s advice and the full course is completed to prevent the development of drug resistance. They are ineffective against viral infections like the cold and flu.
Vaccines: A vaccine works by introducing dead or weakened microbes into a healthy body. This stimulates the body to produce antibodies to fight the invaders, creating a “memory” of how to combat the microbe. If the actual disease-carrying microbe enters the body later, the pre-existing antibodies provide protection. Vaccination has been instrumental in preventing diseases like cholera, tuberculosis, hepatitis, and smallpox. Edward Jenner discovered the smallpox vaccine in 1798.

2.3. Environmental and Agricultural Contributions

Environmental Cleanup: Microorganisms act as nature’s decomposers. They break down dead organic waste—such as decaying plants, animal remains, and vegetable peels—into simple, harmless, and usable substances. This process cleans up the environment and returns nutrients to the ecosystem, which can be used by plants again.
Increasing Soil Fertility: Certain bacteria and blue-green algae are known as “biological nitrogen fixers.” They can absorb nitrogen gas from the atmosphere and convert it into nitrogenous compounds in the soil. This process, called nitrogen fixation, enriches the soil and increases its fertility. The bacterium Rhizobium, which lives in a symbiotic relationship in the root nodules of leguminous plants (pulses), is a key example.

3. The Harmful Impact of Micro Organisms (“Foes”)

While many microbes are beneficial, others are harmful, causing disease, spoiling food, and damaging materials.

3.1. Pathogens and Disease

Disease-causing microorganisms are called pathogens. They can enter the human body through air, water, or food, or be transmitted through direct contact with an infected person or via an animal carrier.

Human Diseases: Microbial diseases that spread from an infected person to a healthy one are called communicable diseases.
- Viral Diseases: Common cold, influenza (flu), measles, chicken pox, polio, Hepatitis A.
- Bacterial Diseases: Tuberculosis (TB), cholera, typhoid, anthrax.
- Protozoan Diseases: Dysentery, malaria.
Disease Carriers: Some insects and animals act as carriers, transferring pathogens without getting sick themselves. The housefly can transfer pathogens from garbage to uncovered food. The female Anopheles mosquito is a carrier for the malaria parasite (Plasmodium), and the female Aedes mosquito carries the dengue virus.
Animal and Plant Diseases: Pathogens also affect other organisms. Anthrax, caused by the bacterium Bacillus anthracis (discovered by Robert Koch in 1876), is a dangerous disease affecting both humans and cattle. Foot and mouth disease in cattle is caused by a virus. In plants, microbes cause diseases like citrus canker (bacteria), rust of wheat (fungi), and yellow vein mosaic of bhindi (virus), which reduce crop yields.

3.2. Food Spoilage and Poisoning

Microorganisms that grow on food can spoil it by producing toxic substances. This spoilage is indicated by a bad smell, bad taste, and changed color. Consuming such food leads to food poisoning, which can cause serious illness and even death.

4. Control and Prevention Strategies

4.1. Food Preservation Techniques

To prevent food spoilage from microbial attack, various preservation methods are used:

Method	Description	Examples of Use
Chemical Preservatives	Salts and edible oils check microbial growth. Common preservatives like sodium benzoate and sodium metabisulphite are also used.	Pickles (salt, oil, vinegar), jams, squashes.
Preservation by Salt	Covering food with dry salt checks the growth of bacteria.	Meat, fish, amla, raw mangoes.
Preservation by Sugar	Sugar reduces moisture content, inhibiting bacterial growth.	Jams, jellies, squashes.
Heat Treatment	Boiling milk kills many microorganisms.	Storing milk at home.
Cold Treatment	Low temperatures in a refrigerator inhibit the growth of microbes.	Storing various cooked and fresh foods.
Pasteurisation	Milk is heated to about 70°C for 15-30 seconds and then suddenly chilled. This process, discovered by Louis Pasteur, prevents microbial growth.	Commercially packaged milk.
Storage and Packing	Sealing food in airtight packets prevents the attack of microbes.	Dry fruits, vegetables.

4.2. Disease Prevention

The spread of communicable diseases can be prevented by:

Maintaining personal hygiene and good sanitary habits.
Keeping patients in isolation and their belongings separate.
Getting vaccinations at a suitable age.
Consuming properly cooked food and boiled drinking water.
Controlling disease carriers, for example, by preventing mosquitoes from breeding in collected water.

5. The Nitrogen Cycle

The atmosphere is composed of 78% nitrogen gas, an essential constituent of proteins, chlorophyll, and nucleic acids. However, plants and animals cannot use atmospheric nitrogen directly. The nitrogen cycle describes how nitrogen is converted into usable forms and circulated through the ecosystem.

Nitrogen Fixation: Certain bacteria and blue-green algae in the soil, along with lightning, “fix” atmospheric nitrogen, converting it into usable nitrogen compounds.
Assimilation: Plants absorb these compounds from the soil through their roots and use them to synthesize proteins and other compounds.
Consumption: Animals obtain nitrogen by eating plants.
Decomposition: When plants and animals die, bacteria and fungi in the soil convert their nitrogenous wastes back into nitrogenous compounds, which can be reused by plants.
Denitrification: Certain other bacteria convert some of these nitrogenous compounds back into nitrogen gas, which is released into the atmosphere, maintaining a constant percentage.

Knowledge Review Quiz

Multiple Choice Questions (20)

Which of these is NOT one of the four major groups of microorganisms? (a) Bacteria (b) Fungi (c) Viruses (d) Algae
The bacterium Lactobacillus is used in the making of: (a) Bread (b) Curd (c) Antibiotics (d) Alcohol
The process of converting sugar into alcohol is called: (a) Pasteurisation (b) Nitrogen Fixation (c) Fermentation (d) Vaccination
Who discovered the vaccine for smallpox in 1798? (a) Louis Pasteur (b) Alexander Fleming (c) Robert Koch (d) Edward Jenner
Which of the following diseases is caused by a protozoan? (a) Tuberculosis (b) Malaria (c) Chicken Pox (d) Typhoid
Yeast is used in the large-scale production of: (a) Sugar (b) Alcohol (c) Oxygen (d) Salt
Which of the following is an antibiotic? (a) Sodium benzoate (b) Streptomycin (c) Yeast (d) Vinegar
The female Anopheles mosquito is a carrier for the parasite that causes: (a) Dengue (b) Cholera (c) Malaria (d) Typhoid
The bread or idli dough rises because of the production of: (a) Oxygen (b) Heat (c) Carbon dioxide (d) Water vapor
The bacterium Rhizobium is involved in: (a) Causing disease (b) Food poisoning (c) Nitrogen fixation (d) Decomposing waste
Which method is used to preserve jams and jellies? (a) Preservation by salt (b) Preservation by sugar (c) Preservation by oil (d) Pasteurisation
Polio and chicken pox are caused by: (a) Bacteria (b) Fungi (c) Protozoa (d) Viruses
Who discovered the process of fermentation in 1857? (a) Edward Jenner (b) Robert Koch (c) Alexander Fleming (d) Louis Pasteur
Disease-causing microorganisms are called: (a) Antibodies (b) Antibiotics (c) Pathogens (d) Carriers
The most common carrier of communicable diseases mentioned in the text is: (a) Ant (b) Housefly (c) Butterfly (d) Mosquito
The process of heating milk to about 70°C and then suddenly chilling it is called: (a) Fermentation (b) Vaccination (c) Salting (d) Pasteurisation
Citrus canker is a plant disease caused by: (a) A virus (b) A bacterium (c) A fungus (d) An insect
What percentage of our atmosphere is nitrogen gas? (a) 21% (b) 50% (c) 78% (d) 90%
Which of these is NOT a communicable disease? (a) Cholera (b) Common cold (c) Food poisoning (d) Tuberculosis
Robert Koch discovered the bacterium that causes: (a) Smallpox (b) Malaria (c) Anthrax (d) Polio

Fill in the Blanks (20)

Microorganisms can be seen with the help of a __________.
Blue-green algae fix __________ directly from the air to enhance soil fertility.
The source of antibiotic medicines is __________.
When a disease-carrying microbe enters our body, the body produces __________ to fight the invader.
The female Aedes mosquito acts as a carrier of the __________ virus.
The process of __________ involves the conversion of dead organic waste into manure by microbes.
Typhoid and tuberculosis are __________ diseases.
__________ discovered penicillin in 1929.
Microorganisms that spoil food can produce __________ substances, causing food poisoning.
Use of oil and vinegar prevents spoilage of pickles because __________ cannot live in such an environment.
The bacterium involved in the fixation of nitrogen in leguminous plants is __________.
Common ailments like cold, influenza, and most coughs are caused by __________.
Sodium benzoate and sodium metabisulphite are common chemical __________.
A worldwide campaign against __________ has led to its eradication from most parts of the world.
The tiny, black rounded structures seen on moist bread are a type of fungus called __________.
The process in the nitrogen cycle where bacteria convert nitrogenous compounds back into nitrogen gas is called __________.
Polio drops given to children are actually a __________.
Bubbles of __________ gas from yeast fill the dough and increase its volume.
__________ are microorganisms that live only inside the cells of a host organism.
Anthrax is a dangerous human and __________ disease.

Short Answer Questions (10)

What are the four major groups of microorganisms? Provide one example for each.
Explain why viruses are different from other microorganisms.
Describe the process of fermentation and name its discoverer.
What are antibiotics? What precautions should be taken while taking them?
How does a vaccine work to protect a person from a disease?
List three ways in which microorganisms are beneficial to the environment and agriculture.
What is food poisoning and how is it caused by microorganisms?
Explain the method of pasteurisation and why it is used.
What are communicable diseases? Give two examples and describe one way they are transmitted.
Briefly outline the key steps of the nitrogen cycle.

Understanding Micro Organisms: Friends and Foes

1. The World of Micro Organisms

The Beneficial Role of Microorganisms (“Friends”)

3. The Harmful Impact of Micro Organisms (“Foes”)

4. Control and Prevention Strategies

5. The Nitrogen Cycle

Knowledge Review Quiz

Multiple Choice Questions (20)

Fill in the Blanks (20)

Short Answer Questions (10)

Suggested Answers

Like this:

1. The World of Micro Organisms

The Beneficial Role of Microorganisms (“Friends”)

3. The Harmful Impact of Micro Organisms (“Foes”)

4. Control and Prevention Strategies

5. The Nitrogen Cycle

Knowledge Review Quiz

Multiple Choice Questions (20)

Fill in the Blanks (20)

Short Answer Questions (10)

Suggested Answers

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