Microorganisms are tiny living organisms that cannot be seen with the naked eye. They are found everywhere—in air, water, soil, food, and even inside the human body. Some microorganisms are beneficial because they help make food, recycle nutrients, and produce medicines, while others cause diseases. In this easy Class 8 Biology guide, you will learn the definition of microorganisms, their types, examples, importance, harmful effects, and interesting facts in simple language.
| Human Disease | Causative Microorganism | Mode of Transmission | Preventive Measures |
|---|---|---|---|
| Tuberculosis | Bacteria | Air | Isolate the patient; maintain personal hygiene; vaccination at suitable age. |
| Measles / Polio | Virus | Air / Water | |
| Cholera / Typhoid | Bacteria | Water / Food | Drink boiled drinking water; consume properly cooked food. |
| Malaria | Protozoa | Mosquito (Vector) | Use mosquito nets/repellents; do not allow water to stagnate nearby. |
Microorganisms are omnipresent—they exist almost everywhere on Earth. They can survive in all types of environments, ranging from ice-cold climates and deep-sea hydrothermal vents to scalding hot springs and dry, barren deserts. They are also found inside the bodies of animals and humans.
Some microorganisms live alone (like Amoeba), while others live in vast colonies (like fungi and bacteria).
🔬 The 4 Major Groups of Microorganisms
Microorganisms are broadly classified into four major biological groups based on their cellular structure and characteristics.
1. Bacteria
Bacteria are single-celled (unicellular) prokaryotic organisms. They come in various shapes, such as spherical (coccus) or rod-shaped (bacillus).
Examples: Lactobacillus (used to make curd), Rhizobium (helps in nitrogen fixation).
2. Fungi
Fungi are mostly multicellular, saprophytic or parasitic organisms that lack chlorophyll and cannot perform photosynthesis.
Examples: Bread mould, Penicillium, Yeast (used in baking).
3. Protozoa
Protozoa are unicellular eukaryotic organisms, many of which are heterotrophic and move around to capture food. Some are parasites that cause diseases.
Examples: Amoeba, Paramecium, Plasmodium (the malarial parasite).
4. Algae
Algae are simple, plant-like organisms that contain chlorophyll and can synthesize their own food through photosynthesis. They can be unicellular or multicellular.
Examples: Chlamydomonas, Spirogyra.
⚠️ The Curious Case of Viruses
Viruses are also microscopic entities, but they are considered unique and distinct from the four major groups of microorganisms.
Why they are different: Viruses do not show any characteristics of life outside a host body. They reproduce only inside the cells of a host organism, which may be a bacterium, a plant, or an animal.
Because they behave as non-living objects outside a host and living organisms inside a host, they are considered to be on the borderline between the living and non-living worlds.
Examples: Influenza virus, HIV, Bacteriophage.
🤝 Friendly Microorganisms (Where They Help Us)
Microorganisms play a vital role in our daily lives, agriculture, and industries:
Making Curd and Bread: The bacterium Lactobacillus promotes the formation of curd from milk by multiplying in it. Yeast reproduces rapidly and produces carbon dioxide (CO_2) gas during respiration. This gas bubbles out, causing dough to rise, which is the secret behind fluffy bread, cakes, and pastries.
Commercial Use (Fermentation): Microbes are used for the large-scale production of alcohol, wine, and acetic acid (vinegar). Yeast converts natural sugars present in grains like barley, wheat, and rice into alcohol—a process known as fermentation.
Medicinal Use (Antibiotics): When you fall ill, doctors may give you antibiotics. These are medicines manufactured using specific fungi and bacteria to kill or stop the growth of disease-causing microorganisms.
Note: The first antibiotic, Penicillin, was discovered by Alexander Fleming in 1929 from the mould Penicillium notatum.
Vaccines:
Vaccines consist of dead or weakened microbes introduced into a healthy body. The body fights them off by producing defensive proteins called antibodies. If the real microbe attacks later, the body remembers how to defeat it.
Increasing Soil Fertility: Certain bacteria like Rhizobium and Blue-green algae can fix nitrogen gas from the atmosphere into the soil, enriching it with nutrients and increasing its fertility.
Cleaning the Environment: Microorganisms act as natural decomposers. They break down organic waste, dead plants, and animal remains into simple harmless substances, recycling nutrients back into nature.
🪰 Harmful Microorganisms (Pathogens)
Microorganisms that cause diseases in humans, animals, and plants are called pathogens.
1. Human Diseases
Pathogens enter our body through the air we breathe, the water we drink, or the food we eat. Microbial diseases that can spread from an infected person to a healthy person through air, water, food, or physical contact are called communicable diseases (e.g., Cholera, Common Cold, Chickenpox, Tuberculosis).
Carriers: Some insects and animals act as carriers of disease-causing microbes.
The Female Anopheles mosquito carries the parasite of malaria (Plasmodium).
The Female Aedes mosquito acts as a carrier of the Dengue virus.
2. Animal and Plant Diseases
Anthrax is a dangerous human and cattle disease caused by a bacterium.
Microbes also ruin crops, reducing agricultural yield. Examples include Citrus canker (bacterial disease in air) and Rust of wheat (fungal disease carried by seeds/air).
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