Microorganism: Friend and Foe - CBSE Class 8 Science

What are Microorganisms?

Microorganisms, or microbes, are living organisms that are so small they cannot be seen with the naked eye. To see them, we need a special instrument called a microscope. They are found almost everywhere on Earth—in scorching hot deserts, icy cold polar regions, salty seas, marshy lands, and even inside the bodies of other living organisms, including humans. While some live alone, others grow in groups called colonies. These tiny powerhouses are essential for life on our planet. They are broadly classified into four major groups: Bacteria, Fungi, Protozoa, and Algae. Viruses are also considered microbes, but they are unique because they are on the borderline between living and non-living things. They can only reproduce by invading the cells of a host organism like a bacterium, plant, or animal.

The Major Groups of Microorganisms

Bacteria

Single-celled organisms that come in different shapes like rod-shaped (bacilli), spherical (cocci), and spiral (spirilla). Examples include Lactobacillus, which helps make curd, and E. coli.

Fungi

These are non-green, plant-like organisms that cannot make their own food. They can be single-celled like Yeast or multi-cellular like Bread Mould and mushrooms. They thrive in dark, warm, and moist conditions.

Protozoa

A group of single-celled organisms which are animal-like. Most are heterotrophic. Amoeba and Paramoecium are common examples. Some cause serious diseases like malaria (caused by Plasmodium) and dysentery (caused by Entamoeba).

Algae

Simple, plant-like organisms containing chlorophyll, which allows them to make their own food through photosynthesis. They can be unicellular (like Chlamydomonas) or multicellular (like Spirogyra).

Viruses

Viruses are microscopic infectious agents that are much smaller than bacteria. They are non-cellular and can only replicate inside the living cells of other organisms. Outside a host cell, they are inactive. They cause diseases like the common cold, influenza (flu), polio, and COVID-19.

Our Friends: How Microorganisms Help Us

• Making Curd, Bread, and Cake: The bacterium Lactobacillus is our friend in the kitchen! It multiplies in milk and converts it into curd. Yeast, a type of fungus, is essential for baking. When yeast respires, it releases carbon dioxide gas, which causes dough to rise, making breads, cakes, and idlis soft and fluffy.
• Commercial Production of Alcohol and Vinegar: Yeast is used on a large scale to produce alcohol and wine from sugars present in grains like barley, wheat, and rice, and fruit juices. This process of converting sugar into alcohol is known as fermentation. Bacteria are also used to produce acetic acid from alcohol, which we know as vinegar.
• Making Medicines (Antibiotics and Vaccines): Some beneficial bacteria and fungi are used to produce antibiotics, which are medicines that kill or stop the growth of disease-causing microbes. Penicillin, Streptomycin, and Tetracycline are common antibiotics. Vaccines also use weakened or dead microbes to trigger our immune system to produce antibodies, preparing our body to fight off future infections from that specific microbe.
• Increasing Soil Fertility: Certain bacteria, like Rhizobium found in the root nodules of leguminous plants (like peas and beans), and blue-green algae can 'fix' atmospheric nitrogen into usable nitrogenous compounds in the soil. This enriches the soil and increases its fertility, acting as a natural fertilizer.
• Cleaning the Environment: Microbes act as nature's decomposers. They break down complex organic waste from dead plants, animals, and faeces into simple, harmless substances. This not only cleans up our surroundings but also recycles vital nutrients back into the ecosystem.

The Nitrogen Cycle Explained

1. Step 1: Nitrogen Fixation — Our atmosphere is 78% nitrogen gas (N₂), but plants and animals cannot use it directly. Nitrogen fixation is the process where atmospheric nitrogen is converted into usable compounds like ammonia. This is done by lightning strikes and more importantly, by nitrogen-fixing bacteria (like Rhizobium) and blue-green algae.
2. Step 2: Nitrogen Assimilation — Plants absorb these nitrogen compounds (like nitrates) from the soil through their roots. They then use this nitrogen to build essential molecules like proteins and nucleic acids. Animals get nitrogen by eating these plants or other animals.
3. Step 3: Ammonification — When plants and animals die, or when animals excrete waste, decomposers (like bacteria and fungi) break down the organic matter. This process releases the nitrogen back into the soil in the form of ammonium compounds.
4. Step 4: Nitrification — Nitrifying bacteria in the soil convert the ammonium compounds first into nitrites (NO₂⁻) and then into nitrates (NO₃⁻). Nitrates are the form of nitrogen most easily absorbed by plants.
5. Step 5: Denitrification — Denitrifying bacteria complete the cycle. They take nitrates from the soil and convert them back into nitrogen gas (N₂), which is then released into the atmosphere. This process ensures that the amount of nitrogen in the atmosphere remains relatively constant.

Important Exam Tip: Antibiotics vs. Viruses

A very common exam question is about why antibiotics don't work against viral infections like the common cold or flu. Remember this key difference: Antibiotics are designed to disrupt the life processes of bacteria. For example, some antibiotics block the bacteria's ability to build a cell wall, causing them to die. Viruses are completely different. They are not true cells and lack their own metabolic machinery. They invade our own body's cells and use our cells' machinery to make copies of themselves. Since viruses don't have cell walls or the same metabolic pathways as bacteria, antibiotics have nothing to target. This is why taking antibiotics for a viral illness is ineffective and can contribute to antibiotic resistance.

Practice Questions with Solutions

• Q: What are antibiotics? What precautions must be taken while taking them? A: Step 1: Define antibiotics. Antibiotics are medicines produced by certain microorganisms (like bacteria and fungi) that kill or stop the growth of other disease-causing microorganisms. Step 2: List the precautions. First, antibiotics should only be taken on the advice of a qualified doctor. Second, one must complete the full course prescribed by the doctor. Third, one should not take antibiotics when not needed, such as for a viral infection like a cold, as this can make the drug less effective in the future. Final answer: Antibiotics are medicines that fight bacterial infections. They should always be taken under a doctor's supervision, the full course must be completed, and they should not be used unnecessarily.
• Q: Explain the process of fermentation with an example. Who discovered it? A: Step 1: Define fermentation. Fermentation is the process by which microorganisms like yeast convert sugars into alcohol and carbon dioxide in the absence of oxygen. Step 2: Provide an example. A common example is the use of yeast in the baking and brewing industries. Yeast is added to fruit juices or grain mixtures. It converts the sugar into alcohol (for making wine and beer) and carbon dioxide. Step 3: Name the discoverer. The process of fermentation was discovered by the famous scientist Louis Pasteur in 1857. Final answer: Fermentation is the conversion of sugar into alcohol by yeast. For example, it is used to make wine from grape juice. Louis Pasteur discovered this process in 1857.
• Q: What are pathogens? Name two ways they can enter our bodies. A: Step 1: Define pathogens. Pathogens are disease-causing microorganisms. They are the 'foes' in the microbial world. Step 2: List entry methods. Pathogens can enter the body of a healthy person in several ways. Two common ways are: 1. Through the air we breathe: Microbes causing diseases like the common cold, influenza, and tuberculosis can spread through droplets from an infected person's sneeze or cough. 2. Through contaminated water or food: Pathogens causing diseases like cholera, typhoid, and food poisoning can enter our body when we consume infected water or food. Final answer: Pathogens are microbes that cause diseases. They can enter our bodies through the air we breathe and through contaminated food or water.
• Q: Why is it important to preserve food? Name two methods of food preservation. A: Step 1: Explain the need for preservation. Food preservation is important to prevent spoilage by microorganisms. Microbes grow on food and produce toxic substances, making it unfit for consumption and causing food poisoning. Preservation increases the shelf life of food, reduces wastage, and ensures food availability in off-seasons. Step 2: Name two preservation methods. 1. Chemical Method: Using preservatives like sodium benzoate and sodium metabisulphite (commonly used in jams and squashes) to check the growth of microbes. 2. Pasteurization: Heating milk to about 70°C for 15 to 30 seconds and then suddenly chilling and storing it. This process, discovered by Louis Pasteur, kills most harmful microbes. Final answer: Food preservation is crucial to prevent microbial spoilage, which can cause illness and waste food. Two methods are using chemical preservatives like sodium benzoate and pasteurization of milk.

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