Microorganisms: Friend And Foe - Class 8 Science (NCERT)

What Are Microorganisms?

Microorganisms, often called microbes, are living organisms that are so tiny they cannot be seen without the aid of a microscope. They are ubiquitous, meaning they are found everywhere – in the air, water, soil, inside our bodies, and even in extreme environments like hot springs and ice. Despite their small size, they play crucial roles in our ecosystem and everyday life. We can broadly classify microorganisms into several major groups:

1. Bacteria: These are single-celled organisms with a simple structure. They can be rod-shaped, spherical, or spiral. Examples include Lactobacillus (used in making curd) and Rhizobium (helps in nitrogen fixation).
2. Fungi: These can be single-celled (like yeast) or multicellular (like moulds and mushrooms). They are heterotrophic, meaning they cannot make their own food and often grow on decaying matter. Examples include yeast (used in baking) and Penicillium (source of penicillin).
3. Protozoa: These are single-celled animal-like microorganisms. They can live in water, soil, or as parasites inside other organisms. Examples include Amoeba and Plasmodium (causes malaria).
4. Algae: These are simple plant-like organisms, often found in water. They can be single-celled or multicellular and contain chlorophyll, allowing them to perform photosynthesis. Examples include Chlamydomonas and Spirogyra.
5. Viruses: These are unique because they are considered to be on the border between living and non-living. They are much smaller than bacteria and can only reproduce inside the cells of other living organisms. Examples include the influenza virus and HIV.

Our Microscopic Friends: Beneficial Microorganisms

Not all microorganisms are harmful; many are incredibly useful to humans and the environment. They contribute significantly to various processes, from food production to medicine and agriculture. Let's explore some of their friendly roles:

• Food Production: Microorganisms like Lactobacillus bacteria are essential for converting milk into curd. Yeast is used in baking bread, pastries, and cakes, making them soft and fluffy due to the production of carbon dioxide gas during fermentation. It's also vital in the production of alcohol and wine.
• Commercial Use: Yeast is extensively used in the large-scale production of alcohol, wine, and acetic acid (vinegar). The process of converting sugar into alcohol by yeast is called fermentation.
• Medicinal Use: Many antibiotics, which are medicines that kill or stop the growth of disease-causing microorganisms, are produced from fungi and bacteria. For example, penicillin was discovered from the fungus Penicillium. Vaccines, which help our body develop immunity against diseases, also often involve weakened or dead microbes.
• Environmental Cleanup (Decomposition): Microorganisms like bacteria and fungi act as decomposers. They break down dead organic matter (plants and animals) into simpler substances, returning nutrients to the soil. This process cleans up the environment and makes the soil fertile.
• Nitrogen Fixation: Certain bacteria, like Rhizobium, live in the root nodules of leguminous plants (like peas and beans). They convert atmospheric nitrogen into usable forms for plants, enriching the soil and reducing the need for chemical fertilizers.
• Sewage Treatment: Microorganisms help break down organic waste in sewage treatment plants, making water safer to release into rivers and oceans.

Our Microscopic Foes: Harmful Microorganisms

While many microbes are our allies, a significant number are our adversaries, causing diseases in humans, animals, and plants. These disease-causing microorganisms are called pathogens. They can enter our bodies through various means, leading to infections and illnesses.

• Diseases in Humans: Pathogens can cause a wide range of diseases. For example, bacteria cause diseases like tuberculosis, cholera, and typhoid. Viruses are responsible for common cold, flu, polio, measles, and chickenpox. Protozoa cause malaria (by Plasmodium) and dysentery (by Amoeba). Fungi can cause skin infections like ringworm.
• Diseases in Animals: Microorganisms also affect livestock. For instance, Anthrax, a dangerous human and cattle disease, is caused by bacteria. Foot and Mouth disease in cattle is caused by a virus.
• Diseases in Plants: Many microbes spoil crops and reduce yields. Bacteria cause citrus canker in citrus fruits. Fungi cause rust of wheat and blight of potatoes. Viruses cause mosaic diseases in various plants. These plant diseases can be controlled by spraying certain chemicals.
• Food Spoilage: Harmful microbes can grow on food items, producing toxic substances. This makes the food inedible and sometimes poisonous. This is why food preservation is so important.

Methods of Food Preservation

1. Chemical Methods — Adding chemical preservatives like sodium benzoate and sodium metabisulphite inhibits microbial growth. These are commonly used in jams, squashes, and pickles to prevent spoilage.
2. Preservation by Common Salt — Salt removes water from food through osmosis, creating an environment unsuitable for microbial growth. This method is used for preserving meat, fish, raw mangoes, tamarind, and amla.
3. Preservation by Sugar — Sugar also reduces moisture content in food, inhibiting the growth of bacteria that spoil food. Jams, jellies, and squashes are preserved this way.
4. Preservation by Oil and Vinegar — Oil and vinegar create an acidic and oxygen-deprived environment where bacteria cannot survive. Pickles are a classic example where these are used as preservatives.
5. Heat and Cold Treatments — Heating food to a high temperature kills many microbes (e.g., boiling milk). Cooling food (refrigeration) slows down microbial growth. Pasteurization (heating milk to about 70°C for 15 to 30 seconds and then suddenly cooling it) kills harmful microbes without significantly altering its taste.
6. Storage and Packing — Airtight containers and vacuum-sealed packaging prevent exposure to air and moisture, which are necessary for microbial growth. This method is used for preserving dry fruits and vegetables.

Examples of Communicable Diseases and Their Transmission

• Tuberculosis (TB): Caused by bacteria. Transmitted through air (droplets from coughing/sneezing by an infected person).
• Measles: Caused by virus. Transmitted through air (droplets).
• Chickenpox: Caused by virus. Transmitted through air (droplets) and direct contact.
• Polio: Caused by virus. Transmitted through contaminated food and water.
• Cholera: Caused by bacteria. Transmitted through contaminated food and water.
• Typhoid: Caused by bacteria. Transmitted through contaminated food and water.
• Malaria: Caused by protozoa (Plasmodium). Transmitted by female Anopheles mosquito bite.
• Dengue: Caused by virus. Transmitted by female Aedes mosquito bite.
• Common Cold: Caused by virus. Transmitted through air (droplets) and direct contact with infected surfaces.

Exam Tip: Remembering Key Concepts

When studying Microorganisms, it's crucial to distinguish between their 'friend' and 'foe' roles. Don't just memorise lists; try to understand the why behind each example. For instance, think about how Lactobacillus makes curd (fermentation) or how salt preserves food (removes moisture). Also, pay close attention to the modes of transmission for different diseases. A common mistake is confusing viral diseases with bacterial ones or misidentifying the vectors (like mosquitoes) for diseases like malaria and dengue. Creating a table for beneficial and harmful microorganisms with specific examples and their actions can be very helpful for quick revision.

Practice Questions with Solutions

• Q: What are antibiotics? Give two examples and explain their function.
  A: Step 1: Define antibiotics as medicines that kill or stop the growth of disease-causing microorganisms.
  Step 2: Provide examples such as Penicillin and Streptomycin.
  Step 3: Explain their function: They work by targeting specific processes in bacteria or fungi, thus curing infections without harming the host cells. For example, Penicillin interferes with bacterial cell wall formation.
  Final answer: Antibiotics are medicines that destroy or stop the growth of disease-causing microorganisms. Examples include Penicillin and Streptomycin. They function by inhibiting the vital processes of pathogens, helping to cure infections.

• Q: Why is pasteurization important for milk? Describe the process.
  A: Step 1: Explain the purpose of pasteurization: It kills harmful microbes in milk without significantly changing its nutritional value or taste.
  Step 2: Describe the process: Milk is heated to about 70°C for 15 to 30 seconds.
  Step 3: Immediately cool the milk: After heating, it is quickly cooled to prevent the remaining microbes from growing.
  Final answer: Pasteurization is important for milk because it kills most harmful microorganisms present, making it safe for consumption and increasing its shelf life. The process involves heating milk to about 70°C for 15-30 seconds, and then rapidly cooling it to prevent microbial growth.

• Q: Name two common methods of food preservation and explain how each works.
  A: Step 1: Identify two methods, e.g., Preservation by Common Salt and Preservation by Sugar.
  Step 2: Explain Preservation by Common Salt: Salt draws out water from food through osmosis, creating a dry environment where microbes cannot grow. It is used for meat and pickles.
  Step 3: Explain Preservation by Sugar: Sugar also reduces the moisture content in food, making it difficult for bacteria and fungi to thrive. This method is common for jams and jellies.
  Final answer: Two common methods of food preservation are Preservation by Common Salt and Preservation by Sugar. Common salt works by drawing out water from food, preventing microbial growth. Sugar reduces the moisture content, creating an environment unsuitable for microorganisms.

• Q: How do certain bacteria help in increasing soil fertility?
  A: Step 1: Recall the role of specific bacteria in soil.
  Step 2: Identify nitrogen-fixing bacteria like Rhizobium.
  Step 3: Explain their function: These bacteria live in the root nodules of leguminous plants and convert atmospheric nitrogen gas into usable nitrogen compounds (like nitrates) that plants can absorb.
  Final answer: Certain bacteria, like Rhizobium, help increase soil fertility through a process called nitrogen fixation. They convert atmospheric nitrogen into nitrogenous compounds, which are essential nutrients for plant growth, thereby enriching the soil.

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