Micro-Organisms

Overview & Fundamental Concept

Complete AISSEE Class 9 General Science guide for Micro-Organisms (Topic 14). Contains pointwise microbiology facts, disease-vector comparison matrices, antibiotic/vaccine discovery tables, solved models, and 10 practice drills.

• Micro-organisms (Microbes) Definition: Living organisms that are so microscopic in size that they cannot be seen with the unaided human eye; they can only be observed using a microscope.
• Five Major Classifications of Microbes:
  • 1. Bacteria: Single-celled (unicellular) prokaryotic organisms. Have cell walls but lack a membrane-bound nucleus. Shapes: Rod-shaped (Bacillus), Spherical (Coccus), Spiral (Spirillum), and Comma-shaped (Vibrio). Examples: Lactobacillus, Rhizobium, Vibrio cholerae.
  • 2. Fungi: Non-green, heterotrophic eukaryotic organisms (saprophytes or parasites) with chitinous cell walls. Can be unicellular (Yeast) or multicellular (Bread mould / Rhizopus, Penicillium, Aspergillus, Mushrooms).
  • 3. Protozoa: Microscopic, unicellular animal-like eukaryotes. Lack cell walls and capture food via heterotrophy. Examples: Amoeba (moves via pseudopodia), Paramecium (moves via cilia, slipper-shaped), Plasmodium (malarial parasite), Euglena (photosynthetic with flagellum).
  • 4. Algae: Simple, plant-like autotrophic organisms containing chlorophyll. Possess cell walls and carry out photosynthesis. Can be unicellular (Chlamydomonas) or multicellular/filamentous (Spirogyra, seaweeds).
  • 5. Viruses: Ultramicroscopic acellular entities on the borderline between living and non-living things.
   • Why Borderline? Outside a living cell, viruses show zero metabolic activity and can be crystallized like non-living minerals. Inside a living host cell (bacterial, plant, or animal), they hijack host machinery and reproduce rapidly.
   • Diseases caused by viruses: Common cold, Influenza (Flu), Polio, Chickenpox, Measles, Rabies, COVID-19, and AIDS.

• 1. Commercial Food Production:
  • Curd Formation: Milk contains the sugar lactose. The bacterium Lactobacillus multiplies in warm milk and converts lactose into lactic acid, coagulating milk protein (casein) into curd.
  • Baking Industry: Yeast (Saccharomyces cerevisiae) reproduces rapidly in bread dough and produces Carbon Dioxide ($CO_2$) gas during anaerobic respiration. The gas bubbles get trapped in the dough, causing it to rise and making bread, cakes, and idlis soft and spongy.
  • Commercial Alcohol & Wine (Fermentation): Yeast is grown on natural sugars present in barley, wheat, rice, and fruit juices to convert sugar into alcohol.
  • Louis Pasteur (1857): French scientist who discovered the biological process of Fermentation in 1857.
• 2. Soil Fertility & Nitrogen Fixation:
  • Plants cannot absorb atmospheric nitrogen ($N_2$) directly.
  • Rhizobium Bacteria: Live in the symbiotic root nodules of leguminous plants (peas, gram, beans) and convert atmospheric nitrogen into water-soluble nitrates that plants absorb.
  • Blue-Green Algae (Cyanobacteria: Nostoc, Anabaena) & Azotobacter: Free-living biological nitrogen fixers in soil.
• 3. Medicinal Uses: Antibiotics & Vaccines:
  • Antibiotics: Medicines produced by microorganisms that kill or stop the growth of disease-causing bacteria. Examples: Streptomycin, Tetracycline, Erythromycin.
  • Alexander Fleming (1928): Discovered the world's first antibiotic — Penicillin in 1928. While working on Staphylococcus bacteria, he noticed that a green mould (Penicillium notatum) growing on an unwashed culture plate prevented bacteria from growing around it.
  • Vaccines & Edward Jenner (1798): When a weakened or dead microbe (vaccine) is introduced into a healthy body, the immune system produces protective proteins called Antibodies. The body remembers how to fight the microbe if it attacks again.
  • Edward Jenner discovered the Smallpox vaccine in 1798 (the first vaccine in medical history). Smallpox has since been completely eradicated globally.
  • Robert Koch (1876): Discovered the rod-shaped bacterium Bacillus anthracis, which causes fatal Anthrax disease in cattle and humans. He also discovered the bacterium causing Tuberculosis in 1882.

• Pathogens: Disease-causing micro-organisms that enter our body through air, water, food, or direct physical contact.
• Communicable Diseases: Microbial diseases that can spread from an infected person to a healthy person through air, water, food, vectors, or physical contact (e.g. cholera, common cold, chickenpox, tuberculosis).
• Disease Carriers (Vectors):
  • 1. Housefly (Musca domestica): Carries pathogens on its hairy body from garbage and uncollected faeces to uncovered food, spreading Cholera, Typhoid, and Dysentery.
  • 2. Female Anopheles Mosquito: Acts as the carrier (vector) for the parasite of Malaria (Plasmodium, a protozoan).
  • 3. Female Aedes Mosquito: Acts as the carrier (vector) for the Dengue virus.
  • Control: Mosquitoes breed in stagnant water. Keep coolers, tyres, and flowerpots dry and clean to prevent breeding.
• Human Diseases Caused by Micro-organisms:
  • Bacterial Diseases: Tuberculosis (air), Cholera (contaminated water/food), Typhoid (contaminated water).
  • Viral Diseases: Measles (air), Chickenpox (air/contact), Polio (air/water), Hepatitis A (water).
  • Protozoan Diseases: Malaria (carried by female Anopheles mosquito), Amoebiasis.
  • Fungal Diseases: Ringworm (skin contact).
• Common Plant Diseases:
  • 1. Citrus Canker: Caused by Bacteria; transmitted through Air (affects citrus trees like lemons/oranges).
  • 2. Rust of Wheat: Caused by Fungi (Puccinia); transmitted through Air and infected Seeds.
  • 3. Yellow Vein Mosaic of Bhindi (Okra): Caused by a Virus; transmitted by the insect vector Whitefly.

• Food Poisoning: Caused by the consumption of food spoilt by micro-organisms. Pathogens (like Salmonella and Clostridium botulinum) grow on food and produce potent toxins that cause severe vomiting, abdominal pain, diarrhea, and even death.
• Food Preservation: The scientific processing of food to prevent spoilage and inhibit microbial growth while retaining nutritional quality.
• Major Food Preservation Methods:
  • 1. Chemical Preservatives: Sodium Benzoate and Sodium Metabisulphite are common food preservatives added to jams, squashes, and ketchups to inhibit bacterial and fungal proliferation.
  • 2. Preservation by Common Salt: Meat, fish, amla, raw mangoes, and tamarind are preserved by salting. High salt concentration draws out water from microbial cells by exosmosis, dehydrating and killing bacteria.
  • 3. Preservation by Sugar: Jams, jellies, and squashes are preserved by sugar. Sugar reduces the moisture content, preventing bacterial growth.
  • 4. Preservation by Oil and Vinegar: Pickles are preserved using mustard oil and vinegar (acetic acid) because bacteria cannot survive in an acidic, anaerobic oily environment.
  • 5. Heat and Cold Treatments:
   • Boiling: Kills microorganisms by denaturing their enzymes.
   • Refrigeration: Low temperature in a refrigerator inhibits the growth and metabolic activity of microbes.
  • 6. Pasteurisation of Milk (Louis Pasteur):
   • Milk is heated to approximately 70°C for 15 to 30 seconds and then suddenly chilled and stored.
   • The sudden thermal shock kills harmful vegetative pathogens without altering the taste or nutritional quality of milk. Pasteurised milk can be consumed directly without boiling.
  • 7. Storage and Packing: Dry fruits and potato chips are sealed in airtight, airtight packets flushed with nitrogen gas to prevent microbial attack.