Chapter 2: Microorganisms: Friend and Foe

2.1 Microorganisms

Microorganisms: Living organisms too small to be seen with the unaided eye, also called microbes.
Observation: Can be observed using magnifying glasses (e.g., fungus on bread) or microscopes (e.g., soil and pond water organisms).
Classification: Primarily classified into four major groups: bacteria, fungi, protozoa, and algae.
Viruses: Microscopic but different from other microorganisms; reproduce only inside host cells.
Diseases caused by Viruses: Common cold, influenza, coughs, polio, chickenpox.
Diseases caused by Protozoa: Dysentery, malaria.
Diseases caused by Bacteria: Typhoid, tuberculosis.

2.2 Where do Microorganisms Live?

Habitat: Live in diverse environments - ice-cold climates, hot springs, deserts, marshy lands.
Presence: Found both outside and inside the bodies of animals, including humans.
Structure: Can be single-celled (e.g., bacteria, some algae, protozoa) or multicellular (e.g., many algae, fungi).
Existence: Some grow on other organisms, while others exist freely.

2.3 Microorganisms and Us

2.3.1 Friendly Microorganisms

Beneficial Uses: Essential for various purposes, including food preparation and environmental cleaning.
Food Preparation: Used in making curd, bread, and cake.
Alcohol Production: Used for large-scale production of alcohol and wine.
Environmental Cleaning: Decompose organic waste (vegetable peels, animal remains) into harmless substances.
Medicine Production: Used in manufacturing medicines.
Agriculture: Increase soil fertility by fixing nitrogen.

2.3.2 Making of Curd and Bread

Curd Formation: Bacteria like Lactobacillus convert milk into curd.
Other Food Items: Bacteria involved in making cheese, pickles, and other food items.
Fermentation: Yeast helps in fermenting rice batter for idlis and dosas.
Dough Rising: Yeast reproduces rapidly, producing carbon dioxide which makes dough rise for bread, pastries, and cakes.

2.3.3 Commercial Use of Microorganisms

Alcohol and Wine: Yeast is used for commercial production of alcohol and wine from grains or fruit juices.
Acetic Acid: Production of vinegar.
Fermentation Process: Conversion of sugar into alcohol by yeast. Discovered by Louis Pasteur in 1857.

2.3.4 Medicinal Use of Microorganisms

Antibiotics: Medicines that kill or stop the growth of disease-causing microorganisms. Produced from bacteria and fungi (e.g., penicillin, streptomycin, tetracycline, erythromycin).
Discovery of Penicillin: Alexander Fleming discovered penicillin from mould in 1929.
Livestock and Plants: Antibiotics are used to check microbial infections in animals and control plant diseases.
Precautions: Antibiotics should be taken only on doctor's advice and the full course completed. Not effective against viral infections like cold and flu.
Vaccines: Protect against diseases by stimulating the body's immune system. Dead or weakened microbes are introduced, prompting antibody production.
Vaccination Examples: Cholera, tuberculosis, smallpox, hepatitis, polio.
Smallpox Eradication: Edward Jenner discovered the smallpox vaccine in 1798.

2.3.5 Increasing Soil Fertility

Nitrogen Fixation: Bacteria (like Rhizobium) and blue-green algae fix atmospheric nitrogen into usable compounds, enriching the soil.
Biological Nitrogen Fixers: Microorganisms that perform this function.

2.3.6 Cleaning the Environment

Decomposition: Microorganisms decompose dead organic matter (plants, animals) into simple substances.
Manure Production: Plant waste is converted into manure, releasing nutrients for plants.
Waste Disposal: Microbes break down harmful and smelly substances, cleaning the environment.
Non-biodegradable materials: Materials like polythene bags and plastic toys are not decomposed by microbes.

2.4 Harmful Microorganisms

2.4.1 Disease Causing Microorganisms in Humans

Pathogens: Disease-causing microorganisms.
Entry into Body: Through air, water, food, or direct contact.
Communicable Diseases: Spread from an infected person to a healthy one via air, water, food, or physical contact (e.g., cholera, common cold, chickenpox, tuberculosis).
Spread of Cold: Viruses spread through fine droplets in the air from sneezing.
Prevention: Keep handkerchief on mouth/nose while sneezing, maintain distance from infected persons.
Carriers: Insects and animals that transmit diseases (e.g., housefly, female Anopheles mosquito carrying malaria parasite).
Mosquito Control: Prevent water stagnation to avoid mosquito breeding.
Table 2.1: Common Human Diseases
- Tuberculosis: Bacteria, Air, Isolation, Vaccination.
- Measles: Virus, Air, Vaccination.
- Chicken Pox: Virus, Air/Contact, Vaccination.
- Polio: Virus, Air/Water, Vaccination.
- Cholera: Bacteria, Water/Food, Boiled water, Vaccination.
- Typhoid: Bacteria, Water/Food, Boiled water, Vaccination.
- Hepatitis A: Virus, Water, Boiled water, Vaccination.
- Malaria: Protozoa, Mosquito, Mosquito nets, Insecticides.

2.4.2 Disease Causing Microorganisms in Animals

Examples: Anthrax (caused by bacterium Bacillus anthracis) in humans and cattle; Foot and mouth disease (caused by virus) in cattle.

2.4.3 Disease Causing Microorganisms in Plants

Impact: Reduce crop yield (e.g., citrus canker, rust of wheat, yellow vein mosaic of bhindi).
Control: Can be controlled by chemicals that kill microbes.
Table 2.2: Common Plant Diseases
- Citrus canker: Bacteria, Air.
- Rust of wheat: Fungi, Air, seeds.
- Yellow vein mosaic of bhindi: Virus, Insect.

2.4.4 Food Poisoning

Cause: Consumption of food spoilt by microorganisms that produce toxic substances.
Consequences: Can cause serious illness or even death.

2.5 Food Preservation

Purpose: To prevent food spoilage by microorganisms.
Spoilage Signs: Emits bad smell, bad taste, changed colour.
Chemical Method: Use of preservatives like common salt, edible oils, sodium benzoate, sodium metabisulphite.
Preservation by Common Salt: Used for meat, fish, amla, raw mangoes.
Preservation by Sugar: Used for jams, jellies, squashes; reduces moisture content.
Preservation by Oil and Vinegar: Prevents spoilage of pickles, vegetables, fruits, fish, meat.
Heat and Cold Treatments: Boiling kills microbes; refrigeration inhibits their growth.
Pasteurisation: Heating milk to about 70°C for 15-30 seconds and then chilling it. Prevents microbial growth without boiling (discovered by Louis Pasteur).
Storage and Packing: Sealed, air-tight packets prevent microbial attack on dry fruits and vegetables.

2.6 Nitrogen Fixation

Rhizobium: Bacterium found in the root nodules of leguminous plants.
Symbiotic Relationship: Lives with leguminous plants (e.g., beans, peas) and fixes atmospheric nitrogen.
Lightning: Can also contribute to nitrogen fixation.

2.7 Nitrogen Cycle

Atmospheric Nitrogen: Approx. 78% of Earth's atmosphere. Essential for proteins, chlorophyll, nucleic acids, vitamins.
Direct Use: Plants and animals cannot use atmospheric nitrogen directly.
Fixation: Bacteria and blue-green algae convert atmospheric nitrogen into usable compounds.
Plant Uptake: Plants absorb these nitrogen compounds from the soil.
Animal Consumption: Animals obtain nitrogen by feeding on plants.
Decomposition: Bacteria and fungi convert nitrogenous wastes from dead organisms into usable compounds for plants.
Return to Atmosphere: Some bacteria convert nitrogenous compounds back into nitrogen gas, returning it to the atmosphere.
Constant Percentage: This cycle maintains the atmospheric nitrogen level.