Microorganisms: Friend and Foe — Summary Notes

Microorganisms (microbes) are living organisms that are too small to see with the unaided eye.
Examples: bacteria, fungi, protozoa, and some algae. Viruses are also microscopic but are different from other microorganisms; they reproduce only inside host cells (bacterium, plant, or animal cell).
Microorganisms can be seen only with magnification; some (e.g., bread mould) are visible with a magnifying glass.
Major groups: bacteria, fungi, protozoa, algae; viruses are microscopic but distinct.
Common diseases: viruses cause cold, influenza; polio and chicken pox are viral diseases; protozoa cause dysentery and malaria; bacteria cause typhoid and tuberculosis (TB).

They can be single-celled or multicellular.
Found everywhere: ice-cold climates, hot springs, deserts, marshy lands, bodies of animals including humans.
Some grow on other organisms; others live freely.

Beneficial roles:
- Food and drink: fermentation and production of curd, bread, cheese; Lactobacillus in curd formation; bacteria and yeast in fermentation of rice idlis/dosa batter; bread rising due to CO₂ from yeast.
- Environment: decomposition of organic wastes into harmless substances by bacteria; cleanup of the environment.
- Agriculture: nitrogen fixation by certain bacteria to enrich soil fertility (biological nitrogen fixers).
- Medicines: antibiotics produced by bacteria and fungi (e.g., Streptomycin, tetracycline, erythromycin).
- Industry: large-scale production of alcohol, wine, acetic acid (vinegar) using yeasts and other microbes.
- Vaccines: vaccines protect against diseases (e.g., Jenner’s smallpox vaccine; polio vaccination campaigns such as Pulse Polio).
Important concepts:
- Antibiotics should be used under doctor’s advice and must be completed as prescribed; misuse can reduce effectiveness and kill beneficial bacteria; not effective against viruses (e.g., cold/flu).
- Vaccines work by exposing dead/weakened microbes to stimulate antibody production and memory; they help prevent diseases.
- Nitrogen fixation: some microbes (including Rhizobium in legume roots and certain cyanobacteria) convert atmospheric N₂ into usable nitrogen compounds for plants; essential for protein/nucleic acid synthesis.

Pathogens cause diseases in humans, animals, and plants; many are transmissible via air, water, food, or direct contact.
Communicable diseases: spread from infected to healthy individuals (e.g., cholera, measles, TB).
Carriers: insects like houseflies and mosquitoes (Anopheles for malaria, Aedes for dengue) help transmit pathogens.
Prevention:
- Keep food covered; maintain hygiene; avoid standing water to curb mosquito breeding.
- Use protective measures during sneezing; isolate patients if needed; vaccination where available.
Plant and animal diseases: various microbes cause diseases in crops and livestock; control often involves targeted measures (not detailed here).

Microorganisms spoil food, causing bad smell, taste, and color; some produce toxins.
Preservatives (chemical): salts and acids (e.g., sodium benzoate, sodium metabisulphite).
Methods:
- Salting to preserve meats/fish and certain fruits.
- Sugar to preserve jams, jellies, squashes by reducing moisture.
- Oil and vinegar to create environments unfavourable to bacteria.
- Heat treatment and refrigeration slow or stop microbial growth; pasteurization: milk heated to 70^ ext{°C} for 15–30 seconds and then cooled.
- Packaging in airtight conditions prevents microbial entry.

Nitrogen is essential in proteins, chlorophyll, nucleic acids, vitamins; atmosphere contains 78 ext{
%} nitrogen.
Nitrogen fixation converts atmospheric N_2 into usable nitrogen compounds for plants.
Key players:
- Rhizobium bacteria in root nodules of leguminous plants (e.g., beans, peas).
- Some cyanobacteria (blue-green algae) also fix atmospheric nitrogen.
The fixed nitrogen enters the soil, is taken up by plants, moves through the food chain to animals, and returns to the soil via decomposition.

Four major stages (brief):
- Nitrogen fixation: N_2
  ightarrow ext{usable nitrogen compounds} by bacteria/cyanobacteria.
- Assimilation: plants take up nitrogen compounds from soil.
- Decomposition: microorganisms decompose nitrogenous wastes to reusable forms.
- Denitrification: some bacteria convert nitrogenous compounds back to N_2, returning it to the atmosphere.
The atmospheric nitrogen level remains relatively constant due to this cycle.
Importance: nitrogen is a building block of proteins, chlorophyll, nucleic acids, and vitamins.

Microorganisms are extremely small and live in diverse environments, including inside living beings.
They can be unicellular or multicellular and include bacteria, fungi, protozoa, and some algae; viruses are microbes but are not cells.
Some microbes are beneficial (fermentation, food production, soil fertility, medicine, environmental cleanup) while others are harmful (pathogens, food spoilage).
Nitrogen fixation by bacteria/cyanobacteria makes nitrogen available to plants; the nitrogen cycle maintains atmospheric nitrogen levels.
Food preservation uses chemical preservatives, salt, sugar, oil/vinegar, heat, cold, and packaging to prevent microbial growth; pasteurization is a key process.
Vaccines and antibiotics are important medical tools derived from microorganisms; responsible use is essential to maintain effectiveness.