Microorganisms
Microorganisms — Friend and Foe
What you'll learn
- What microorganisms are and their main types.
- How microorganisms are useful to humans.
- How microorganisms cause disease (pathogens).
- Food preservation methods to prevent microbial spoilage.
- Antibiotics and vaccines — how they work.
Key concepts
What are microorganisms?
- Microorganisms (microbes): living organisms too small to be seen with the naked eye; require a microscope.
- Found everywhere: soil, water, air, inside our bodies, in extreme environments (hot springs, ice caps).
- First observed by Antonie van Leeuwenhoek (1670s) using his hand-made microscope.
Major types of microorganisms
| Type | Features | Examples |
|---|---|---|
| Bacteria | Single-celled; prokaryote (no nucleus); found in soil, water, air, body | Lactobacillus (curd), Rhizobium (legume roots), Vibrio cholerae (cholera) |
| Fungi | Eukaryote; not plants; decomposers; some multicellular | Aspergillus (bread mould), Penicillium (antibiotic), yeast (Saccharomyces) |
| Protozoa | Single-celled; eukaryote; most live in water or as parasites | Plasmodium (malaria), Amoeba, Paramecium |
| Algae | Mostly aquatic; photosynthetic; range from single-cell to large seaweed | Chlamydomonas, Spirogyra, Ulva (sea lettuce) |
| Viruses | NOT truly living (no cells, no metabolism alone); need a host cell to reproduce | Influenza, COVID-19 (SARS-CoV-2), Polio, HIV, Measles |
- Virus vs Bacteria: Viruses are much smaller; antibiotics don't work on viruses; viruses use host cell machinery to replicate.
Microorganisms as friends — useful roles
In food production
| Product | Microorganism | How |
|---|---|---|
| Curd (yoghurt) | Lactobacillus bacteria | Ferments lactose in milk → lactic acid → milk curdles |
| Bread / Idli / Dosa | Yeast (Saccharomyces cerevisiae) | Ferments sugars → CO₂ gas → dough rises |
| Cheese | Various bacteria + fungi (Penicillium in blue cheese) | Fermentation changes milk proteins and fats |
| Vinegar | Acetobacter bacteria | Oxidises alcohol → acetic acid |
| Alcohol (beer, wine) | Yeast | Ferments sugar → ethanol + CO₂ |
| Idli/Dosa batter | Lactic acid bacteria + yeast | Natural fermentation; also increases nutritional value |
In medicine
- Antibiotics: chemicals produced by microorganisms (usually fungi) that kill or inhibit bacteria.
- Penicillin: discovered by Alexander Fleming (1928) from Penicillium mould; revolutionised medicine.
- Other antibiotics: streptomycin, tetracycline, erythromycin.
- Important: antibiotics work on bacteria, NOT on viruses (don't take antibiotics for a cold/flu!).
- Antibiotic resistance: overuse leads to resistant "superbugs" — global health crisis.
- Vaccines: use killed/weakened pathogens to train immune system.
- Edward Jenner (1796): first vaccine — cowpox to protect against smallpox.
- Louis Pasteur: germ theory; developed vaccines for cholera, anthrax, rabies.
- India's vaccines: OPV (polio), BCG (TB), MMR (measles-mumps-rubella), COVID-19 vaccines (Covaxin, Covishield).
In agriculture
- Biological nitrogen fixation: Rhizobium bacteria in legume root nodules convert atmospheric N₂ to usable ammonia → reduces need for chemical fertilisers.
- Biofertilisers: blue-green algae (Cyanobacteria) fix nitrogen in paddy fields.
- Decomposers: fungi and bacteria break down dead organic matter → nutrients return to soil → supports plant growth.
- Biopesticides: Bacillus thuringiensis (Bt) produces toxins that kill specific insect pests; used instead of harmful chemical pesticides.
In the environment
- Decomposition: microorganisms break down dead plants, animals, and waste → recycle carbon, nitrogen, phosphorus.
- Sewage treatment: bacteria break down organic waste in sewage treatment plants.
- Bioremediation: certain bacteria can break down oil spills and toxic chemicals.
Microorganisms as foes — diseases they cause
Diseases caused by bacteria
| Disease | Bacterium | Spread | Symptoms |
|---|---|---|---|
| Tuberculosis (TB) | Mycobacterium tuberculosis | Air (droplets) | Cough, fever, weight loss |
| Cholera | Vibrio cholerae | Contaminated water/food | Severe diarrhoea, dehydration |
| Typhoid | Salmonella typhi | Contaminated water/food | High fever, stomach pain |
| Food poisoning | Staphylococcus, Salmonella | Infected food | Vomiting, diarrhoea |
| Anthrax | Bacillus anthracis | Infected animals/soil | Skin sores; can be fatal |
Diseases caused by viruses
| Disease | Virus | Spread |
|---|---|---|
| Common cold / Flu | Rhinovirus / Influenza | Air droplets |
| COVID-19 | SARS-CoV-2 | Air droplets, surfaces |
| Polio | Poliovirus | Contaminated water/faeces |
| Measles | Measles virus | Air droplets |
| Dengue | Dengue virus | Aedes mosquito bite |
| HIV / AIDS | HIV | Blood, sexual contact, mother-to-child |
| Hepatitis B | Hepatitis B virus | Blood, sexual contact |
Diseases caused by protozoa
| Disease | Protozoan | Vector |
|---|---|---|
| Malaria | Plasmodium falciparum/vivax | Anopheles female mosquito |
| Kala-azar (Leishmaniasis) | Leishmania | Sandfly |
| Dysentery (Amoebic) | Entamoeba histolytica | Contaminated water/food |
Diseases caused by fungi
- Ringworm: fungal infection of skin; circular red rash (NOT a worm).
- Athlete's foot: Tinea pedis; affects skin between toes.
- Candidiasis: Candida albicans; mouth, gut, or vaginal infection (especially in immunocompromised people).
Food preservation — keeping microorganisms away
| Method | Principle | Examples |
|---|---|---|
| Heating / Pasteurisation | High temp kills microbes | Milk (72°C for 15s); canned food |
| Refrigeration | Low temp slows microbial growth | Meat, dairy, vegetables |
| Deep freezing | Very low temp stops microbial growth | Frozen food; long storage |
| Salting | Salt draws water out of food (osmosis); microbes dehydrate | Pickles, salted fish, cured meat |
| Sugar | Similar to salt; high sugar concentration draws water out | Jams, jellies, mithai |
| Drying / Dehydration | Removes water; no water → no microbial growth | Dried fruits, papad, spices |
| Vinegar (acidification) | Acidic pH inhibits most bacteria | Pickles, chutneys |
| Chemical preservatives | Sodium benzoate, potassium sorbate added to food | Packaged juices, sauces |
| Vacuum packing | Remove oxygen; aerobic bacteria can't grow | Packaged meats, chips |
Pasteurisation
- Process developed by Louis Pasteur (1850s).
- Heat liquid (milk) to ~72°C for 15 seconds → kills harmful pathogens without changing taste significantly.
- Then rapidly cool to < 4°C.
- Extends shelf life; makes milk safe.
Quick check
- Name the five major types of microorganisms. Which is not truly a living cell?
- How does Lactobacillus help make curd?
- Who discovered penicillin? From which organism was it obtained?
- Name two diseases caused by each: bacteria, viruses, protozoa.
- List four food preservation methods and explain the principle behind each.
Open the Practice tab for graded questions on Microorganisms.
4 topics • Notes • Practice • AI explanations available
1. Bacteria
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2. Virus
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3. Fungi
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4. Icse Protista
Microorganisms — Icse Protista
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