Vast numbers of living organisms are too small to be seen with the naked eye. These are called microorganisms (or microbes). They were discovered after the invention of the microscope by Antonie van Leeuwenhoek in the 17th century.
Major Groups of Microorganisms
Bacteria: Single-celled prokaryotes (no nucleus). Found everywhere — soil, water, air, food, and inside living organisms. Shape types: spherical (cocci), rod-shaped (bacilli), spiral (spirilla). Examples: Lactobacillus (in curd), Rhizobium (nitrogen fixation), Mycobacterium tuberculosis (TB).
Fungi: Eukaryotes with cell walls made of chitin. Can be unicellular (yeast) or multicellular (moulds, mushrooms). Examples: Aspergillus (bread mould), Penicillium (source of penicillin antibiotic), Saccharomyces (yeast, used in baking).
Protozoa: Single-celled eukaryotes. Most are found in water. Examples: Amoeba (irregular shape, uses pseudopodia), Paramecium (slipper-shaped, uses cilia), Plasmodium (causes malaria, transmitted by female Anopheles mosquito).
Algae: Mainly aquatic, photosynthetic. Can be unicellular (Chlamydomonas, Chlorella) or multicellular (Spirogyra, Ulva). They produce a large fraction of Earth's oxygen.
Viruses: Not considered true living organisms as they have no cellular structure and can only reproduce inside a host cell. Cause diseases like influenza, common cold, COVID-19, polio, and dengue.
Beneficial Microorganisms
- Fermentation: Yeast converts sugars to alcohol and CO2 (bread rises; idli/dosa batter ferments).
- Curd/cheese/vinegar production: Bacteria like Lactobacillus acidophilus.
- Nitrogen fixation: Rhizobium in root nodules of legumes converts atmospheric N2 to usable nitrates.
- Decomposition: Bacteria and fungi break down dead organic matter, recycling nutrients.
- Antibiotics: Penicillium notatum produces penicillin; many antibiotics come from soil bacteria.
- Vaccines: Weakened/dead microbes stimulate immunity (Jenner's smallpox vaccine).
Harmful Microorganisms
Diseases caused by microorganisms in humans, animals, and plants:
| Disease | Causative Agent | Type |
|---|---|---|
| Tuberculosis | Mycobacterium tuberculosis | Bacteria |
| Cholera | Vibrio cholerae | Bacteria |
| Malaria | Plasmodium | Protozoa |
| Common cold | Rhinovirus | Virus |
| Athlete's foot | Tinea pedis | Fungus |
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Curd formation — When a small amount of curd (containing Lactobacillus) is added to warm milk and kept warm overnight, the bacteria multiply and convert lactose to lactic acid, curdling the milk. This is fermentation.
Bread making — Yeast (Saccharomyces cerevisiae) is mixed into dough. It ferments sugars and releases CO2, causing the dough to rise and become soft. The equation is: C6H12O6 → 2C2H5OH + 2CO2.
Nitrogen fixation — Rhizobium bacteria live in root nodules of beans, peas, and other legumes. They convert N2 gas from the air into ammonia (NH3), which plants use to make proteins. Farmers rotate crops using legumes to enrich soil with nitrogen naturally.
Malaria transmission — Plasmodium is a protozoan parasite. It enters the human bloodstream through the bite of the female Anopheles mosquito. It reproduces in red blood cells, causing periodic fever, chills, and anaemia. Prevention: use mosquito nets, eliminate stagnant water, take antimalarial medication.
Penicillin discovery — Alexander Fleming (1928) noticed that a mould (Penicillium) contaminating a bacterial culture plate killed the surrounding bacteria. This led to the development of the first antibiotic, penicillin, which revolutionised medicine.
Food preservation — Microorganisms cause food spoilage. Methods to prevent this include: refrigeration (slows bacterial growth), salting (removes water by osmosis), pickling (acidic environment kills microbes), pasteurisation (heating milk to 72 °C for 15 seconds then cooling quickly kills pathogens).
Composting — Garden waste (leaves, vegetable peels) is piled and kept moist. Bacteria and fungi decompose organic matter over weeks into humus-rich compost, returning nutrients to the soil. This is an example of microbes benefiting agriculture.
Key Formula
Fermentation of glucose: C6H12O6 → 2C2H5OH + 2CO2
Common mistakes
Students often confuse bacteria and viruses — bacteria are living cells treated by antibiotics; viruses are non-cellular and antibiotics do NOT work on viruses. Also, not all microorganisms are harmful — many are essential for life on Earth.
Summary
Microorganisms include bacteria, fungi, protozoa, algae, and viruses. They live everywhere and play vital roles in fermentation, nitrogen fixation, antibiotic production, and decomposition. Some cause diseases, but these can be prevented and treated through vaccines, antibiotics, and hygiene practices.