What are microorganisms?
A microorganism (or microbe) is a living thing so small that you need a microscope to see it. The word means "small living thing".
We measure them in micrometres (µm). One micrometre is one-thousandth of a millimetre. A light microscope that makes things look 400 to 1000 times bigger lets us see bacteria, yeast and amoeba. Viruses are smaller still, so we need an electron microscope.
Microbes live almost everywhere: soil, water, air, hot springs, ice, and on and inside our bodies. Some live alone as one cell. Some, like moulds, grow as long threads.
Try it: see microbes at home or in the lab
Put a drop of curd water on a glass slide, add a cover slip and look under a school microscope. You will see tiny rods: those are bacteria. A drop of pond water often shows moving protozoa and green algae.
The main groups of microbes
- Bacteria: one cell, no nucleus. Shapes: round (coccus), rod (bacillus), spiral (spirillum), comma (vibrio). Example: Lactobacillus in curd.
- Fungi: cells with a nucleus and a cell wall. Yeast is one cell; moulds such as bread mould grow as threads called hyphae and spread by spores. Mushrooms are big fungi.
- Protozoa: one cell with a nucleus, animal-like; they move and eat food. Examples: Amoeba, Paramecium, Plasmodium (malaria).
- Algae (tiny ones): plant-like cells with chlorophyll that make food by photosynthesis. Example: Chlamydomonas, diatoms.
- Viruses: not cells. A protein coat around DNA or RNA. They can copy themselves only inside a living host cell. Outside a cell they do nothing.
Prokaryotes and eukaryotes
Bacteria are prokaryotes: their DNA floats in the cell with no nucleus. Fungi, protozoa and algae are eukaryotes: their DNA is inside a nucleus and they have other parts like mitochondria. Eukaryotic microbes often have both sexual and asexual stages in their life cycle; bacteria mostly do not.
How microbes reproduce
- Binary fission (bacteria, amoeba): the cell copies its DNA and splits into two equal cells.
- Budding (yeast): a small bud grows on the parent and breaks off.
- Spores (moulds, some bacteria): tiny tough cells that spread by air and grow when food and water are there.
- Viruses: enter a host cell and make it build hundreds of new viruses.
Fast doubling explains why food spoils quickly. If one bacterium doubles every 20 minutes, after n doublings there are 2n bacteria. In 3 hours (9 doublings) one becomes 512. Cold slows doubling; that is why a fridge keeps food fresh.
Microbiota, health and helpful microbes
Your body carries trillions of microbes, mostly in the gut. Together they are your microbiota. They help digest fibre, make some vitamins (like vitamin K) and crowd out harmful germs. This is symbiosis: both sides gain.
Other symbiotic microbes: Rhizobium bacteria in pea and bean root nodules fix nitrogen for the plant; microbes in a cow's stomach digest grass; lichens are a fungus and an alga living together.
Microbes in biotechnology: yeast for bread and fermentation; bacteria for curd, cheese and vinegar; bacteria engineered to make insulin; microbes that clean sewage and make biogas; moulds and bacteria that make antibiotics and enzymes.
Decomposer microbes break down dead plants and animals and return nutrients to the soil, so life can keep going.
Harmful microbes, antibiotics and resistance
A microbe that causes disease is a pathogen. Examples: cholera and typhoid (bacteria), malaria (protozoan), flu and measles (viruses), ringworm (fungus). Some spread by water, some by air, some by insects such as mosquitoes.
Antibiotics are medicines that kill bacteria or stop them growing. The first, penicillin, came from a mould (Alexander Fleming, 1928). Antibiotics do not work on viruses, because viruses have none of the cell parts antibiotics attack. Colds and flu need rest, fluids and sometimes antiviral medicine.
Antibiotic resistance: if antibiotics are used when not needed, or a course is stopped early, the toughest bacteria survive and multiply. Soon the medicine stops working. So: take antibiotics only when a doctor prescribes them, finish the course, never share them.
We also protect ourselves with hand-washing, clean water, cooking food well, vaccines and keeping food cold.
Investigating microbes safely
In a school lab, grow microbes only on closed agar plates, never open them after growth, wash hands and let the teacher dispose of plates. A fair test changes one thing at a time, for example the temperature, and counts the colonies.
Key formulas and definitions
- Microorganism (microbe): a living thing too small to see without a microscope
- 1 µm (micrometre) = 0.001 mm
- Prokaryote: cell with no nucleus (bacteria)
- Eukaryote: cell with a nucleus (fungi, protozoa, algae)
- Virus: not a cell; protein coat + DNA or RNA; copies only inside a host cell
- Binary fission: number after n doublings = 2ⁿ
- Pathogen: a microbe that causes disease
- Microbiota: all the microbes living on and in a body
- Antibiotic: kills or stops bacteria; no effect on viruses
Worked examples
1. A milk sample has 1 bacterium. Bacteria double every 20 minutes. How many are there after 2 hours?
2 hours = 120 minutes = 120 ÷ 20 = 6 doublings. Number = 2⁶ = 64 bacteria.
2. Riya has a cold caused by a virus. She wants an antibiotic. Is that a good idea?
No. A cold is caused by a virus. Antibiotics act only on bacteria, so they will not help. Taking them for no reason helps resistant bacteria grow. Rest and fluids are the right choice unless a doctor advises otherwise.
3. A student sees a single cell with a nucleus, green colour inside, and no movement. Which group is it most likely?
It has a nucleus, so it is a eukaryote (not a bacterium). Green colour means chlorophyll, so it makes its own food: it is most likely an alga.
4. Why does cooked rice left in a warm kitchen go bad faster than rice in a fridge?
Bacteria double faster when it is warm (around 30–40 °C). In the fridge (about 4 °C) their doubling is very slow, so their number stays small for longer.
Common mistakes
- Thinking all microbes are germs. Most are harmless or helpful; only a few are pathogens.
- Thinking antibiotics cure colds and flu. These are viral; antibiotics only act on bacteria.
- Calling viruses bacteria. Viruses are far smaller and are not cells at all.
- Stopping antibiotics when you feel better. The surviving bacteria can come back and be resistant.