2.5 The World of Microorganisms
Some living things are made of just one cell, or a few. They are so small that no eye can spot them — you need a microscope to magnify them 100 to 400 times. These tiny living beings are called microorganisms (micro = very small, organism = living being), or microbes for short. They are everywhere: in water, soil, air, and even inside your own body.
Activities 2.4 & 2.5 — Hunt for microbes Place a drop of pond or stagnant water on a slide, add a coverslip, and look under the microscope: you will see tiny organisms moving about. Now stir some moist garden soil into water, let it settle, and take a drop from the top — this soil suspension is full of moving creatures too. Two ordinary samples, two glimpses of a hidden living world. (Always handle soil with a spoon or gloves, never bare hands.)
The main families of microbes
When students examined pond water and soil under the microscope, they kept finding the same broad groups. Microorganisms are usually sorted into four families:
| Group | What they are like | Examples |
|---|---|---|
| Bacteria | Single-celled; spherical, rod, comma or spiral shaped; no well-defined nucleus | Lactobacillus, Rhizobium |
| Protozoa | Single-celled; move about, often with special structures | Amoeba (irregular shape), Paramecium |
| Fungi | Single-celled or many-celled; branched threads; no chlorophyll | yeast, bread mould |
| Algae | Single or many-celled; green (contain chlorophyll); make their own food | Spirulina, Chlorella, diatoms |
Unicellular or Multicellular? Bacteria and protozoa (like Amoeba) are unicellular — a whole living being in a single cell. Fungi and algae can be unicellular or multicellular. Plants and animals are always multicellular. So “how many cells?” is one of the first questions a biologist asks about any organism.
What is a virus, then?
Ever Heard Of… Viruses are microscopic but acellular — they are not made of cells at all. A virus can only multiply after it enters a living cell (of a plant, animal or even a bacterium), and in doing so it may cause disease. This is the key difference between viruses and all the other microbes: bacteria, fungi, protozoa and algae are living cells that can grow and divide on their own, but a virus needs a host cell to reproduce.
Spot the Mistake “A virus is just a very small bacterium.” Wrong. A bacterium is a complete (if tiny) cell that can multiply by itself; a virus is not a cell and can only multiply inside a host cell. Same “too small to see,” completely different nature.
How are microbe cells built?
Like plant and animal cells, microbe cells have a cell membrane. Fungal cells also have a cell wall but no chloroplasts, so they cannot photosynthesise. Bacteria are special: they have no well-defined nucleus and no nuclear membrane — instead, their genetic material sits in a region called the nucleoid. That one feature sets bacteria apart from the cells of yeast, protozoa, algae, fungi, plants and animals.
Science Talk A school light-microscope magnifies microbes 100–400 times; an electron microscope reaches about 10,00,000 times and reveals the tiny structures inside a cell. Talk through: why do scientists need such enormous magnification to study what is happening within a single bacterium?