A drop of pond water holds hundreds of living things you cannot see. Some make your curd and your bread; others make you ill. Both are microorganisms.
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Microorganisms fall into five groups. Learn one example of each and most questions answer themselves.
🔬 Group and example
| Group | Example | Note |
|---|---|---|
| Bacteria | Lactobacillus, Rhizobium | single-celled; some useful, some harmful |
| Fungi | yeast, bread mould | make bread rise; also spoil food |
| Protozoa | Amoeba, Plasmodium | Plasmodium causes malaria |
| Algae | Spirogyra, Chlamydomonas | green, make their own food |
| Viruses | influenza, dengue virus | living only inside a host cell |
A virus is the odd one out. Outside a living cell it does nothing at all — it cannot grow, feed or reproduce. That is why viruses are often described as "on the border of living and non-living", and why antibiotics do not work against them.
We would have no curd, no bread and far less soil nitrogen without microorganisms.
💚 What they do for us
| Microorganism | What it does |
|---|---|
| Lactobacillus | turns milk into curd |
| Yeast | makes dough rise and is used in brewing — it produces carbon dioxide |
| Rhizobium | lives in the root nodules of pulses and fixes nitrogen in the soil |
| Penicillium | the mould from which the antibiotic penicillin was obtained |
| Decomposers | break down dead matter and return nutrients to the soil |
Why farmers grow pulses in rotation. Rhizobium in the root nodules of gram, peas and beans takes nitrogen from the air and fixes it in the soil, so the next crop needs less fertiliser.
Diseases spread in a few well-known ways, and each way suggests its own prevention.
🤒 Disease, cause and carrier
| Disease | Caused by | Spread by |
|---|---|---|
| Malaria | Plasmodium (protozoan) | female Anopheles mosquito |
| Dengue | virus | Aedes mosquito, which bites by day |
| Typhoid | bacteria | contaminated water and food |
| Cholera | bacteria | contaminated water |
| Tuberculosis | bacteria | air — coughs and sneezes |
| Common cold | virus | air and touch |
Malaria is caused by a protozoan, not by the mosquito. The mosquito is only the carrier, or vector. The same distinction matters for dengue, where the cause is a virus.
How a vaccine works: it puts a dead or weakened germ into the body so the body learns to make antibodies against it. If the real germ arrives later, the defence is already prepared. Edward Jenner developed the first vaccine, against smallpox.
12 questions on microorganisms and disease. 🍀
