The five kingdoms, and why viruses do not fit

1Five puzzles from one weekend

Over one weekend in Muhanga: a loaf left in a damp cupboard grows grey fur with black dots; mushrooms push up beside a termite mound after rain; the water in a pond turns green; a jug of milk sours into ikivuguto; and your younger brother catches the flu.

Each of these is caused by living things, or by something close to living. Are the mould and the mushroom plants, because they grow from the ground? Is the green scum in the pond a plant or an animal? And what is the flu? To answer, biologists sort all organisms into five kingdoms, then deal with viruses separately.

2Observation: two small organisms

Bread mould. Look at mouldy bread with a hand lens (keep it in a closed plastic bag, and do not breathe in the spores). You will see fine white threads and tiny black balls on stalks. The threads feed on the bread; the black balls are full of spores, which spread the mould. There is no green colour anywhere: a mould cannot make its own food.

Pond water. Put a drop of pond water on a slide and look at it under a microscope. You may see single cells swimming with rows of tiny hairs, and green threads made of cells joined end to end. Each of these living things has a nucleus, yet many are only one cell. They are neither plants nor animals.

3The five kingdoms compared

4Viruses: outside the kingdoms

Figure 1: A bacterium is a complete cell. A virus has no cytoplasm, no ribosomes and no membrane of its own.

A virus is a particle of genetic material (DNA or RNA) inside a protein coat. It is not a cell. Outside a living cell a virus does nothing at all: it does not feed, respire, grow or excrete. It becomes active only when it gets inside a living host cell, where it takes over the cell's machinery to make copies of itself, often killing the cell.

Viruses cause HIV infection, measles, influenza, rabies and COVID-19 in humans, and cassava mosaic disease and maize streak disease in crops.

5Why the kingdom matters at the health centre

Antibiotics kill bacteria by attacking structures such as the cell wall and ribosomes. Viruses have neither, so antibiotics do not cure viral infections such as colds and flu. This is why a nurse will not give antibiotics for a cold, and why taking antibiotics you do not need is harmful: it helps bacteria become resistant. Fungal diseases, such as coffee leaf rust on Rwandan farms, need different treatments again (fungicides), because fungi are a different kingdom with different cells.