Bacteria are the domain you met in Three Domains and the Kingdoms: single cells with no nucleus. They are tiny (a typical one is 1 to 2 µm long) and they live almost everywhere: in soil, water, hot springs, ice, and on and inside you.
Key idea
A bacterium is one cell with a cell wall, a membrane, cytoplasm, ribosomes and a loop of DNA, but no nucleus. Most bacteria are harmless or helpful; only a small share cause disease.
Why it works
Life on Earth depends on bacteria. They break down dead plants and animals so the nutrients return to the soil. Some live in bumps on the roots of beans and peas and turn nitrogen from the air into a form plants can use. Others turn milk into yogurt, and the ones in your gut help break down food. Most strains of E. coli, for example, live harmlessly in the intestines of people and animals.
- Coccus: round (a sphere).
- Bacillus: rod-shaped.
- Spirillum: spiral.
Worked examples
Worked example: What is missing?
Which of these does a bacterium lack: a membrane, ribosomes, a nucleus, DNA?
- A bacterium has a membrane, ribosomes and DNA, like every cell.
- Its DNA lies loose in the cytoplasm.
Answer: A nucleus.
Worked example: Helpful or not?
A farmer plants beans in a field to add nitrogen to the soil. Who does the work?
- Bacteria living in the bean roots turn nitrogen gas from the air into a form plants use.
Answer: Bacteria in the roots.
Common mistake
Thinking all bacteria are germs that make people sick. Only a small share cause disease; many are needed, from the bacteria that recycle dead leaves to the ones that help digest food. Check: name one food made with bacteria (yogurt, cheese, sauerkraut).