What are bacteria?
A bacterium (plural bacteria) is a living thing made of just one cell. It is a prokaryote: its cell has no nucleus. Most are 1 to 5 micrometres (µm) long; 1 µm is one thousandth of a millimetre, so we need a microscope to see them.
Bacteria live almost everywhere: soil, water, air, hot springs, and in and on our bodies. A spoon of soil can hold a billion of them.
Shapes
- Coccus (round): e.g. Streptococcus in chains, Staphylococcus in bunches.
- Bacillus (rod): e.g. Lactobacillus, E. coli.
- Spirillum (spiral, corkscrew).
- Vibrio (comma): e.g. Vibrio cholerae.
Structure of a bacterial cell
- Cell wall: a strong layer made of peptidoglycan that keeps the shape and stops the cell from bursting.
- Capsule (some bacteria): a sticky outer layer that protects against drying and against our white blood cells.
- Cell membrane: controls what goes in and out; respiration enzymes sit here because there are no mitochondria.
- Cytoplasm with 70S ribosomes (smaller than ours) that make proteins.
- Nucleoid: one circular chromosome of DNA, not inside any membrane.
- Plasmids: small extra DNA rings, often with genes for antibiotic resistance.
- Flagellum: a long tail that spins like a propeller. Pili: short hairs for sticking to surfaces and for joining other bacteria.
Gram-positive and Gram-negative
A purple stain (the Gram stain) sorts bacteria. Gram-positive bacteria have a thick peptidoglycan wall and stay purple. Gram-negative bacteria have a thin wall plus an outer membrane and look pink. This helps doctors choose medicines.
Reproduction and horizontal gene transfer
Binary fission
A bacterium copies its DNA loop, grows longer, and a new wall forms across the middle. It splits into two identical daughter cells. In good conditions this takes about 20 minutes, so numbers grow very fast: after n generations one cell becomes 2ⁿ cells.
Endospores
When food or water runs out, some bacteria (e.g. Bacillus, Clostridium) form a tough endospore that survives boiling and drying, then grows again later.
Horizontal gene transfer
Bacteria can get new genes from other bacteria, not only from their parent cell:
- Conjugation: a pilus joins two cells and a copy of a plasmid passes across.
- Transformation: a bacterium picks up loose DNA from a dead cell.
- Transduction: a virus that infects bacteria carries a piece of DNA from one cell to another.
This is why antibiotic resistance spreads so quickly.
How bacteria get food and energy (metabolism)
- Photosynthetic (autotrophs using light): cyanobacteria make food and release oxygen.
- Chemosynthetic (autotrophs using chemicals): nitrifying bacteria get energy by changing ammonia into nitrite and nitrate.
- Saprotrophs (decomposers): feed on dead matter and recycle nutrients.
- Symbionts: live in partnership. Rhizobium in legume root nodules fixes nitrogen; gut bacteria help digestion and make vitamin K.
- Parasites: feed on a living host and may cause disease.
With or without oxygen
Aerobic bacteria need oxygen. Anaerobic bacteria live without it (some are even killed by it) and often ferment sugar, for example into lactic acid.
Roles in biogeochemical cycles
Bacteria are key in the nitrogen cycle (fixing N₂, nitrification, denitrification back to N₂) and the carbon cycle (decomposers release CO₂; cyanobacteria take it in).
Useful and harmful bacteria
Useful
- Food: curd, yoghurt, cheese, vinegar, idli and dosa batter.
- Farming: nitrogen fixation, compost, biogas.
- Medicine and industry: antibiotics such as streptomycin, insulin made by genetically modified E. coli, sewage treatment.
Harmful
- Human diseases: tuberculosis, cholera, typhoid, tetanus, pneumonia.
- Plant diseases such as citrus canker; food spoilage and food poisoning.
Antibiotics kill bacteria or stop them growing (for example by blocking their cell wall or 70S ribosomes). They do not work against viruses. Misuse helps resistant bacteria survive and spread.
Try it
Leave a little milk with a spoon of curd in a warm place overnight. Next day it has set: billions of Lactobacillus made lactic acid. In the 3D, use the slider to see how fast 1 cell becomes 1,024.
Bacteria and archaea: two different domains
Scientists group all life into three domains: Bacteria, Archaea and Eukarya. Archaea are also prokaryotes and look like bacteria under a microscope, but their chemistry is different.
| Feature | Bacteria | Archaea | Eukarya |
|---|---|---|---|
| Nucleus | No | No | Yes |
| Peptidoglycan in wall | Yes | No | No |
| Membrane lipids | Ester-linked | Ether-linked (very stable) | Ester-linked |
| Ribosomes | 70S | 70S (but proteins like eukaryotes) | 80S |
| Typical homes | Everywhere | Hot springs, salt lakes, animal guts (methanogens) | Plants, animals, fungi, protists |
Archaea are not known to cause human disease.
Key formulas and definitions
- Prokaryote: no nucleus, no membrane-bound organelles
- Number after n generations = 2ⁿ × starting number
- Shapes: coccus, bacillus, spirillum, vibrio
- Gene transfer: conjugation, transformation, transduction
- Bacteria wall: peptidoglycan; archaea: no peptidoglycan
- Ribosomes: prokaryotes 70S, eukaryotes 80S
Worked examples
1. A bacterium divides every 20 minutes. How many bacteria come from one cell after 2 hours?
2 hours = 120 minutes = 6 generations. 2⁶ = 64 bacteria.
2. A cell has a cell wall, ribosomes and DNA, but no nucleus and no mitochondria. Is it a plant cell or a bacterium? Why?
A bacterium. Plant cells also have walls, but they have a nucleus and mitochondria. No nucleus means a prokaryote.
3. Why do pea plants grow well in soil with little nitrogen fertiliser?
Rhizobium bacteria live in their root nodules and fix nitrogen gas from air into ammonia that the plant uses. In return the plant gives them sugars. This is symbiosis.
4. A patient stops antibiotics after two days. Explain why resistant bacteria can increase.
The drug kills the weakest bacteria first. Stopping early leaves the toughest ones alive; they multiply by binary fission and can pass resistance plasmids to other bacteria by conjugation.
Common mistakes
- Calling all bacteria harmful. Most are harmless or useful; only a small number cause disease.
- Saying bacteria have a nucleus. Their DNA lies free in the cytoplasm in the nucleoid region.
- Thinking antibiotics cure viral fevers or colds. Antibiotics act on bacterial parts (wall, 70S ribosomes) that viruses do not have.
- Treating archaea as a kind of bacteria. They are a separate domain with different wall and membrane chemistry.