The tiniest living things
Bacteria (singular: bacterium) are tiny single-celled living things. They are so small that about 1000 could line up across the head of a pin. You need a microscope to see them, which makes them microorganisms.
Bacteria are among the oldest life forms on Earth, appearing about 3.5 billion years ago. They live almost everywhere: in soil, water, ice, hot springs, deep underground and inside your body.
What a bacterium is like
Bacteria are prokaryotes: their cells have no nucleus. Their DNA floats freely in the cell, usually as one loop.
| Part | Job |
|---|---|
| Cell wall | Gives shape and protection |
| Cell membrane | Controls what goes in and out |
| Cytoplasm | Jelly-like fluid where chemical reactions happen |
| DNA | Instructions for the cell |
| Ribosomes | Build proteins |
| Flagellum (some) | A whip-like tail for swimming |
| Pili (some) | Hair-like parts for sticking to surfaces |
Shapes of bacteria
| Shape | Name | Example |
|---|---|---|
| Round | Coccus (cocci) | Streptococcus, which causes strep throat |
| Rod | Bacillus (bacilli) | E. coli |
| Spiral | Spirillum (spirilla) | Bacteria that cause Lyme disease |
How bacteria reproduce
Bacteria reproduce by binary fission: one cell copies its DNA and splits into two identical cells. In good conditions, some bacteria divide every 20 minutes. Starting with one bacterium, that could make over 4 billion in 11 hours!
Some bacteria can form tough spores that survive heat, cold and dryness for years.
Helpful bacteria
Most bacteria are harmless, and many are very helpful.
| Job | How bacteria help |
|---|---|
| Decomposers | Break down dead plants and animals, returning nutrients to the soil |
| Nitrogen fixers | Turn nitrogen gas into a form plants can use |
| Digestion | Bacteria in your gut help digest food and make vitamin K |
| Food | Turn milk into yogurt and cheese; make sauerkraut and pickles |
| Medicine | Engineered bacteria make insulin for people with diabetes |
| Cleanup | Some break down oil spills and sewage |
| Oxygen | Cyanobacteria were the first organisms to make oxygen by photosynthesis |
Your microbiome
Your body contains trillions of bacteria, roughly as many bacterial cells as human cells. This community is your microbiome. Gut bacteria help digest food, train your immune system and crowd out harmful germs.
Harmful bacteria
A small number of bacteria cause disease. They can damage cells or release poisons called toxins.
| Disease | Bacterium |
|---|---|
| Strep throat | Streptococcus pyogenes |
| Tuberculosis | Mycobacterium tuberculosis |
| Food poisoning | Salmonella, some E. coli |
| Tetanus | Clostridium tetani |
| Lyme disease | Borrelia burgdorferi (spread by ticks) |
Fighting harmful bacteria
- Hand washing with soap removes bacteria.
- Cooking food properly kills them.
- Refrigerating food slows their growth.
- Vaccines protect against some bacterial diseases, like tetanus and whooping cough.
- Antibiotics are medicines that kill bacteria or stop them growing. Penicillin, discovered by Alexander Fleming in 1928, was the first widely used antibiotic.
Antibiotic resistance
When antibiotics are overused, bacteria that happen to survive multiply, creating resistant strains. Antibiotics don’t work on viruses, so they shouldn’t be used for colds or flu. Always take antibiotics exactly as a doctor prescribes.
Common mistakes
- Thinking all bacteria are germs. Most are harmless or helpful.
- Thinking antibiotics cure colds. Colds are caused by viruses.
- Confusing bacteria and viruses. Bacteria are living cells; viruses are much smaller and need a host cell to reproduce.
Try it yourself
- With an adult, swab a doorknob and touch it to a nutrient plate (agar). Seal it and watch colonies grow over a few days. Don’t open it!
- Make yogurt: add a spoonful of plain yogurt with live cultures to warm milk and keep it warm overnight.
- Calculate: if a bacterium divides every 30 minutes, how many will there be after 4 hours?
(Answer to 3: 2⁸ = 256)
Quick recap
- Bacteria are tiny single-celled prokaryotes found almost everywhere.
- They come in round, rod and spiral shapes.
- They reproduce quickly by binary fission.
- Most bacteria are helpful; a few cause disease.
- Antibiotics kill bacteria but must be used carefully to prevent resistance.