The Microbial World
2. Bacteria
Learning outcomes
- I can describe the basic structure of bacterial cells.
- I can distinguish bacteria from other microorganisms.
- I can explain how bacteria reproduce.
- I can identify beneficial and harmful bacteria.
- I can explain the importance of bacteria in nature and industry.
Introduction
Bacteria are among the oldest and most successful living organisms on Earth. They have existed for more than 3.5 billion years, long before plants, animals, or humans appeared. Today, bacteria are found almost everywhere—from deep oceans and hot springs to the soil beneath our feet and inside the human body.
Although bacteria are often associated with disease, only a small percentage cause illness. In fact, most bacteria are harmless or beneficial. They recycle nutrients, help digest food, produce medicines, and are used in industries that make foods such as yoghurt and cheese. Understanding bacteria helps us appreciate both their importance and their impact on our lives.
What Are Bacteria?
Bacteria are single-celled microorganisms that belong to a group of organisms called prokaryotes.
Unlike plant and animal cells, bacterial cells:
- Do not have a nucleus.
- Do not contain membrane-bound organelles such as mitochondria or chloroplasts.
- Have DNA that floats freely in the cytoplasm.
Despite their simple structure, bacteria can perform all the processes needed for life.
Figure 1. Bacteria are simple single-celled organisms known as prokaryotes.
The Structure of a Bacterial Cell
A typical bacterial cell contains several important parts.
Cell Wall
- Provides support and protection.
- Maintains the cell's shape.
Cell Membrane
- Controls the movement of substances into and out of the cell.
Cytoplasm
- A jelly-like substance where many chemical reactions take place.
DNA
- A single circular chromosome.
- Not enclosed within a nucleus.
- Contains the bacterium's genetic information.
Ribosomes
- Small structures where proteins are made.
Plasmids
Some bacteria contain small circular pieces of DNA called plasmids.
Plasmids often carry genes that provide useful traits, such as antibiotic resistance.
Flagellum (Some Species)
Some bacteria have one or more flagella.
These long, whip-like structures help the bacterium move.
Figure 2. The main structures of a bacterial cell include the cell wall, membrane, cytoplasm, DNA, ribosomes, and sometimes plasmids and flagella.
How Are Bacteria Different from Other Microorganisms?
Bacteria differ from other groups of microorganisms in several ways.
| Bacteria | Other Microorganisms |
|---|---|
| Single-celled | May be single-celled or multicellular |
| No nucleus | Fungi, protozoa, and algae have nuclei |
| Prokaryotic cells | Most other microorganisms are eukaryotic |
| Reproduce by binary fission. | Many reproduce sexually or by spores |
These differences help scientists classify living organisms.
How Do Bacteria Reproduce?
Bacteria reproduce by a process called binary fission.
During binary fission:
- The bacterial DNA is copied.
- The cell grows larger.
- The cell membrane and wall divide.
- Two genetically identical daughter cells are produced.
Binary fission is a form of asexual reproduction because only one parent cell is involved.
Under favourable conditions, some bacteria can divide every 20 minutes, allowing populations to grow very rapidly.
Figure 3. During binary fission, one bacterial cell divides to produce two genetically identical cells.
Beneficial Bacteria
Most bacteria are beneficial.
Examples include:
In the Human Body
Beneficial bacteria in the digestive system help:
- Digest food.
- Produce vitamins such as vitamin K.
- Prevent harmful bacteria from growing.
- Support the immune system.
In Nature
Bacteria:
- Decompose dead organisms.
- Recycle nutrients.
- Improve soil fertility.
- Fix nitrogen in the roots of certain plants.
In Industry
Bacteria are used to produce:
- Yoghurt.
- Cheese.
- Vinegar.
- Medicines.
- Enzymes.
- Biofuels.
Figure 4. Beneficial bacteria support human health, agriculture, food production, and ecosystems.
Harmful Bacteria
A small number of bacteria cause disease.
Examples include:
- Tuberculosis.
- Cholera.
- Salmonella food poisoning.
- Tetanus.
- Strep throat.
These bacteria may produce toxins or damage body tissues.
Fortunately, many bacterial infections can be treated with antibiotics, although antibiotic resistance is becoming an increasing concern.
The Importance of Bacteria in Ecosystems
Bacteria play several essential ecological roles.
Decomposition
They break down dead plants and animals.
This:
- Recycles nutrients.
- Returns minerals to the soil.
- Supports plant growth.
Nitrogen Cycle
Some bacteria convert nitrogen gas into forms plants can use.
This process is called nitrogen fixation.
Without these bacteria, plants would struggle to obtain enough nitrogen to grow.
Figure 5. Bacteria recycle nutrients and play a vital role in the nitrogen cycle.
Bacteria in Modern Industry
Scientists use bacteria to manufacture many useful products.
Examples include:
- Insulin for diabetes treatment.
- Vaccines.
- Industrial enzymes.
- Bioplastics.
- Wastewater treatment.
- Bioremediation (cleaning polluted environments).
Genetically modified bacteria are now used in many areas of biotechnology.
Why Bacteria Matter
Although bacteria are microscopic, they have enormous effects on the world.
They help:
- Maintain ecosystems.
- Produce food.
- Support human health.
- Manufacture medicines.
- Recycle nutrients.
- Drive many important biological processes.
Life on Earth could not function without bacteria.
Worked Example
Question
State whether each statement is True or False.
- Bacteria have a nucleus.
- Bacteria reproduce by binary fission.
- All bacteria cause disease.
- Some bacteria help make yoghurt.
Solution
- False
- True
- False
- True
Real-World Connection
People eat foods containing probiotic bacteria, such as yoghurt and some fermented drinks, to help maintain a healthy balance of microorganisms in their digestive system. These beneficial bacteria can support digestion and may help restore the gut microbiome after treatment with antibiotics.
Did You Know?
Scientists estimate that the human body contains trillions of bacteria, especially in the digestive system. In fact, bacterial cells living in and on your body have a combined mass of about 0.2 kilograms. Most of these bacteria are harmless or beneficial and are essential for maintaining good health.
Key Terms
Bacteria – Single-celled prokaryotic microorganisms that lack a nucleus.
Binary fission – A form of asexual reproduction in which one bacterial cell divides into two genetically identical cells.
Cell wall – A rigid outer layer that supports and protects bacterial cells.
Flagellum – A whip-like structure that helps some bacteria move.
Plasmid – A small circular piece of DNA found in many bacterial cells.
Prokaryote – An organism whose cells do not contain a nucleus or other membrane-bound organelles.
Ribosome – A structure where proteins are made.
Key Takeaways
- Bacteria are single-celled prokaryotes that do not have a nucleus or membrane-bound organelles.
- A bacterial cell contains a cell wall, cell membrane, cytoplasm, DNA, ribosomes, and sometimes plasmids and flagella.
- Bacteria reproduce by binary fission, producing two genetically identical daughter cells.
- Most bacteria are beneficial, helping with digestion, nutrient cycling, food production, and medicine, while only a small number cause disease.
- Bacteria are essential in ecosystems because they decompose organic matter and recycle nutrients.
- Modern industries use bacteria to produce foods, medicines, industrial enzymes, and environmentally friendly technologies.