Revision notes for Edexcel A A Level Biology Bacteria, viruses and pathogen entry. Open the guide for explanations and worked examples. Written against the Edexcel A A Level Biology (9BN0) specification, so the content matches what's examinable rather than general Biology background.
Bacteria, viruses and pathogen entry
What you'll learn
- How bacteria and viruses differ in structure, size and reproduction.
- What a pathogen must achieve before it can cause an infection.
- The main routes pathogens use to enter the human body.
- How first-line defences reduce pathogen entry and transmission.
The basic idea: exposure is not the same as infection
Your body is constantly exposed to microorganisms. Many are harmless, and some are helpful, such as the normal microbiota on your skin and in your gut. A problem begins when a disease-causing agent gets into the body, survives, and multiplies in a suitable place.
Definition
Pathogen, infection and disease
A pathogen is a biological agent that can cause disease. An infection occurs when a pathogen enters the body and multiplies. Disease is the harmful effect on the host, such as tissue damage, fever, coughing, diarrhoea or immune system failure.
Pathogens include bacteria, viruses, fungi and protoctists. In this section, we focus on bacteria and viruses, because they are common examples in A-Level immunity and infection.
Key Idea
The infection sequence
For a pathogen to cause disease, it usually needs to enter the host, avoid or overcome first-line defences, attach to suitable cells or tissues, and replicate.
Bacteria: living prokaryotic cells
Definition
Bacterium
A bacterium is a single-celled prokaryote, meaning it has cytoplasm, a cell-surface membrane, ribosomes and DNA, but no nucleus and no membrane-bound organelles.
A typical bacterium is about 0.5–5 µm long. Many bacteria have a cell wall made from peptidoglycan, also called murein. Some also have a capsule or slime layer, flagella for movement, and pili for attachment to surfaces.
Bacterial DNA is usually a circular chromosome in the cytoplasm. Many bacteria also contain plasmids, which are small circular DNA molecules that may carry useful genes, such as antibiotic resistance genes.
The diagram below compares a typical bacterium with a typical enveloped virus. Notice the huge difference in complexity: the bacterium is a cell, but the virus is not.
Bacteria reproduce by binary fission, where one bacterial cell replicates its DNA and divides into two genetically identical cells. Under favourable conditions, some bacteria can divide very rapidly.
Definition
Toxin
A toxin is a poisonous substance produced by a pathogen that damages host cells or disrupts normal body functions.
Some bacteria cause disease by invading tissues directly. Others produce toxins. For example, bacterial toxins may damage epithelial cells, trigger diarrhoea, or interfere with nerve or immune function.
Viruses: infectious particles, not cells
Definition
Virus
A virus is an acellular infectious particle, meaning it is not made of cells. It contains genetic material inside a protein coat called a capsid, and it can only replicate inside a living host cell.
Viruses are much smaller than bacteria. Many are about 20–300 nm across. Since 1 µm is 1×10−6 m1 \times 10^{-6}\ \text{m}1×10−6 m and 1 nm is 1×10−9 m1 \times 10^{-9}\ \text{m}1×10−9 m, a virus with a diameter of 100 nm is only 0.1 µm across.
Viruses may contain DNA or RNA as their genetic material. Some, such as HIV, have a lipid envelope surrounding the capsid. This envelope often contains attachment proteins that bind to specific molecules on host cells.
Definition
Obligate intracellular parasite
An obligate intracellular parasite is something that can reproduce only inside a living cell. Viruses are obligate intracellular parasites because they lack ribosomes, cytoplasm and many enzymes needed for independent metabolism.
A virus does not divide by binary fission. Instead, it enters a host cell, uses the host cell’s machinery to make viral components, and then new virus particles are assembled and released.
Common Mistake
Thinking antibiotics kill viruses
Antibiotics target bacterial features such as cell wall synthesis or bacterial ribosomes. Viruses do not have these structures, so antibiotics do not work against viral infections.
Comparing bacteria and viruses
| Feature | Bacteria | Viruses |
|---|---|---|
| Basic nature | Living prokaryotic cells | Acellular infectious particles |
| Typical size | About 0.5–5 µm | About 20–300 nm |
| Genetic material | Circular DNA, often with plasmids | DNA or RNA |
| Ribosomes | 70S ribosomes present | No ribosomes |
| Reproduction | Binary fission | Assembly inside host cells |
| Treatment link | Some antibiotics may work | Antibiotics do not work |
The size difference matters in microscopy. Bacteria can often be seen using a light microscope, but most viruses are too small and need an electron microscope.
The magnification relationship is:
magnification=image sizeactual size\text{magnification} = \frac{\text{image size}}{\text{actual size}}magnification=actual sizeimage sizeExample
Calculating actual size from a micrograph
A bacterial cell is 36 mm long on a printed micrograph at a magnification of 18 000. Find its actual length.
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Convert the image size into micrometres: 36 mm is 36 000 µm, because 1 mm is 1000 µm.
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Rearrange and substitute into the magnification equation:
actual size=36 000 μm18 000=2.0 μm\text{actual size} = \frac{36\,000\ \mu\text{m}}{18\,000} = 2.0\ \mu\text{m}actual size=1800036000 μm=2.0 μm -
Check the answer against biological scale: 2.0 µm is a sensible size for a bacterium, but much larger than a typical virus.
Tip
Unit sanity check
If your calculated virus is several micrometres across, or your calculated bacterium is only a few nanometres across, check your unit conversion carefully.
Pathogen entry: getting past the body surface
Definition
Pathogen entry
Pathogen entry is the movement of a pathogen through a body surface or barrier into tissues, body fluids or host cells where it can survive and replicate.
The human body has many first-line defences. These are non-specific barriers that act against many different pathogens before the immune system has to make a targeted response.
The main entry routes are shown below: respiratory tract, digestive tract, breaks in the skin or direct entry into blood, and urogenital mucosa.
Respiratory tract
Pathogens can enter in droplets or aerosols when an infected person coughs, sneezes, talks or breathes out. The respiratory tract is lined with epithelial cells, mucus and cilia.
Mucus traps pathogens. Cilia are tiny hair-like structures that move mucus up towards the throat, where it can be swallowed or coughed out.
A key example is Mycobacterium tuberculosis, the bacterium that causes tuberculosis, often shortened to TB. It can be inhaled in droplets and reach the lungs.
Digestive tract
Pathogens may enter in contaminated food or water. They must survive stomach acid, digestive enzymes, bile, gut mucus and competition from normal gut bacteria.
Some pathogens are adapted to survive acidic conditions or produce toxins that affect the gut lining, causing vomiting or diarrhoea. These symptoms may help transmit the pathogen to new hosts through contaminated surfaces, food or water.
Example
Working out an entry route from evidence
A person develops diarrhoea after drinking untreated water. The pathogen survives stomach acid and attaches to intestinal epithelial cells.
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The source was swallowed water, so the most likely entry route is the digestive tract.
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The pathogen had to overcome digestive first-line defences, especially stomach acid, mucus, gut epithelium and normal microbiota.
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Attachment to intestinal epithelial cells helps explain how it stayed in the gut long enough to multiply and cause symptoms.
Breaks in the skin or direct blood entry
Intact skin is a strong barrier. The outer layer contains keratinised dead cells, and blood clotting helps seal wounds quickly.
However, pathogens can enter through cuts, needle sharing, animal bites or insect vectors. A vector is an organism that transfers a pathogen from one host to another, such as a mosquito transmitting a virus or protoctist.
Urogenital mucosa
Some pathogens enter through mucous membranes of the reproductive or urinary systems. HIV can be transmitted through infected blood, semen, vaginal fluids or breast milk.
Common Mistake
Mucous membranes are not the same as skin
Mucous membranes are moist epithelial surfaces, not dry keratinised skin. They are important entry points because they are thinner and more easily crossed by some pathogens.
Attachment: why pathogens infect particular cells
Pathogens often need to attach to host cells before entering or damaging them. Attachment depends on complementary shapes between pathogen surface molecules and host cell surface molecules.
Definition
Receptor
A receptor is a molecule, often a protein or glycoprotein, on a cell surface that can bind specifically to another molecule.
This specificity helps explain why pathogens infect particular tissues. HIV, for example, binds to CD4 receptors on helper T cells, along with co-receptors. This is one reason HIV damages the immune system so seriously.
Definition
Antigen
An antigen is a molecule, often on the surface of a pathogen, that can be recognised by immune cells or antibodies as non-self.
Once pathogens enter the body, their antigens can be detected by the immune system. That leads into the next part of the topic: non-specific responses and the specific immune response.
Example
Predicting which cells a virus can infect
A virus strain can enter only cells that have both receptor A and receptor B. Cell type 1 has both receptors. Cell type 2 has receptor A only. Cell type 3 has receptor B only.
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Apply the entry rule: the virus needs both receptor A and receptor B, so one receptor alone is not enough.
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Compare each cell type with the requirement: cell type 1 matches fully, while cell types 2 and 3 each lack one required receptor.
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Conclude that only cell type 1 is likely to be infected by this virus strain.
Practical link: studying bacteria safely
Bacteria can be grown on nutrient agar, but this must be done using aseptic technique, which means using sterile methods to prevent contamination by unwanted microorganisms.
In school laboratories, plates are usually incubated at a maximum of 25 °C to reduce the chance of growing human pathogens. Lids are taped but not fully sealed, so dangerous anaerobic conditions are not encouraged.
Viruses cannot be grown on ordinary nutrient agar because they need living host cells to replicate.
Common Mistake
Saying all bacteria are harmful
Most bacteria do not cause disease. Many are harmless or beneficial, such as gut bacteria that help maintain the normal microbiota and compete with pathogens.
Exam technique
In the exam
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When comparing bacteria and viruses, always state that bacteria are cells but viruses are not cells.
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Link entry route to barrier: respiratory pathogens meet mucus and cilia; digestive pathogens meet stomach acid and gut epithelium; blood-borne pathogens may bypass skin.
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For application questions, explain the sequence: exposure, entry, attachment, replication, then symptoms or transmission.
Self review
Check yourself
- Why can antibiotics treat some bacterial infections but not viral infections?
- Which first-line defences would a respiratory pathogen need to overcome?
- How does receptor specificity help explain why HIV infects helper T cells?
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