Pathogens — Bacteria and Viruses

This lesson covers bacterial and viral pathogens as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the structure of bacteria and viruses, how they cause disease and be able to describe specific examples including Salmonella, HIV and TMV.

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What Are Pathogens?

A pathogen is a microorganism that causes disease. Pathogens are parasites — they live on or inside a host organism and cause damage to the host while benefiting themselves (by gaining nutrients or a place to reproduce).

There are four types of pathogen: bacteria, viruses, fungi and protists. This lesson focuses on bacteria and viruses.

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Bacteria

Structure of a Bacterial Cell

Bacteria are prokaryotic organisms. They are much smaller than eukaryotic cells (typically 1–5 μm in length) and have a simpler structure:

Feature	Description
Cell wall	Rigid outer layer that gives the cell its shape (NOT made of cellulose — bacterial cell walls are made of peptidoglycan)
Cell membrane	Controls what enters and leaves the cell
Cytoplasm	Where chemical reactions occur
Ribosomes	Smaller than eukaryotic ribosomes; site of protein synthesis
Chromosomal DNA	A single circular loop of DNA, free in the cytoplasm (no nucleus)
Plasmid DNA	Small, extra circles of DNA; can carry genes for antibiotic resistance
Flagellum (some bacteria)	A tail-like structure used for movement

graph TD
    subgraph "Bacterial Cell Structure"
        A["Cell wall<br/>peptidoglycan"]
        B[Cell membrane]
        C[Cytoplasm]
        D["Circular DNA<br/>chromosomal"]
        E["Plasmids<br/>small extra DNA loops"]
        F["Ribosomes<br/>smaller than eukaryotic"]
        G["Flagellum<br/>for movement"]
    end

How Bacteria Cause Disease

Bacteria make you feel ill in two main ways:

1. Damaging cells directly — bacteria reproduce rapidly by binary fission (splitting in two). As they multiply, they destroy host cells and tissues.
2. Producing toxins — many bacteria release toxins (poisons) as waste products. These toxins damage cells and tissues, causing symptoms such as fever, inflammation and vomiting.

Exam Tip: Not all bacteria are harmful. Many bacteria are beneficial — e.g. gut bacteria help digest food. Only a small proportion of bacteria are pathogenic.

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Example: Salmonella

Salmonella is a bacterium that causes food poisoning.

Feature	Detail
Type of pathogen	Bacterium
How it spreads	Ingesting contaminated food (especially undercooked poultry, eggs, meat)
How it causes disease	Salmonella bacteria produce toxins in the gut
Symptoms	Fever, abdominal cramps, vomiting, diarrhoea
Prevention	Thorough cooking of food, good hygiene, vaccination of poultry

In the UK, poultry are vaccinated against Salmonella to reduce the number of cases of food poisoning.

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Viruses

Structure of a Virus

Viruses are not cells. They are much smaller than bacteria (typically 20–300 nm — about 1/100th the size of a bacterium). They cannot carry out life processes on their own and must infect a living cell to reproduce.

Feature	Description
Protein coat (capsid)	A protective outer shell made of protein
Genetic material	Either DNA or RNA (not both) enclosed within the capsid
No cell membrane	Viruses do not have their own cell membrane
No cytoplasm or ribosomes	Viruses cannot carry out metabolic reactions on their own

Some viruses also have an outer lipid envelope derived from the host cell membrane (e.g. HIV).

How Viruses Cause Disease

Viruses cause disease by a process called the lytic pathway:

1. The virus attaches to a specific host cell using proteins on its surface.
2. The virus injects its genetic material (DNA or RNA) into the host cell.
3. The host cell's machinery is hijacked — it is forced to make copies of the virus's genetic material and protein coats.
4. The new virus particles assemble inside the host cell.
5. The host cell bursts open (lyses), releasing hundreds of new virus particles.
6. Each new virus particle can go on to infect other cells.

graph LR
    A["Virus attaches<br/>to host cell"] --> B["Injects genetic<br/>material"]
    B --> C["Host cell makes<br/>copies of virus"]
    C --> D["New viruses<br/>assemble"]
    D --> E["Host cell<br/>bursts / lyses"]
    E --> F["New viruses<br/>infect other cells"]

This is why viral diseases spread so rapidly — each infected cell produces many new copies of the virus.

Exam Tip: Viruses are described as obligate intracellular parasites because they can only reproduce inside a living host cell. A key exam distinction is that antibiotics do NOT work against viruses — only against bacteria.

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Example: HIV (Human Immunodeficiency Virus)

HIV is a virus that attacks the immune system, specifically the T helper lymphocytes (a type of white blood cell).

Feature	Detail
Type of pathogen	Virus
How it spreads	Sexual contact, sharing needles, mother to child during pregnancy/birth/breastfeeding, contaminated blood products
How it causes disease	HIV infects and destroys T helper lymphocytes, weakening the immune system over time
Symptoms (early)	Flu-like illness shortly after infection
Progression	If untreated, the number of T helper cells drops so low that the body cannot fight off other infections — this late stage is called AIDS (Acquired Immune Deficiency Syndrome)
Treatment	Antiretroviral drugs — these do not cure HIV but control the virus, keeping the person healthy and reducing transmission
Prevention	Use of condoms, not sharing needles, screening blood products, antiretroviral drugs to prevent mother-to-child transmission

Exam Tip: HIV is the virus; AIDS is the condition that develops if HIV is left untreated. They are not the same thing. Many people with HIV never develop AIDS because antiretroviral drugs keep the virus under control.

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Example: Tobacco Mosaic Virus (TMV)

TMV is a virus that infects plants, particularly tobacco plants (but also tomatoes, peppers and other crop plants).

Feature	Detail
Type of pathogen	Virus
Host	Plants (especially tobacco, tomato)
How it spreads	Direct contact between plants, contaminated tools, hands, insects
How it causes disease	TMV infects plant cells and causes a distinctive mosaic pattern of discolouration on leaves
Symptoms	Patches of yellow, light green and dark green on leaves (mosaic pattern); reduced photosynthesis; stunted growth
Impact	Reduced photosynthesis means less glucose is produced, so the plant grows more slowly and crop yields decrease

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Comparing Bacteria and Viruses