Types of Pathogen

A pathogen is any microorganism that causes disease in its host. Communicable (infectious) diseases are caused by pathogens that can be transmitted from one individual to another. OCR A-Level Biology A specification 4.1.1 (a) requires you to know the four main groups of pathogen — bacteria, viruses, protoctista and fungi — and understand how each causes disease at the cellular level. This lesson sets the foundations for the whole module: you should leave it able to classify named pathogens, describe how they damage their hosts, and recognise the characteristic diseases they cause in plants and animals.

Key Definitions:

• Pathogen — a microorganism that causes disease.
• Communicable disease — a disease that can be transmitted from one organism to another.
• Host — an organism that harbours and supports a pathogen.
• Virulence — the degree of pathogenicity; the ability of a pathogen to cause disease.

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The Four Groups of Pathogen

flowchart TD
    A[Pathogens] --> B[Bacteria]
    A --> C[Viruses]
    A --> D[Protoctista]
    A --> E[Fungi]
    B --> B1[Prokaryotic, cellular]
    C --> C1[Acellular, obligate intracellular]
    D --> D1[Eukaryotic, single-celled]
    E --> E1[Eukaryotic, hyphal/yeast]

1. Bacteria

Bacteria are prokaryotic single-celled organisms. They have no membrane-bound nucleus or organelles; their DNA is a single circular chromosome in the cytoplasm, often with smaller plasmids. A peptidoglycan cell wall surrounds the cell membrane. Bacteria reproduce by binary fission and can double in number every 20 minutes under ideal conditions.

Bacteria can be classified by:

• Shape: cocci (spheres), bacilli (rods), spirilla (spirals), vibrio (commas).
• Gram stain: Gram-positive bacteria have thick peptidoglycan walls that retain crystal violet dye; Gram-negative bacteria have thin walls with an outer lipopolysaccharide membrane.

Bacteria cause disease in two main ways:

1. Producing toxins. Exotoxins (secreted) and endotoxins (released when the cell lyses) damage host cells directly — for example, Vibrio cholerae releases choleragen, which causes intestinal cells to leak chloride ions and water.
2. Invading and destroying host tissues. For example, Mycobacterium tuberculosis survives inside macrophages and forms granulomas in the lungs.

2. Viruses

Viruses are acellular, non-living particles that cannot reproduce independently. A virus particle (virion) consists of:

• A nucleic acid core (DNA or RNA, single- or double-stranded).
• A protein coat (capsid) made of capsomeres.
• Sometimes an outer lipid envelope with protein spikes (e.g., HIV, influenza).

Viruses are obligate intracellular parasites — they can only replicate inside a living host cell. The viral genome hijacks the host's ribosomes, enzymes and nucleotides to synthesise new viral components, which assemble into new virions. The host cell is usually destroyed in the process, either by lysis or by apoptosis. Viruses cause disease by:

• Lysing infected cells.
• Integrating their DNA into host chromosomes (e.g., retroviruses like HIV).
• Stimulating damaging immune responses.

Exam Tip: Never describe a virus as "living" or "dying". Viruses are not classified as living organisms because they cannot metabolise, grow, or reproduce independently. In exam answers use phrases like "viral particles" and "replicate" rather than "live" or "grow".

3. Protoctista

Protoctista (protists) are eukaryotic, mostly single-celled organisms. They have a true nucleus, membrane-bound organelles and, often, flagella or cilia for movement. Some are parasitic; others are photosynthetic. Pathogenic protoctista typically invade host cells, feeding on their contents before rupturing the cell and moving on.

The best-known example is Plasmodium, which causes malaria. Plasmodium falciparum is the most virulent species. It has a complex life cycle that alternates between the female Anopheles mosquito (the vector) and humans, invading first liver cells and then red blood cells.

4. Fungi

Fungi are eukaryotic organisms with chitin cell walls. They can be:

• Unicellular (yeasts, e.g., Candida).
• Multicellular, composed of threadlike hyphae forming a mycelium (e.g., Trichophyton).

Fungi in plants often live in the vascular tissue, blocking it and secreting extracellular enzymes that digest host tissues. In animals, fungi typically cause superficial infections such as ringworm and athlete's foot, but can also cause severe systemic disease in immunocompromised patients. Fungal spores are an important means of transmission, especially in crop diseases.

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Comparing the Four Groups

Feature	Bacteria	Viruses	Protoctista	Fungi
Cell type	Prokaryotic	Acellular	Eukaryotic	Eukaryotic
Genetic material	Circular DNA + plasmids	DNA or RNA	Linear DNA in nucleus	Linear DNA in nucleus
Cell wall	Peptidoglycan	None	Usually absent	Chitin
Reproduction	Binary fission	Requires host	Varied (often inside host cells)	Spores, budding, or hyphal growth
Size	0.5–5 μm	20–300 nm	1–200 μm	Variable (μm to m for mycelia)
Example disease	TB, cholera	HIV, influenza	Malaria	Ringworm, black sigatoka

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How Pathogens Cause Disease

At the cellular level, pathogens damage their hosts in several ways:

1. Damaging host DNA, membranes or proteins through toxins or enzymes (e.g., diphtheria toxin inhibits protein synthesis by inactivating elongation factor EF-2).
2. Competing for nutrients — parasitic pathogens consume glucose, amino acids and vitamins needed by the host.
3. Blocking transport systems — for example, fungal hyphae block xylem vessels, wilting the plant.
4. Disrupting immune responses — HIV destroys T helper cells; Mycobacterium tuberculosis hides inside macrophages.
5. Triggering excessive inflammation — sepsis is often caused by an overwhelming host response to bacterial endotoxins.

Exam Tip: When asked "how does the pathogen cause symptoms?" always link the mechanism (e.g., toxin, intracellular replication) to the tissue damaged (e.g., gut epithelium, alveolar macrophages) to the observable symptom (e.g., diarrhoea, coughing). Three-step chains pick up the marks.

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Common Exam Mistakes

• Calling viruses "cells". They have no cytoplasm, ribosomes, or membranes of their own, and should never be described as cells.
• Confusing "bacteria" and "virus". OCR often gives an electron micrograph and asks you to identify which kingdom a pathogen belongs to — look for ribosomes and a cell wall (bacteria) versus capsid symmetry (virus).
• Forgetting that some fungi are unicellular. Yeasts like Candida are just as much fungi as moulds.
• Thinking all protoctista are pathogenic. Most are harmless single-celled organisms; only a few species cause disease.
• Confusing "pathogen" with "parasite". All pathogens are technically parasites of a sort, but not all parasites are classed as pathogens (e.g., many helminths are parasites but are studied separately).

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Quick Recap

• The four groups of pathogen are bacteria (prokaryotic), viruses (acellular), protoctista (eukaryotic, often intracellular) and fungi (eukaryotic, chitin-walled).
• Bacteria cause disease mainly through toxins and tissue invasion; viruses hijack host cells and destroy them; protoctista invade and digest host cells; fungi damage vascular tissue (plants) or skin (animals).
• You must be able to give named examples for each group and link cellular mechanisms to symptoms.

Reference: OCR A-Level Biology A (H420) specification 4.1.1 (a).