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6.2.2Topic 6 · Immunity, Infection and Forensics

Entry Routes of Pathogens — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 10 of 19 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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6.2.2 Entry Routes of Pathogens — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the routes by which pathogens enter the body, the physical and chemical barriers that prevent entry, and the ways in which disease is transmitted.For a pathogen to cause disease it must reach …

  • Transmission
  • Direct contact
  • Droplets and aerosols
  • Contaminated food and water
  • normal flora
  • Exam-style question 1

Section 1 of 8Transmission of pathogens

For a pathogen to cause disease it must reach a susceptible host, enter it, and multiply. Transmission may be direct or indirect.

Transmission of pathogens
Direct contact
Direct contact touching, sexual contact, or contact with blood (HIV, hepatitis B), and transfer from mother to fetus across the placenta or in breast milk.
Droplets and aerosols
Droplets and aerosols coughing, sneezing and breathing release droplets containing pathogens (influenza, tuberculosis, measles).
Contaminated food and water
Contaminated food and water pathogens in food or drinking water (cholera, Salmonella, typhoid), often spread by faecal contamination.
Vectors
Vectors animals that carry the pathogen between hosts, such as the Anopheles mosquito (malaria) and houseflies.
Fomites
Fomites objects such as bedding, towels and surgical instruments.

The ease with which a disease spreads depends on population density, hygiene, sanitation, the infectious dose and the speed of reproduction of the pathogen.

Section 2 of 8Entry routes

RouteBarrier or defenceExamples of entry
SkinKeratinised outer layer, sebum, skin flora competing with pathogensCuts, wounds, insect bites (malaria); some fungi invade directly (athlete's foot)
Gas exchange systemMucus traps particles; cilia move it to the throat; macrophages in alveoliInhaled droplets: TB, influenza, whooping cough
Digestive systemStomach acid (pH 1 to 2), lysozyme in saliva, gut floraContaminated food and water: cholera, Salmonella
Urinogenital tractUrine flow, acidic vagina maintained by lactic acid bacteriaSexually transmitted infections: chlamydia, gonorrhoea, HIV
EyesTears containing lysozyme, blinkingConjunctivitis

Section 3 of 8Defences in detail

The skin is a physical barrier: its dead keratinised cells are tightly packed, waterproof and difficult for most pathogens to penetrate. Sebaceous glands release sebum, which has a mildly acidic effect and contains fatty acids that inhibit microbes. Blood clots seal wounds quickly, with platelets releasing clotting factors to produce a fibrin mesh.

In the airways, goblet cells secrete mucus that traps pathogens and dust, and ciliated epithelial cells beat to move the mucus up to the throat, where it is swallowed and killed by stomach acid.

Defences in detail
Smoking damages cilia and increases the chance of infection.
Lysozyme, an enzyme in tears, saliva and mucus, hydrolyses the peptidoglycan in bacterial cell walls, making bacteria burst.

The normal flora (microbiome) of the skin, gut and vagina compete with pathogens for nutrients and space and may produce substances that harm them. The use of broad-spectrum antibiotics can disturb these communities, allowing an infection such as Clostridioides difficile to take hold.

Section 4 of 8Reducing transmission

Public health measures break the chain of infection: clean water, sewage treatment, safe food handling, handwashing, vaccination, use of condoms, control of vectors by nets and insecticide, isolation of infected patients and good ventilation.

Reducing transmission
In hospitals, sterilisation of equipment and barrier nursing reduce spread of infections such as MRSA.
Different routes require different strategies; for example, a waterborne disease such as cholera is controlled mainly by sanitation, while an airborne one requires measures such as masks and ventilation.

If a pathogen passes these barriers, the non-specific and then the specific immune responses are triggered (next lessons).

Section 5 of 8Case studies: cholera, TB and malaria

Cholera is spread through water contaminated with faeces. Vibrio cholerae survives stomach acid in large numbers, colonises the small intestine and releases a toxin that causes epithelial cells to secrete chloride ions and water, producing severe watery diarrhoea.

1
Treatment is oral rehydration, and prevention depends on clean water and sanitation.
2
Tuberculosis is an airborne bacterial infection: droplets are inhaled and the bacteria are taken up by macrophages in the lungs but survive inside them.
3
Overcrowding, poor ventilation and a weak immune system, as in HIV infection, increase transmission.
4
Malaria is caused by the protoctist Plasmodium and is transmitted by the bite of a female Anopheles mosquito, which injects the parasite into the blood when it feeds.
5
Control includes insecticide-treated bed nets, removing breeding sites and drugs.

These cases show that the route of entry determines the best control measure, and that the same disease can be controlled in different ways. When asked to suggest how to reduce the spread of a named disease, state the route and give a measure that blocks it.

Section 6 of 8Exam-style questions

  1. Describe how the gas exchange system helps to prevent infection by pathogens. [4]4 marks

  2. Suggest why people who smoke are more likely to suffer from respiratory infections. [3]3 marks

  3. Explain why the stomach is an unsuitable environment for most pathogens, and suggest how some pathogens still cause gut infections. [3]3 marks

Section 7 of 8Key terms

TermMeaning
TransmissionThe passing of a pathogen from one host to another.
VectorAn organism that carries a pathogen between hosts.
LysozymeAn enzyme that hydrolyses bacterial cell walls.
Normal floraHarmless microorganisms that live on and in the body.
MucusA sticky secretion that traps particles and microbes.
FomiteAn inanimate object that carries pathogens.
Communicable diseaseA disease that can pass from one host to another.

Section 8 of 8Exam technique

1
Saying the skin is waterproof and a barrier without noting the keratin and normal flora.
2
Thinking that all mucus is made by cilia. Goblet cells make mucus; ciliated cells move it.
3
Forgetting that lysozyme breaks down peptidoglycan, not the cell walls of all microbes.
4
Confusing a vector with a pathogen; a vector transmits it.

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