Microorganisms and Disease
Pathogens and Types of Disease
Pathogens are microorganisms that cause infectious disease. The four main types:
| Type | Example disease | Treatment |
|---|---|---|
| Bacteria | Tuberculosis (TB), food poisoning (Salmonella) | Antibiotics |
| Viruses | Influenza, COVID-19, HIV | Antiviral drugs; vaccination |
| Fungi | Athlete's foot, ringworm | Antifungal creams |
| Parasites/Protists | Malaria (Plasmodium) | Antimalarial drugs |
Pathogens spread via: droplets (coughing/sneezing), contaminated food/water, direct contact, vectors (e.g. Anopheles mosquito for malaria).
Non-communicable diseases (e.g. cancer, cardiovascular disease) are NOT caused by pathogens — they have genetic/lifestyle/environmental causes.
The Immune System
The body has two lines of defence:
Physical/chemical barriers (non-specific):
- Skin — physical barrier; sebum (oily secretion) is antimicrobial
- Mucus — traps pathogens in airways; cilia sweep mucus to throat
- Stomach acid (HCl) — kills swallowed pathogens
Immune response (specific) — white blood cells:
- Phagocytes (neutrophils/macrophages): engulf and digest pathogens (phagocytosis). Non-specific.
- Lymphocytes (B cells): produce antibodies (Y-shaped proteins) specific to antigens on the pathogen surface. Antigen-antibody complex forms → pathogen destroyed/neutralised. Memory cells remain → faster response on re-exposure (immunity).
Antigens: proteins on the pathogen surface that trigger the immune response. Antibodies: complementary to specific antigens; lock-and-key specificity.
Vaccination
A vaccine contains weakened/dead pathogens or their antigens. It stimulates the immune system to produce antibodies and memory cells without causing disease. On re-exposure, memory cells activate rapidly — the person is immune.
Herd immunity: if enough of a population is vaccinated, the pathogen cannot spread — even unvaccinated individuals are protected.
WJEC required knowledge: MMR vaccine (measles, mumps, rubella); influenza vaccine changed annually (virus mutates).
Antibiotics
Antibiotics kill or inhibit bacteria only — they do NOT work against viruses. Mechanism: disrupt bacterial cell wall synthesis or protein synthesis.
Antibiotic resistance: bacteria can evolve resistance by natural selection:
- Random mutation occurs in some bacteria.
- Mutant bacteria survive antibiotic treatment (resistant).
- Reproduce rapidly → resistant strain spreads (MRSA — methicillin-resistant Staphylococcus aureus).
Strategies to reduce resistance: complete full antibiotic courses, prescribe only when necessary, do not use antibiotics for viral infections.
Microbial Cultures — Required Practical
Growing bacteria in the lab (aseptic technique):
- Sterilise equipment (autoclave petri dishes/media at 121°C under pressure).
- Inoculate agar plate with bacteria using inoculation loop (flamed until red-hot then cooled).
- Seal plate with tape (do NOT fully seal — allows gas exchange; prevents anaerobic pathogens).
- Incubate at 25°C in school labs (not 37°C body temperature — prevents culturing dangerous human pathogens).
- Measure zones of inhibition (clear areas) around antibiotic discs.
Calculating inhibition zone: measure diameter or radius; calculate area (A = πr²) to compare antibiotic effectiveness.
⚠Common mistakes
- Antibiotics kill bacteria, NOT viruses — stating they work on viruses is a common error.
- Vaccines contain antigens, not antibodies — the body makes its own antibodies.
- Phagocytosis is non-specific; antibody production is specific.
- School cultures incubated at 25°C (not 37°C) — safety reason.
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