Microorganisms as pathogens — A/L Biology

What must I remember about Microorganisms as pathogens?

• Infectious diseases are caused by transmissible biological pathogens (viruses, bacteria, fungi, protozoa, helminths) capable of colonizing host tissues, evading host defenses, and causing pathological dysfunction.
• Koch's Postulates form the classical microbiological criteria for establishing causal etiology between a specific microbe and a disease: 1) Microbe present in every disease case, 2) Isolated and grown in pure culture, 3) Inoculation into healthy host reproduces disease, 4) Same microbe re-isolated from inoculated host.
• Transmission routes: Airborne / droplet aerosol (Mycobacterium tuberculosis, Influenza, SARS-CoV-2), Fecal-oral waterborne/foodborne (Vibrio cholerae, Salmonella typhi, Hepatitis A), Vector-borne (Plasmodium falciparum via Anopheles, Dengue virus via Aedes), Direct contact/sexual (HIV, Treponema pallidum), and Zoonotic spillover (Rabies virus).
• Pathogenicity mechanisms: Production of Exotoxins (heat-labile, highly potent proteins actively secreted by Gram-positive and Gram-negative bacteria, e.g., Tetanus toxin, Cholera toxin, Botulinum) versus Endotoxins (heat-stable lipopolysaccharide LPS component of outer membrane in Gram-negative bacteria, e.g., Salmonella, inducing septic shock and fever).
• Epidemiological dynamics: Endemic (constant background presence in a region), Epidemic (sudden regional outbreak exceeding normal expectancy), and Pandemic (global cross-border dissemination across multiple continents).

Test yourself (1 minute)

1. Exotoxins vs Endotoxins clinical comparison

   Exotoxins are lethal in microgram doses, target specific cellular receptors (e.g., neurotoxins, enterotoxins), and can be neutralized into protective toxoid vaccines; Endotoxins produce generalized inflammatory fevers, require high doses, and cannot be converted into toxoids.

2. Exceptions to Koch's Postulates

   Obligate intracellular microbes (e.g., Treponema pallidum, Mycobacterium leprae, all viruses) cannot be cultured on cell-free synthetic media; asymptomatic carrier states exist (e.g., typhoid Mary).

3. Incubation Period definition

   The time interval between initial exposure/entry of pathogen and the first appearance of clinical signs and symptoms.

Practise Microorganisms as pathogens questions today

Source: NIE G.C.E. (A/L) Biology Syllabus & Resource Book, Unit 9

What comes next?

Next in the syllabus

Where does Microorganisms as pathogens fit in the syllabus?

Year 2 · Term 3