Pathogenesis: Adhesion, Invasion and Virulence Factors
Causing disease takes a sequence of steps: reaching a surface, sticking to it, getting past defences and obtaining nutrients such as iron. Each step depends on specific molecules, and losing one of them can make a pathogen harmless.
What a learner can do afterwards
- Orders the steps of an infection and names a factor used at each.
- Explains why sticking to a particular tissue decides which disease results.
- Predicts the effect of deleting one adhesion gene on the ability to infect.
1 · Read
Picture infection as an obstacle course with four gates in fixed order. You reach a surface, stick to it with gripping molecules called adhesins, push past the defences, and secure food such as iron. Each gate needs its own molecules, and losing any single gate can turn a killer harmless.
Two cousins from one germ family show why sticking decides disease. The cousin whose adhesins grip the lung lining causes breathing illness. The cousin whose adhesins grip the gut causes diarrhoea. The sticking molecules, not the whole germ, pick the tissue and therefore the disease.
Each gate has its own toolkit. Capsules shield germs from phagocytes, the cells that would eat them. Enzymes dissolve tissue barriers, siderophores steal iron from you, and toxins disable your cells from a distance. A true virulence factor shows a clear gap: the germ still grows in a dish yet causes far weaker disease.
When you meet a suspect gene, ask what deleting it breaks. If the mutant grows normally yet fails to grip and infect its usual site, you found a true virulence factor. Since every later gate needs that first grip, blocking sticking is the cheapest place to stop infection.
Infection must run reach, stick, enter, and feed in order, so breaking any single gate can stop disease.
More to read
15.2 How Pathogens Cause Disease
Causing disease is a sequence, and each step needs its own molecules. The pathogen reaches a surface, sticks to it with adhesins, pushes past defences, and secures nutrients such as iron. Losing any single step can turn a killer harmless, which is why each factor is a drug and vaccine target. Order matters for diagnosis too: where the chain breaks predicts which stage fails.
15.3 Virulence Factors of Bacterial and Viral Pathogens
Virulence factors are the named tools at each gate. Capsules block phagocytes, enzymes dissolve tissue barriers, toxins disable cells at a distance, and siderophores steal iron from the host. Bacterial and viral versions differ in detail but share the logic of defeating one host defence per tool. Deleting one tool gene measurably weakens disease even when the microbe still grows in a dish. That gap between growing and harming is the signature of a true virulence factor.
15.4 Virulence Factors of Eukaryotic Pathogens
Eukaryotic pathogens run the same playbook with bigger toolkits. Fungi and parasites also adhere, invade, and feed, adding tricks such as shape shifting and antigenic variation to outrun immunity. Their size lets them physically damage tissue that bacteria would only poison. The sequence still holds: stick first, then enter, then eat. Blocking adhesion remains the cheapest intervention point because everything downstream depends on that grip.
22.4 Bacterial Diseases in Humans
Which tissue a pathogen sticks to decides which disease results. Adhesins that grip the lung line cause respiratory illness from the same microbe family whose gut gripping cousins cause diarrhoea. Tissue tropism is therefore a property of the sticking molecules, not of the whole organism. Delete one adhesion gene and the pathogen may survive yet fail to infect its usual site.
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Where it sits
Learn first
Nothing; this is a starting point.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.