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The ability to cause disease


The extent of pathogenicity

Entry into host: - different portals

Mucous membranes
Respiratory tract
Gastrointestinal tract
Genitourinary tract -
Parenteral Route


lethal dose for 50% of inoculated hosts


infectious dose for 50% of inoculated hosts

Using examples, explain how microbes adhere to host cells

Capsules - prevent phagocytosis
Cell Wall Components - help adherence
Enzymes - destroy blood cells, other functions
Cytoskeleton - invasins - 'basket around bacteria"

Explain how capsules and cell wall components contribute to pathogenicity.

Capsules - can prevent phagocytosis

When the body produces antibodies against the capsule then phagocytosis is efficient

Antigenic variation

Some pathogens alter their surface antigens - alternative genes for surface proteins - so they change by the time the host mounts a full response


cause clotting of fibrin in
blood - clot may prevent phagocytosis
e.g. in boils caused by Staphylococcus


break down fibrin - dissolve
clots that are used to prevent spread
of the pathogen
- Staphylokinase has been used
therapeutically to dissolve clots
causing heart attacks

cause clotting of fibrin in



breaks down hyaluronic acid - a polysaccharide that bonds cells in connective tissue - helps bacteria to spread
- causes blackening of infected area
Used with drugs to help their dispersal throughout the body


breaks down collagen in connective tissue and muscle - helps spread of bacteria
Siderophores:- help scavenge iron from

IgA proteases

- some pathogens produce proteases that can destroy these antibodies

Some pathogens alter their surface antigens - alternative genes for surface proteins - so they change by the time the host mounts a full response

Antigenic variation

an example of Antigenic variation

As each clone of Trypanosome is removed by the immune system a new clone takes its place

Describe how bacteria use the host cell's cytoskeleton to enter the cell.

use actin to move to the host cell then they Penetration into the Host Cell Cytoskeleton Invasins

Provide an example of direct damage, and compare this to toxin production.

Many bacteria cause direct cytopathic effects on host cells by growing inside cells and rupturing them. Some cause damage as they "pass through" host cells.
Most damage however is caused by toxins -poisonous substances produced by certain m/o's


toxins in the blood

Contrast the nature and effects of exotoxins and endotoxins

proteins produced inside bacteria and released into medium
Endotoxins derived from an outer portion of the cell wall in Gram -ve bacteria (outer membrane)

What is the portion of the polysachharide in endotoxins?

Lipid portion called Lipid A


(inactive toxin) injected to boost immune system

Diphtheria toxin

2 polypeptides A (Active) and B (Binding)
work together. B binds surface and promotes transport of entire protein into cell
A inhibits protein synthesis

Streptococcus pyogenes

Membrane-disrupting. Erythrogenic
Diseases Caused by Exotoxins

Clostridium botulinum

Diseases Caused by Exotoxins ..A-B toxin. Neurotoxin
Toxin produced during sporulation released late in growth
toxin binds in nerve/muscle junction and prevents transmission of impulses

C. tetani

A-B toxin. Neurotoxin
Diseases Caused by Exotoxins
binds to nerve cells in central nervous system (CNS) that normally terminate contractions
uncontrollable contractions ("lockjaw")

Vibrio cholerae

A-B toxin. Enterotoxin
Diseases Caused by Exotoxins

Staphylococcus aureus

Superantigen. Enterotoxin
Diseases Caused by Exotoxins

Using examples, describe the roles of plasmids and lysogeny in pathogenicity.

R (resistance plasmids) can carry genetic information that allow bacteria to degrade antibiotics.
In addition plasmids may encode instructions to make toxins

Lysogenic Conversion
When a virus enters the lysogenic cycle (integrates in host DNA) then it may carry extra genes that alter the host

what is a virus?

Pathogenic properties depend on access and damage to host during replication

Grow inside cells - avoid immune response - AIDS virus enters cells of the immune system

List nine cytopathic effects of viral infections.

1: Inhibition of macromolecular synthesis

2: Release of lysosome contents

3: Inclusion bodies (site of viral synthesis/assembly)

4: Formation of fused cells (Syncytium)

5: Change in cell function - e.g. when measles virus attaches to CD46 cells they reduce IL-12 production
6: Some viral infected cells produce interferon

7: Viral infections can induce antigenic changes on host cell surface

8: Changes to host chromosomes, oncogenes may be added

9: Cancer causing viruses inhibit "contact inhibition" between host cells thus promoting cancer.

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