Pacterial Pathogenesis
An exotoxin is a soluble protein excreted by a
microorganism and can cause damage to the host by
destroying cells or disrupting normal cellular metabolism.
Both gram negative and gram positive bacteria produce
exotoxins.
Most exotoxins can be destroyed by heating. Some examples
include botulinum toxin produced by Clostridium botulinum,
the Corynebacterium diphtheriae exotoxin which is produced
during life threatening symptoms of diphtheria.
Click here to learn about diptheria.
What are some of the symptoms of diptheria?
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What organs are affected by the dipteria exotoxin?
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2. _______________________
3. _______________________
What is so interesting about the disease is that only
bacteria that contain a bacteriophage can cause the
disease.
Microbiological diagnosis involves isolation of
Corynebacterium diphtheriae on a selective medium such as
Hoyle's medium - containing potassium tellurite.
What color does the colony form overnight?
1. _______________________
Endotoxins are toxic components of the outer membrane of
some gram negative bacteria. They all characteristically
cause fever and aches. Examples include Salmonella thypi
which causes typhoid fever.
About 50% of all human diseases are caused by bacteria.
These pathogens must invade the host and then evade the
host’s immune defenses in order to grow and thrive.
Summer 2008 Workshop in Biology and Multimedia for High School
Teachers
Harvard University Life Sciences – HHMI Outreach Program
Some bacteria ordinarily are found on the host but it is
only when the host’s immune system fails that they become
pathogenic. These organisms are said to be opportunistic.
The USDA Food and Drug Administration has a web site where
they publish the “Bad Bug Book”
Click on this site and study the list of pathogenic
organisms.
In the space below
Click on Clostridium botulinum
In the space provided first describe the nature of the
microbe. How does it stain/ Does it produces spores? Etc.
Next, describe the toxin and its mode of action? What is
the name of the disease caused by this microbe?
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Now click on Shigella spp. and give a similar overview of
this organism.
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Feel free to check out other pathogens.
By the late 1800 microbiologists like Louis Pasteur and
John Lister began to link disease to pathogenic microbes.
However, it was Robert Koch who was the first to determine
a definitive link between a specific organism and certain
diseases.
Koch identified the microbe responsible for tuberculosis
(Mycobacterium tuberculosi), as well as the pathogenic
agent of anthrax (Bacillus anthracis).
Aside:
Summer 2008 Workshop in Biology and Multimedia for High School
Teachers
Harvard University Life Sciences – HHMI Outreach Program
Tuberculosis (abbreviated as TB for tubercle bacillus or
Tuberculosis) is a common and deadly infectious disease
caused by mycobacteria, mainly Mycobacterium tuberculosis.
One third of the world's current population has been
infected by TB, and new infections occur at a rate of one
per second.
World Health Organization (WHO). Tuberculosis Fact sheet
N°104 - Global and regional incidence. March 2006,
Retrieved on 6 October 2006.
http://www.who.int/mediacentre/factsheets/fs104/en/index.html
Anthrax is an acute disease in humans and animals caused by
the bacterium Bacillus anthracis which is highly lethal in
some forms.
Koch established guidelines or four criteria which are
known today as Koch’s postulates.
1. Find the same organism in each diseased individual.
2. Isolate the pathogen from one diseased individual and
grow it in a pure culture.
3.Use this cultured pathogen to induce the disease in a
healthy experimental animal (implying its import to do
controls)
4. Isolate the organism from the newly infected
experimental animal.
Review /Food for thought
What is a pathogen?
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What is meant by an “opportunistic” organism?
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Distinguish between an exotoxin and an endotoxin. Give an
example of each.
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What kind of a molecule is an exotoxin?
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Summer 2008 Workshop in Biology and Multimedia for High School
Teachers
Harvard University Life Sciences – HHMI Outreach Program
What are the usual symptoms of an endotoxin?
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What is the agent of tuberculosis?
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What is the agent of anthrax?
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List Koch’s postulates that are used to substantiate a
specific pathogen as the cause of
a disease?
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Can you think of a difficulty that arises in applying
Koch’s postulates to human pathogens?
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Summer 2008 Workshop in Biology and Multimedia for High School
Teachers
Harvard University Life Sciences – HHMI Outreach Program
Summer 2008 Workshop in Biology and Multimedia for High School
Teachers
Harvard University Life Sciences – HHMI Outreach Program