Microbes and Health:
“What causes Yogurtness”?
Bacteria, Bacteria, Bacteria
• The single most successful life form on earth
• Prokaryotic organisms
• Exist in soil, water, in and on animals, plants
and humans
• Can orginize as
single units, pairs, long strings, helical shapes,
twisted spirochetes
• Divide by binary fission (some every 20 min!)
• Colonies originate from one bacterial cell
(clonal growth) and can have different
shapes
• Gram’s stain dye is taken up by bacteria with
thick cell walls (Gram + or -)
http://www.neatorama.com
Three types based upon their shapes…
Bacteria Classification
Coccus
Bacillus
Spirillum
Spherical shape
Oval or rod shaped
Spiral shaped
“strepto” from Greek streptos meaning ‘twisted chain’
“staphylo” from Greek staphule meaning ‘bunch of grapes’
Good Bacteria,
Bad Bacteria
• Bacteria as Pathogens
Cholera – Vibrio cholerae
Typhoid fever – Salomonella typhi
Anthrax – Bacillus anthracis
Tuberculosis – Mycobacterium tuberculosis
• Beneficial Bacteria
Rhizobia – soil bateria important for nitrogen fixation
Human bacterial flora – 500-100,00 species of
bacteria live in the human body
Lactobacillus species – convert milk to lactic acid
Salmonella typhimurium
Digestion of oil spills -
Marine bacteria: Acinetobacter calcoaceticus RAG-1
Genetic engineering – use of E.coli in industry and
reasearch
http://www.accessexcellence.org/AE/AEC/AEF/1994/brown_oil.html
Koch’s
Postulates
Robert Koch
• Robert Koch (pronounced “coke”)
- German physician and
bacteriologist
- Lived 1843-1910
• Developed a criteria for
determining whether a given
bacteria is the cause of a given
disease:
Known as Koch’s Postulates
Koch’s Postulates
1. The microorganism must be found in all
organisms suffering from the disease, but not in
healthy organisms.
2. The microorganism must be isolated from a
diseased organism and grown in pure culture.
3. The cultured microorganism should cause
disease when introduced into a healthy
organism.
4. The microorganism must be again isolated from
the inoculated, diseased experimental host and
identified as identical to the original specific
causative agent.
Robert Koch
 The
German doctor Robert Koch is
considered the founder of modern
bacteriology. His discoveries made a
significant contribution to the
development of the first ‘magic bullets’ chemicals developed to attack specific
bacteria - and Koch was awarded a
Nobel Prize in 1905
 Koch
developed a new experimental
method to test whether a particular microorganism is the cause of a disease.
Building on Pasteur's work on germ theory,
Koch used experiments to prove that the
bacterium Bacillus anthracis was the
cause of anthrax. How did he do this
given a need to follow a scientific
method?
Basic Experimental Procedure
 The
bacterium could be observed in the
tissue of anthrax victims. He extracted this
bacterium from a sheep which had died
of anthrax, grew it and injected a mouse
with it. The mouse developed the disease
as well. Koch repeated this process over
20 generations of mice, before he
announced in 1876 that he had proved
this bacterium caused anthrax.
Koch’s Postulates
1. The microorganism must be found in
abundance in all organisms suffering from
the disease but should not be found in
healthy animals!
(This process itself could be quite time
consuming to isolate the given
bacteria or virus.)
Koch’s Postulates
2. The microorganism must be isolated from
a diseased organism and grown in pure
culture.
Koch’s Postulates
3. The cultured microorganism should
cause disease when introduced into a
healthy organism.
Koch’s Postulates
4.
The microorganism must be re-isolated
from the inoculated, diseased
experimental host and identified as
being identical to the original specific
causative agent. (Recall Koch used 20
generations of mice to prove
conclusively that he had the correct
bacterium.)
Koch Continues His Work!

Koch continued to improve his methods and
techniques. By solidifying liquids such as broth
with gelatine and agar, for instance, he
created a solid medium for growing bacteria
which was easier to handle than the liquids
used by Pasteur. Koch's assistant Julius
Richard Petri (1852-1921) developed the Petri
dish, which made the observation of bacteria
even easier.
http://www.sciencemuseum.org.uk/broughttolife/people/robertkoch.aspx
Koch and Beyond
 Koch
and his team also developed ways
of staining bacteria to improve the
bacteria’s visibility under the microscope,
and were able to identify the bacterial
causes of tuberculosis (1882) and cholera
(1883). Adopting Koch's method, other
researchers were able to identify the
bacteria that caused diseases such as
typhus (1880), tetanus (1884) and the
plague (1894).
Microbes &
Health (Koch’s
Postulates)
A study of
‘yogurtness”
Microbes and Health:
A study of “yogurtness”
Scientists often use model systems to
simulate diseases in humans. In this activity,
you will use a model to test Koch’s
postulates!
Microbes and Health:
A study of “yogurtness”
Milk will represent a healthy individual and
yogurt will represent a diseased individual!
+++++++++++++
At times, milk will develop into a condition
that causes it to thicken and turn into
yogurt; the condition will be referred to as
the “yogurtness” disease!
Microbes and Health:
A study of “yogurtness”
You will play the role of a medical investigator
to determine the cause of the yogurtness. You
suspect that the yogurtness disease may be
caused by something that is found in the yogurt.
+++++++++++++
You will use Koch’s postulates to support or
refute the hypothesis that microbes found in
yogurt are the cause of the yogurtness disease!
Microbes and Health:
A study of “yogurtness”
+++++++++++++++++++++++++++++++++++
Of course, it is important to remember that
this activity is a simulated investigation; real
yogurt is a very healthy food. Microbes
found in yogurt are harmless and do not
cause disease in healthy humans.
+++++++++++++++++++++++++++++++++++
Bacteria in
Yogurt
Streptococcus thermophillus
lactic acid bacteria are
found in yogurt
lactic acid lowers the pH
in milk causing
casein (milk protein) to
denature and the milk to
curdle
Lactobacillus bulgaricus
Lactobacillus acidophilus
Lactobacillus casei
Bifidobacterium Bifidum
lactose
pyruvic acid
lactic acid
Procedures
Overview
Postulate 1
The microorganism
must be found in all
organisms suffering
from the disease, but
not in healthy
organisms.
1. Compare yogurt and milk and define the
symptoms of “yogurtness”:
- microscopic observations
- textures, consistency
- smell
- pH
Milk simulates a “healthy” sample
Yogurt simulates a “diseased” sample
Postulate 2
The microorganism must
be isolated from a
diseased organism and
grown in pure culture.
2. Observe the cultures using a microscope and
compare the different types of colonies.
3. Inoculate 3 separate petri dishes:
Heathy individual- milk
Diseased individual- yogurt
Control bacteria- E.coli (control)
4. Grow cultures overnight at 370C
Postulate 3
The cultured
microorganism should
cause disease when
introduced into a healthy
organism.
5. Inoculate fresh milk with bacteria colonies from
the petri dishes
6. Incubate overnight 370C
7. Assess symptoms of the subject (pH, smell,
texture). Are these the same symptoms of
“yogurtness”?
Postulate 4
The microorganism must
be again isolated from
the inoculated,
diseased experimental
host and identified as
being identical to the
original specific
causative agent
8. Observe yogurt and milk under the
microscope: Can the bacteria be matched to
the original culture?
Got Yogurt?