Robert Hooke (1665)
Antonie van Leeuwenhoek (1674)
Where do “little animalcules” come from?
several
days
fresh
rainwater
no
animalcules
many
animalcules
Spontaneous generation
“So with animals, some spring from parent animals
according to their kind, whilst others grow
spontaneously and not from kindred stock; and of
these instances of spontaneous generation some
come from putrefying earth or vegetable matter, as is
the case with a number of insects, while others are
spontaneously generated in the inside of animals out
of the secretions of their several organs.”
--Aristotle (ca. 350 BC)
Spontaneous generation
“If you press a piece of underwear soiled with sweat,
together with some wheat in an open-mouthed jar,
after about 21 days the odor changes and the
ferment, coming out of the underwear and
penetrating through the husks of wheat, changes the
wheat into mice.”
-- Dr. Jan Baptista von Helmont (ca. 1600)
Spontaneous generation
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Mice from wheat and underwear
Flies from decaying meat
Microbes from rainwater
Francesco Redi (1665)
John Needham (1745)
boil
chicken broth
cool
seal
microorganims
grow
Louis Pasteur (1859)
Scientific Method
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Hypothesis based on observation or experience
Testable hypothesis
Predictions based on hypothesis
Experiment tests predictions
Data support or falsify the hypothesis (never proven)
Hypothesis with a body of strong support = theory
Theory ≈ Fact
Always provisional
Louis Pasteur (1859)
Hypothesis: Given a nutrient broth and exposure to air,
microorganisms will arise spontaneously
Louis Pasteur (1859)
Where do diseases come from?
Then
evil spirits
punishment for sin
bad air
imbalances
Now
bacteria
viruses
fungi
genetics
environment
the Germ Theory of Disease
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Diseases are caused by microbes
Bacteria, viruses, protists, fungi
Pathogens
Inhaled, ingested, etc.
Robert Koch (1884)
streak on growth medium
solidified with agar
clinical sample
with mixture of bacteria
pure culture
of a single type of bacteria
individual colonies
each from one cell
Koch’s Postulates
An organism is the cause of a disease if:
 It is found in all cases of the disease
and not in healthy individuals
 It can be isolated and grown in pure
culture
 Introducing the pure culture into a
healthy animal produces the same
disease
 The same organism can be re-isolated
from the diseaed animal
Why is it so important to identify the cause
of a disease?
“It is now in the power of man to cause all
parasitic diseases to disappear from the world.”
-- Louis Pasteur
Antibiotics
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President James Garfield shot on July 2, 1881
Bullet could not be found and removed
Died 80 days later of a bacterial infection
Assassin Charles Guiteau: “The doctors killed him. I
simply shot him.”
Alexander Fleming (1929)